Geology

Question 1

What does AAPG stand for?

Answer 1

American Association of Petroleum Geologists

Question 2

A measure of the ability of a single fluid(such as water, gas or oil)
to flow through a rock formation when the formation is totally filled
with that fluid. The permeability measure of a rock filled with a
single fluid is different from the permeability measure of the same
rock filled with two or more fluids.

Answer 2

absolute permeability

Question 3

the percentage of the total bulk volume of a rock sample that is

composed of pore spaces or voids.

Answer 3

absolute porosity

Question 4

to enlarge by the addition of external parts or particles

Answer 4

accrete

Question 5

to treat oil-bearing limestone or other formations with acid for the

purpose of increasing production

Answer 5

acidize

Question 6

a record of the measurement of porosity, done by comparing the
depth to the time it takes for a sonic impulse to travel through a
given length of formation. Synonym sonic logging

Answer 6

acoustic log

Question 7

a well-logging method in which sound impulses are generated and

transmitted into the formations opposite the wellbore.

Answer 7

acoustic survey

Question 8

the process of recording the acoustic characteristics of subsurface
formations, based on the time required for a sound wave to travel
a specific distance through rock. The rate of travel depends on
the composition of the formation, its porosity, and its fluid content.
Synonym sonic logging

Answer 8

acoustic well logging

Question 9

a force of attraction that causes molecules of one substance to

cling to those of a different substance

Answer 9

adhesion

Question 10

the adhesion of a thin film of gas or liquid to the surface of a solid

Answer 10

adsorption

Question 11

a state or county tax based on the value of a property

Answer 11

ad valorem tax

Question 12

a sediment deposited by wind

Answer 12

aeolian deposit

Question 13

requiring free atmospheric oxygen for normal activity

Answer 13

aerobic

Question 14

bacteria that require free oxygen for their life processes. Aerobic
bacteria can produce slime or scum, which accumulates on metal
surfaces, causing oxygen-concentration cell corrosion.

Answer 14

aerobic bacteria

Question 15

What does AFE stand for?

Answer 15

authority for expenditure

Question 16

a surveying instrument consisting of sighting device, index and

reading or recording device

Answer 16

alidade

Question 17

hydrocarbons that have a straight chain of carbon atoms.

Answer 17

aliphatic hydrocarbons

Question 18

a series of open-chained hydrocarbons. The two major classes are
the series with saturated bonds and the series with unsaturated
bonds.

Answer 18

aliphatic series

Question 19

Give a synonym for paraffin

Answer 19

alkane

Question 20

the amount of oil or gas that can be produced legally from a well
per unit of time. In a state using proration, this figure is established
monthly by its conservation agency.

Answer 20

allowable

Question 21

a large sloping sedimentary deposit at the mouth of a canyon.
Especially in arid climates, composed of gravel and sand.

Answer 21

alluvial fan

Question 22

what does AAPG stand for?

Answer 22

American Association of Petroleum Geologists

Question 23

active in the absence of free oxygen

Answer 23

anaerobic

Question 24

bacteria that do not require free oxygen to live or that are not
destroyed by its absence. Under certain conditions, anaerobic
bacteria can cause scale to form in water handling facilities in
oilfields or hydrogen sulfide to be produced from sulfates.

Answer 24

anaerobic bacteria

Question 25

something that is similar to something else. Representing a range
of numbers by directly measurable variable quantities, such as
voltages.

Answer 25

analog

Question 26

finely crystalline, generally light-colored extrusive igneous rock
composed largely of plagioclase feldspar with smaller amounts of
dark colored minerals

Answer 26

andesite

Question 27

an unconformity in which formations above and below are not

parallel.

Answer 27

angular unconformity

Question 28

the common name for anhydrous calcium sulfate, CaSO4.

Answer 28

anhydrite

Question 29

an arched, inverted-trough configuration of folded rock layers.

Answer 29

anticline

Question 30

A body of coarse, poorly sorted sediments formed by the coales-
cence of alluvial or detrital fans. Can also be formed by the coales-
cence of submarine debris fans along the base of the continental

slope.

Answer 30

apron

Question 31

a permeable body of rock capable of yielding groundwater to wells

and springs.

Answer 31

aquifer

Question 32

the formula for evaluating the quantity of hydrocarbons in a forma-
tion. The form of the equation depends on its specific use.

Answer 32

Archie’s Equation

Question 33

Archie’s Equation

Answer 33

Sw^2=aRw/(O^mRt)

Question 34

a sandstone in which less than 15% of the total volume is silt and

clay.

Answer 34

arenite

Question 35

a sandstone composed largely of feldspar grains and deriving from

granitic source rock.

Answer 35

arkose

Question 36

hydrocarbons derived from or containing a benzene ring. Many
have an odor. Single-ring “ “ are the benzene series (benzene,

ethylbenzenes, an toluene). “ “also include naphthalene and an-
thracene.

Answer 36

aromatic hydrocarbons

Question 37

any method used to raise oil to the surface through a well after
reservoir pressure has declined to the point at which the well no
longer produces by means of natural energy, Sucker rod pumps,
gas lift, hydraulic pumps, and submersible electric pumps are the
most common means of artificial lift.

Answer 37

artificial lift

Question 38

a hard brown or black material composed principally of hydrocar-
bons. It is insoluble in water but soluble in gasoline and can be

obtained by heating some petroleums, coal tar, or lignite tar. It is
used for paving and roofing and in paints.

Answer 38

asphalt

Question 39

petroleum with a high proportion of naphthenic compounds, which
leave relatively high proportions of asphaltic residue when refined

Answer 39

asphaltic crude

Question 40

one of a group of solid, liquid, or semisolid materials that are pre-
dominantly mixtures of heavy hydrocarbons and their nonmetallic

derivatives and are obtained either from natural bituminous de-
posits or from the residues of petroleum refining.

Answer 40

asphaltic material

Question 41

natural gas that overlies and contacts crude oil in a reservoir. Also

called free gas.

Answer 41

associated gas

Question 42

a coral island consisting of a reef surrounding a lagoon

Answer 42

atoll

Question 43

an estimate of costs prepared by a lease operator and sent to
each nonoperator with a working interest for approval before work

is undertaken. Normally used in connection with well drilling op-
erations.

Answer 43

authority for expenditure

Question 44

the depositional environments associated with a shallow lagoon
shoreward from a coastal barrier island. These environments are
highly variable and may include tidal channels, salt marshes, shell
reefs, and mangrove swamps, among others.

Answer 44

backbarrier complex

Question 45

that part of the seashore that lies between high-tide and

storm-flood level.

Answer 45

backshore

Question 46

Give a synonym for open-hole completion.

Answer 46

barefoot completion

Question 47

an extrusive igneous rock that is dense, fine grained, and often

dark gray to black.

Answer 47

basalt

Question 48

1. a local depression in the earth’s crust in which sediments can
   accumulate to form thick sequences of sedimentary rock. 2.the
   area drained by a stream and its tributaries. 3.a geologic structure
   in which strata are inclined toward a common center.

Answer 48

basin

Question 49

a specific layer of earth or rock that presents a contrast to other
layers of different material lying above, below, or adjacent to it.

Answer 49

bed

Question 50

the surface that separates each successive layer of a stratified
rock from the preceding layer. It is here that minor changes in
sediments or depositional conditions can be observed.

Answer 50

bedding plane

Question 51

the gravel and coarse sand that are rolled and bounced along the

bottom of a flowing stream.

Answer 51

bed load

Question 52

solid rock exposed at the surface or just beneath the soil.

Answer 52

bedrock

Question 53

involving chemical reactions in living organisms.

Answer 53

biochemical

Question 54

a part of a stratigraphic unit that differs in its fossil fauna and flora

from the rest of the unit.

Answer 54

biofacies

Question 55

produced by living organisms

Answer 55

biogenic

Question 56

a reef or mound built by small organisms and their remains, such

as coral, plankton, and oysters.

Answer 56

bioherm

Question 57

the total mass of living organisms per unit volume per unit of time

Answer 57

biomass

Question 58

the thin zone of air, water, and soil where all terrestrial life exists.

Answer 58

biosphere

Question 59

relating to life, biologic; relating to the actions of living organisms.

Answer 59

biotic

Question 60

a type of mica that is high in magnesium and dark in color.

Answer 60

biotite

Question 61

an uncontrolled flow of gas, oil or other well fluids into the at-
mosphere. This can occur when formation pressure exceeds the

pressure applied to it by the column of drilling fluid. A kick warns
of the possibility of a blowout.

Answer 61

blowout

Question 62

a hole made by drilling or boring a wellbore.

Answer 62

borehole

Question 63

money paid by a contributing company in exchange for the infor-
mation received from the drilling on the completion of a well to a

specified depth, regardless of whether the well is a producer of oil
or gas or is a dry hole.

Answer 63

bottomhole money

Question 64

the part of a marine delta that lies farthest from shore. It consists
of silt and clay extending well out from the toe of the steep delta
face. Such beds grow slowly, out of reach of the effects of river
current and wave action.

Answer 64

bottomset bed

Question 65

a conglomerate rock composed largely of angular fragments

greater than 2 millimeters in diameter

Answer 65

breccia

Question 66

the breaking of solid rock into coarse, angular fragments by fault-
ing or crushing.

Answer 66

brecciation

Question 67

a test in which a well is shut in for a prescribed period of time
and a bottomhole pressure bomb run in the well to record the
pressure. From these data and from knowledge of pressures in
nearby wells, the effective drainage radius or the presence of
permeability barriers or other production deterrents surrounding
the wellbore can be estimated.

Answer 67

buildup test

Question 68

a paraffin hydrocarbon, C4H10

Answer 68

butane

Question 69

containing or composed largely of calcium carbonate, or calcite

CaSO3.

Answer 69

calcareous

Question 70

Another term for calcium carbonate

Answer 70

calcite

Question 71

a chemical compound of calcium, sulfur, and oxygen, CaSO4

Answer 71

calcium sulfate

Question 72

a record showing variations in wellbore diameter by depth, indicat-
ing undue enlargement due to caving in, washout, or other causes.

Answer 72

caliper log

Question 73

the rise and fall of liquids in small diameter tubes or tubelike

spaces, caused by the combined action of surface tension (co-
hesion) and wetting (adhesion)

Answer 73

capillarity

Question 74

a pressure or adhesive force caused by the surface tension of
water. This pressure causes the water to adhere more tightly to
the surface of small pore spaces than to larger ones. Capillary
pressure in a rock formation is comparable to the pressure of water
that rises higher in a small glass capillary than it does in a larger
tube.

Answer 74

capillary pressure

Question 75

deducted from income over the years of useful life of an item

purchased

Answer 75

capitalized

Question 76

a disklike plate of anhydrite, gypsum, limestone, or sulfur overlying
most salt domes in the Gulf Coast region. Impermeable rock
overlying an oil or gas reservoir that tends to prevent migration
of oil or gas out of the reservoir.

Answer 76

caprock

Question 77

a salt of carbonic acid. a compound containing the carbonate

radical CO3

Answer 77

carbonate

Question 78

a mud that forms on the seafloor by the accumulation of calcite

particles. It may eventually become limestone.

Answer 78

carbonate mud

Question 79

a sedimentary rock composed primarily of calcium carbonate or

calcium magnesium carbonate (dolomite)

Answer 79

carbonate rock

Question 80

a chemical reaction that produces carbonates. In geology, a form
of chemical weathering in which a mineral reacts with carbon
dioxide ( in solution as carbonic acid) to form a carbonate mineral.

Answer 80

carbonation

Question 81

of or relating to carbon, carbonic acid, or carbon dioxide.

Answer 81

carbonic

Question 82

to convert into carbon or a carbonic residue.

Answer 82

carbonize

Question 83

the difference between inflow and outflow of funds over a period
of time. This can be positive (profit) or negative (losses)

Answer 83

cash flow

Question 84

an economic analysis that relates investments to subsequent

revenues and also makes possible a comparison between invest-
ments. It usually also includes the general plan to be used for the

figuring of federal income tax on investments.

Answer 84

cash flow analysis

Question 85

the theory that the earth’s landforms assumed their present con-
figuration in a brief episode at the beginning of geologic histo-
ry-possibly in a single great catastrophic event- and have re-
mained relatively unchanged since that time.

Answer 85

catastrophism

Question 86

a natural cavity in the earth’s crust that is large enough to per-
mit human entry. Commonly formed in limestone by groundwater

leaching.

Answer 86

cavern

Question 87

the crystallization or precipitation of soluble minerals in the pore

spaces between clastic particles, causing them to become con-
solidated into sedimentary rock. Precipitation of a binding material

around grains or minerals in rocks.

Answer 87

cementation

Question 88

the time period from 65 million years ago until the present. It is
marked by rapid evolution of mammals and birds, flowering plants,
grasses, and shrubs, and little change in invertebrates.

Answer 88

cenozoic era

Question 89

a rock of precipitated silica whose crystalline structure is not
easily discernible and that fractures conchoidally (like glass). Flint,
jasper, and chat are forms of chert.

Answer 89

chert

Question 90

an analytical instrument that separates mixtures of substances
into identifiable components by means of chromatography.

Answer 90

chromatograph

Question 91

a method of separating a solution of closely related compounds by
allowing it to seep through an adsorbent so that each compound
is adsorbed in a separate layer.

Answer 91

chromatography

Question 92

a sedimentary rock composed of fragments of preexisting rocks.

The principal distinction among clastics is grain size. Conglomer-
ates, sandstones, and shales are clastic rocks

Answer 92

clastic rock

Question 93

sediments formed by the breakdown of large rock masses by
climatological processes, physical or chemical. The rocks formed
from these sediments.

Answer 93

clastics

Question 94

rock texture in which individual rock, mineral, or organic fragments
are cemented together by an amorphous or crystalline mineral
such as calcite.

Answer 94

clastic texture

Question 95

a term used for particles smaller than 1/256 millimeter (4 microns)
is size, regardless of mineral composition. A grou of hydrous
aluminum silicate minerals (clay minerals) . A sediment of fine
clastics

Answer 95

clay

Question 96

a carbonaceous, rocklike material that forms from the remains
of plants that were subjected to biochemical processes, intense
pressure, and high temperatures. It is used as fuel.

Answer 96

coal

Question 97

the attractive force between the same kinds of molecules.

Answer 97

cohesion

Question 98

a decrease in the volume of a stratum due to pressure exerted by

overlying strata, evaporation of water, or other causes.

Answer 98

compaction

Question 99

synonym for draped anticline

Answer 99

compaction anticline

Question 100

to finish work on a well and bring it to productive status

Answer 100

complete a well

Question 101

a light hydrocarbon liquid obtained by condensation of hydrocar-
bon vapors. It consists of varying proportions of butane, propane,

pentane, and heavier fractions, with little or no methane or ethane.

Answer 101

condensate

Question 102

a small-diameter conductive line used in electric wireline opera-
tions, such as electric well logging and perforating, in which the

transmission of electrical current is required.

Answer 102

conductor line

Question 103

a short string of large-diameter casing used to keep the wellbore
open and to provide a means of conveying the up-flowing drilling
fluid from the wellbore to the mud pit. A boot.

Answer 103

conductor pipe

Question 104

a sedimentary rock composed of pebbles of various sizes held
together by a cementing material such as clay. ___________
are similar to sandstone by are composed mostly of grains more
than 2 millimeters in diameter. Most conglomerates are found in
discontinuous, thin, isolated layers; they are not

Answer 104

Conglomerates

Question 105

water retained in the pore spaces, or interstices, of a formation

from the time the formation was created.

Answer 105

connate water

Question 106

in geology, any sharp or well-defined boundary between two
different bodies of rock. A bedding plane or unconformity that
separates formations.

Answer 106

contact

Question 107

a type of metamorphism that occurs when an intrusive body of
igneous rock changes the rocks immediately around it, primarily
by heating and by chemical alteration.

Answer 107

contact metamorphism

Question 108

according to a theory proposed by Alfred Wegener, a German
meteorologist, in 1910, the migration of continents across the
ocean floor like rafts drifting at sea.

Answer 108

continental drift

Question 109

the transition zone between the continental slope and the oceanic

abyss

Answer 109

continental rise

Question 110

a zone, adjacent to a continent, that extends from the lower
waterline to the continental slope, the point at which the seafloor
begins to slope off steeply into the oceanic abyss.

Answer 110

continental shelf

Question 111

of or relating to the geologic period from about 135 million to 65
million years ago at the end of the Mesozoic era, or to the rocks
formed during this period, including the extensive chalk deposits
for which it was named.

Answer 111

cretaceous

Question 112

What is the thickness of the outer layer of the Earth, the crust?

Answer 112

5-30 miles (10-50 km)

Question 113

What elements is the Earth’s crust mainly composed of?

Answer 113

oxygen, aluminum, silicon

Question 114

the test involving the observation of a formation sample immersed
in a solvent under UV light. If any hydrocarbons, which fluoresce
under UV light, are in the sample, they will dissolve and appear as
streamers or streaks of color different from the solvent.

Answer 114

cut fluorescence test

Question 115

the fragments of rock dislodged by the bit and brought to the

surface in the drilling mud.

Answer 115

cuttings

Question 116

What are the dimensions of a darcy?

Answer 116

L squared

Question 117

What does DCFROR stand for? Economics

Answer 117

Discounted Cash Flow Rate of Return

Question 118

a graphic representation of predicted financial gains or losses for

the outcomes of several courses of action

Answer 118

decision tree

Question 119

the action of earth stresses that results in folding, faulting, shear-
ing, or compression of rocks

Answer 119

deformation

Question 120

a graph that compares flowing bottomhole pressure of a well
with production in barrels of oil per day to show the relationship
between drawdown and the producing rate. Its main purpose is to
find the most efficient flow rate for the well.

Answer 120

deliverability plot

Question 121

to exhaust a supply.

Answer 121

deplete

Question 122

a reduction in U.S taxes for owners of an economic interest in
minerals in place to compensate for exhaustion of an irreplaceable
capital asset. Economic interest includes mineral interest, working
interest in lease, royalty overriding royalty, production payment
interest, net profits interest.

Answer 122

depletion allowance

Question 123

the laying down of sediments or other potential rock-forming ma-
terial.

Answer 123

deposition

Question 124

a well drilled in proven territory in a field to complete a pattern of

production. An exploitation well.

Answer 124

development well

Question 125

Of or relating to the geologic period about 400 million to 350 million
years ago in the Paleozoic era, or to rocks formed during this
period, including those of Devonshire, England. Where outcrops
of such rock were first identified.

Answer 125

devonian

Question 126

the chemical and physical changes that sedimentary deposits un-
dergo (compaction, cementation, recrystallization, and sometimes

replacement) during and after lithification.

Answer 126

diagenesis

Question 127

a dome or anticlinal fold in which a mobile plastic core has ruptured
the more brittle overlying rock. Also called piercement dome.

Answer 127

diapir

Question 128

the process or processes of deformation of the earth’s crust that

Answer 128

diastrophism

Question 129

any of the algae of the class Bacillariophyceae, noted for symmet-
rical and sculptured siliceous cell walls. After death, the cell wall

persists and forms diatomite. Diatoms appeared in the Cretaceous
period.

Answer 129

diatom

Question 130

a rock of biochemical origin, which is composed of the siliceous

(glassy) shells of microscopic algae called diatoms.

Answer 130

diatomite

Question 131

The difference between two fluid pressures.

Answer 131

differential pressure

Question 132

intrusive, or platonic, generally coarse-grained igneous rock com-
posed largely of plagioclase feldspar with smaller amounts of dark

colored minerals. Also called black granite.

Answer 132

diorite

Question 133

the depth of liquid in a storage tank.

Answer 133

dip

Question 134

oilwell-surveying method that determines direction and angle of
formation dip in relation to borehole. Records data that permit
computation of amount and direction of formation dip relative to
hole axis and this provides information about geologic structure of
the formation. Also called dipmeter log or dip log.

Answer 134

dipmeter survey

Question 135

upward or downward displacement of a fault plane

Answer 135

dip slip

Question 136

intentional deviation of a wellbore from the vertical

Answer 136

deviated drilling

Question 137

an unconformity above and below which rock strata are parallel. A
disconformity may or may not be parallel to these strata.

Answer 137

disconformity

Question 138

the rate that causes the sum of the discounted outflows and
inflows of funds to equal the net cash outlay in year zero of a
project.

Answer 138

DCFROR

Question 139

the first oil or gas well drilled in a new field that reveals the
presence of a hydrocarbon-bearing reservoir. Subsequent wells
are development wells or exploitation wells.

Answer 139

discovery well

Question 140

in a flowing stream of water, those products of weathering that are

carried along in solution

Answer 140

dissolved load

Question 141

a contact of sale of oil or gas to a purchaser who is directed to pay
for the oil or gas products according to the proportions set out in
the division order. The purchaser may require execution thereof by
all owners of interest in the property.

Answer 141

division order

Question 142

a type of sedimentary rock similar to limestone but containing
more than 50 % magnesium carbonate; sometimes a reservoir
rock for petroleum.

Answer 142

dolomite

Question 143

The shrinking of the solid volume of rock as limestone turns into

dolomite, the conversion of limestone to dolomite rock by re-
placement of a portion of the calcium carbonate with magnesium

carbonate.

Answer 143

dolomitization

Question 144

a geologic structure resembling an inverted bowl, a short anticline

that dips or plunges on all sides.

Answer 144

dome

Question 145

Where did the slang term doodlebugger come from?

Answer 145

The charges detonated by the seismologist create puffs of smoke
that resemble the loose dirt thrown into the air by the antlion or
doodlebug when building its trap.

Question 146

the direct erosive action of flowing water on a streambed.

Answer 146

downcutting

Question 147

lower on the formation angle than a particular point

Answer 147

downdip

Question 148

frictional deformation of the layers above or below an overthrust

fault

Answer 148

drag fold

Question 149

an anticline composed of sedimentary deposits atop a reef or atoll,
along whose flanks greater thicknesses of sediments have been
deposited and compacted than atop the reef itself. Also called
compaction anticline

Answer 149

draped anticline

Question 150

the difference between static and flowing bottomhole pressures.
The distance between the static level and the pumping level of the
fluid in the annulus of a pumping well.

Answer 150

drawdown

Question 151

a record that describes each formation encountered and lists the

drilling time relative to depth, usually 5-10 ft. intervals.

Answer 151

driller’s log

Question 152

the conventional method for formation testing.

Answer 152

drill stem test

Question 153

the energy of expanding gas, inflowing water, or other natural
or artificial mechanisms that forces crude oil out of the reservoir
formation and into the wellbore.

Answer 153

drive

Question 154

any well that does not produce oil or gas in commercial quantities.
A “ -“ may flow water, gas, or even oil, but not in amounts large
enough to justify production.

Answer 154

dry hole

Question 155

money paid by a contributing company on the basis of so much
per foot drilled by the primary company in return for information
gained from the drilling. The contribution is paid only if the well is
a dry hole in all formations encountered in drilling.

Answer 155

dry hole money

Question 156

an accumulation of windborne sand in that part of the seashore

that lies above stormflood level.

Answer 156

dune field

Question 157

science of the relationship between organisms and their environ-
ment.

Answer 157

ecology

Question 158

a measure of the ability of a single fluid to flow through a rock when

another fluid is also present in the pore spaces.

Answer 158

effective permeability

Question 159

the ratio of effective permeability to absolute permeability. The “ -
“ of a rock is 1 when only that fluid is present, and 0 when another
fluid prevents all flow of that fluid.

Answer 159

relative permeability

Question 160

the percentage of the bulk volume of a rock sample that is com-
posed of interconnected pore spaces that allow the passage of

fluids through the sample.

Answer 160

effective porosity

Question 161

effective porosity formula

Answer 161

Interconnected pore volume + dead end pore volume

Question 162

absolute porosity formula

Answer 162

disconnected pore volume + connected pore volume + dead end

pore volume

Question 163

disconnected pore volume + connected pore volume + dead end

pore volume

Answer 163

absolute porosity

Question 164

symbol for oxidation-reduction potential

Answer 164

Eh

Question 165

a mixture in which one liquid, termed the dispersed phase, is

uniformly distributed (usually as minute globules) in another liq-
uid, called the continuous phase or dispersion medium; in a wa-
ter-in-oil emulsion, the reverse hold.

Answer 165

emulsion

Question 166

a record of certain electrical characteristics (such as resistivity and
conductivity) of formations traversed by the borehole. It is made to
identify the formation, determine the nature and amount of fluids
they contain, and estimate their depth. Also called an electric log
or electric survey.

Answer 166

electric well log

Question 167

The introduction of artificial drive and displacement mechanisms
into a reservoir to produce a portion of the oil unrecoverable by
primary recovery methods. These methods include waterflooding,

chemical flooding, most types of gas injection, and thermal recov-
ery.

Answer 167

enhanced oil recovery (EOR)

Question 168

drawn in and transported by the flow of a fluid.

Answer 168

entrained

Question 169

a shallow arm of the ocean that extends from the continental shelf
deep into the interior of the continent. Also called epicontinental
sea.

Answer 169

epeiric sea

Question 170

a division of geologic time; a subdivision of a geologic period,

longer than an age and shorter than a period.

Answer 170

epoch

Question 171

one of the major divisions of geologic time that divides an eon.

Answer 171

era

Question 172

the process by which material (such as rock or soil) is worn away

or removed (as by wind or water)

Answer 172

erosion

Question 173

a coastal indentation or bay into which a river empties and where

fresh water mixes with seawater.

Answer 173

estuary

Question 174

a paraffin hydrocarbon, C2H6; under atmospheric conditions, a

gas, one of the components of natural gas.

Answer 174

ethane

Question 175

a sedimentary rock formed by precipitation of dissolved solids from
water evaporating in enclosed basins. Examples are gypsum and
salt.

Answer 175

evaporite

Question 176

a risk analysis process that multiplies expected gain or loss of a
decision by its probability of occurrence and averages all possible
outcomes to choose the action with the highest expected benefit.

Answer 176

expected value concept

Question 177

deducted from income in the year in which the expenditure is

incurred.

Answer 177

expensed

Question 178

a well drilled to permit more effective extraction of oil from a

reservoir. Sometimes called a development well.

Answer 178

exploitation well

Question 179

the search for reservoirs of oil and gas, including aerial and
geophysical surveys, geologic studies, core testing, and drilling of
wildcats.

Answer 179

exploration

Question 180

when explosives are used to fracture a formation. The rubble pre-
vents fracture healing, making the use of proppant unnecessary.

Answer 180

explosive fracturing

Question 181

the emission of magma at the earth’s surface.

Answer 181

extrusion

Question 182

igneous rock formed from lava poured out on the earth’s surface.

Answer 182

extrusive rock

Question 183

part of a bed of sedimentary rock that differs significantly from

other parts of the bed.

Answer 183

facies

Question 184

coarse-grained, poorly sorted sedimentary rock derived from sed-
iments deposited in alluvial fans; a type of conglomerate.

Answer 184

fanglomerate

Question 185

a contract between a lessee and a third party to assign leasehold
interest to the third party, conditional on the third party’s drilling a
well within the expiration date of the primary term of the lease.

Answer 185

farmout

Question 186

a break in the earth’s crust along which rocks on one side have

been displaced.

Answer 186

fault

Question 187

a surface along which faulting has occurred.

Answer 187

fault plane

Question 188

a subsurface hydrocarbon trap created by faulting, in which an
impermeable rock layer has moved opposite the reservoir bed or
where impermeable gouge has sealed the fault and stopped fluid
migration.

Answer 188

fault trap

Question 189

the principle that fossils in a stratigraphic sequence succeed one

another in a definite, recognizable order.

Answer 189

faunal succession

Question 190

a group of silicate minerals that includes a wide variety of potas-
sium, sodium, and aluminum silicates. Feldspar makes up about

60% of the outer 15 km of the earth’s crust.

Answer 190

feldspar

Question 191

A subsurface condition in which the pore pressure of a geolog-
ic formation exceeds or is less than the expected, or normal,

formation pressure. When impermeable rocks such as shales
are compacted rapidly, their pore fluids cannot always escape
and must then support the total overlying rock column, leading
to abnormally high formation pressures. Excess pressure, called
overpressure or geopressure, can cause a well to blowout or
become uncontrollable during drilling. Severe underpressure can
cause the drillpipe to stick to the underpressured formation.

Answer 191

abnormal pressure

Question 192

The accumulation of stratigraphic sequences by deposition that
stacks beds atop each other, building upwards during periods of
balance between sediment supply and accommodation.

Answer 192

aggradation

Question 193

Predictable variation of a property of a material with the direction
in which it is measured, which can occur at all scales. For a crystal

of a mineral, variation in physical properties observed in differ-
ent directions is anisotropy. In rocks, variation in seismic velocity

measured parallel or perpendicular to bedding surfaces is a form
of anisotropy. Often found where platy minerals such as micas
and clays align parallel to depositional bedding as sediments are
compacted, anisotropy is common in shales.

Answer 193

anisotropy

Question 194

Describing rocks or sediments containing particles that are silt-
or clay-sized, less than 0.625 mm in size. Most have a high

clay-mineral content, and many contain a sufficient percentage of
organic material to be considered a source rock for hydrocarbon.

Answer 194

agrillaceous

Question 195

The measurement of age in years. The determination of this of
rocks, minerals and fossils, in years before the present, is the basis
for the field of geochronology. The measurement of the decay
of radioactive isotopes, especially uranium, strontium, rubidium,

argon and carbon, has allowed geologists to more precisely de-
termine the age of rock formations. Tree rings and seasonal sedi-
mentary deposits called varves can be counted to determine ab-
solute age. Although the term implies otherwise, “absolute” ages

typically have some amount of potential error and are inexact.
Relative age, in contrast, is the determination of whether a given
material is younger or older than other surrounding material on
the basis of stratigraphic and structural relationships, such as
superposition, or by interpretation of fossil content.

Answer 195

absolute age

Question 196

Different from what is typical or expected, or different from what is
predicted by a theoretical model. The difference or anomaly may
refer to the measurement of the difference between an observed
or measured value and the expected values of a physical property.
Anomalies can be of great interest in hydrocarbon and mineral
exploration because they often indicate hydrocarbon and mineral
prospects and accumulations, such as geologic structures like
folds and faults. Geochemical anomalies at the surface of the
Earth can also indicate an accumulation of hydrocarbons at depth.
Geophysical anomalies, such as amplitude anomalies in seismic
data and magnetic anomalies in the Earth’s crust, can also be
associated with hydrocarbon accumulations.

Answer 196

anomalous

Question 197

A solid or nearly solid form of bitumen that can melt upon heat-
ing and contains impurities such as nitrogen, oxygen and sulfur.

Asphalt forms naturally when the light components or volatiles of
petroleum have been removed or evaporated.

Answer 197

asphalt

Question 198

The measurement of the permeability, or ability to flow or transmit
fluids through a rock, conducted when a single fluid, or phase, is

present in the rock. The symbol most commonly used for perme-
ability is k, which is measured in units of darcies or millidarcies.

Answer 198

absolute permeability

Question 199

A telescopic surveying device used to construct surface topo-
graphic and geologic maps in the field. This device is mounted

on a plane table, which has a sheet of paper on which to draw
the map, and an object or location is sighted through the alidade.
The edge of the alidade is aligned in the azimuthal direction of the
object or location. The vertical angle from which elevation of the
location can be calculated is measured using the calibrated arc of
the alidade.

Answer 199

alidade

Question 200

The relatively plastic layer of the upper mantle of the Earth on

which the tectonic plates of the lithosphere move. The asthenos-
phere is approximately 200 km [124 miles] thick and, owing to its

depth below the Earth’s surface, warm (~ 1400 oC) [2640 oF] but
not molten. Here the mantle deforms by plastic flow in response
to applied pressures above 100 MPa [14,500 psi]. This zone is
considered coincidental, at least below oceanic crust, with the
low-velocity zone of the upper mantle.

Answer 200

asthenosphere

Question 201

The measurement of pressure relative to the pressure in a vacu-
um, equal to the sum of the pressure shown on a pressure gauge

and atmospheric pressure.

Answer 201

absolute pressure

Question 202

The condition of an environment in which free oxygen is lacking

or absent.

Answer 202

anoxic

Question 203

The orientation of a planar or linear feature in three-dimensional
space. Planar features that are not horizontal, such as tilted strata,
are described by their strike, or the azimuth of the intersection of
the plane with a horizontal surface, and the dip, or the magnitude
of its inclination from a horizontal reference. The trend and plunge of linear features, such as the axis of a fold, describe the azimuth
of the line and its deviation from horizontal.

Answer 203

attitude

Question 204

The deepest area of the ocean basins. The depositional energy is
low and fine-grained sediments are deposited slowly by waning
turbidity currents or from suspension in the water. The water is
thousands of meters deep (>2,000 m) [>6,520 ft] so it is cold and
sunlight is minimal.

Answer 204

abyss

Question 205

Pertaining to materials, particularly rock masses, that formed

somewhere other than their present location, and were trans-
ported by fault movements, large-scale gravity sliding, or similar

processes. Autochthonous material, in contrast, formed in its pre-
sent location. Landslides can result in large masses of allochtho-
nous rock, which typically can be distinguished from autochtho-
nous rocks on the basis of their difference in composition. Faults

and folds can also separate allochthons from autochthons.

Answer 205

allochthonous

Question 206

Pertaining to an anticline, an arch-shaped fold in rock in which
rock layers are upwardly convex. The oldest rock layers form the
core of the fold, and outward from the core progressively younger
rocks occur. Anticlines form many excellent hydrocarbon traps,
particularly in folds with reservoir-quality rocks in their core and
impermeable seals in the outer layers of the fold. A syncline is the
opposite type of fold, having downwardly convex layers with young
rocks in the core.

Answer 206

anticlinal

Question 207

In plate tectonics, a failed rift arm. At the junctions of tectonic

plates, three intersecting lithospheric plates typically are sepa-
rated by “arms.” Arms might be areas of rifting, convergence or

transform faults (similar to a strike-slip fault). The arm along which
the motion that spreads the plates apart ceases is termed the
failed arm, or aulacogen. Spreading or rifting along the other arms
of the triple junction can form new oceanic basins, whereas the
aulacogen can become a sediment-filled graben.

Answer 207

aulacogen

Question 208

Pertaining to the depositional environment of the deepest area
of the ocean basins, the abyss. The depositional energy is low,
the abyssal plain is flat and nearly horizontal, and fine-grained
sediments are deposited slowly by waning turbidity currents or
from suspension in the water. The water is thousands of meters
deep (> 2000 m) [6520 ft], so the water is cold and sunlight is
minimal.

Answer 208

abyssal

Question 209

Pertaining to minerals or rock fragments that formed in one lo-
cation but were transported to another location and deposited.

Clastic sediments in a rock such as sandstone are allogenic, or
formed elsewhere.

Answer 209

allogenic

Question 210

A type of structural hydrocarbon trap whose closure is controlled

by the presence of an anticline.

Answer 210

anticlinal trap

Question 211

Pertaining to minerals or materials that grow in place with a rock,
rather than having been transported and deposited. These include
quartz, chlorite and other pore-filling minerals or cements that
grow during diagenesis. Evaporite minerals are authigenic, or
formed in situ.

Answer 211

authigenic

Question 212

Sequence stratigraphic term for the amount of space available
for sediment accumulation. Dominant influences on the amount
of accommodation, or accommodation space, include subsidence
and eustasy.

Answer 212

accomodation

Question 213

Pertaining to the subaerial (as opposed to submarine) environ-
ment, action and products of a stream or river on its floodplain,

usually consisting of detrital clastic sediments, and distinct from
subaqueous deposition such as in lakes or oceans and lower

energy fluvial deposition. Sediments deposited in an alluvial en- vironment can be subject to high depositional energy, such as
fast-moving flood waters, and may be poorly sorted or chaotic.

Answer 213

alluvial

Question 214

An arch-shaped fold in rock in which rock layers are upwardly

convex. The oldest rock layers form the core of the fold, and out-
ward from the core progressively younger rocks occur. Anticlines

form many excellent hydrocarbon traps, particularly in folds with
reservoir-quality rocks in their core and impermeable seals in the
outer layers of the fold. A syncline is the opposite type of fold,
having downwardly convex layers with young rocks in the core.

Answer 214

anticline

Question 215

Materials, especially rock masses, that formed in their present

location and have not been transported. Fault surfaces can sep-
arate indigenous rocks from allochthonous rocks, although some

allochthonous rocks are clearly delineated by their differing com-
position.

Answer 215

autochthon

Question 216

The phase in the development of a petroleum system during which
hydrocarbons migrate into and remain trapped in a reservoir.

Answer 216

accumulation

Question 217

Material deposited in an alluvial environment, typically detrital

sediments that are poorly sorted.

Answer 217

alluvium

Question 218

A minor, secondary fault, usually one of a set, whose sense of
displacement is opposite to its associated major and synthetic
faults. Antithetic-synthetic fault sets are typical in areas of normal
faulting.

Answer 218

antithetic fault

Question 219

A boundary of colliding lithospheric plates. The present subduction
zones of the Pacific Rim, the older mountains of the Alps, and the
Himalayas represent active margins.

Answer 219

active margin

Question 220

The condition of an environment in which free oxygen is lacking or
absent. A description of organisms that can survive in the absence
of oxygen, particularly bacteria.

Answer 220

anaerobic

Question 221

The angle that a plane makes with the horizontal measured in any
randomly oriented section rather than perpendicular to strike.

Answer 221

apparent dip

Question 222

In folded rocks, the imaginary surface bisecting the limbs of the
fold. This is called the axial plane when the fold is symmetrical
and the lines defined by the points of maximum curvature of each
folded layer, or hinge lines, are coplanar.

Answer 222

axial surface

Question 223

Pertaining to the environment of deposition of sediments by wind,

such as the sand dunes in a desert. Because fine-grained sedi-
ments such as clays are removed easily from wind-blown deposits,

eolian sandstones are typically clean and well-sorted.

Answer 223

aeolian

Question 224

A surface that separates younger strata from eroded, dipping,
older strata and represents a gap in the geologic record.

Answer 224

angular unconformity

Question 225

The phase of petroleum operations that immediately follows suc-
cessful exploratory drilling. During this, delineation wells might be

drilled to determine the size of the oil or gas field and how to
develop it most efficiently.

Answer 225

appraisal

Question 226

The angle between the vertical projection of a line of interest onto
a horizontal surface and true north or magnetic north measured
in a horizontal plane, typically measured clockwise from north.

Answer 226

azimuth

Question 227

Predictable variation of a property of a material with the direction
in which it is measured, which can occur at all scales. For a crystal

of a mineral, variation in physical properties observed in differ-
ent directions is aeolotropy (also known as anisotropy). In rocks,

variation in seismic velocity measured parallel or perpendicular to
bedding surfaces is a form of aeolotropy. Often found where platy
minerals such as micas and clays align parallel to depositional
bedding as sediments are compacted, aeolotropy is common in
shales.

Answer 227

aeolotropy

Question 228

A member of the evaporite group of minerals and the soft rock
comprising this formed by precipitation of calcium sulfate from

evaporation of seawater. Anhydrite can also form through the de-
hydration of gypsum, another sulfate mineral found in evaporites.

Anhydrite may occur as a cap rock above salt domes.

Answer 228

anhydrite

Question 229

A body of rock whose fluid saturation, porosity and permeability
permit production of groundwater. A water-bearing

Answer 229

aquifer

Question 230

Pertaining to the angle between the vertical projection of a line of
interest onto a horizontal surface and true north or magnetic north
measured in a horizontal plane, typically measured clockwise from
north.

Answer 230

azimuthal

Question 231

The surface or near-surface, unconsolidated sedimentary layer
that has been subject to weathering and whose pores are air-filled
instead of liquid-filled. An aerated layer typically has a low seismic
velocity.

Answer 231

aerated layer

Question 232

Having directionally dependent properties. For a crystal of a min-
eral, variation in physical properties observed in different direc-
tions is anisotropy. In rocks, variation in seismic velocity mea-
sured parallel or perpendicular to bedding surfaces is a form

of anisotropy. Often found where platy minerals such as micas
and clays align parallel to depositional bedding as sediments are
compacted, anisotropy is common in shales.

Answer 232

anisotropic

Question 233

Describing sandy-textured rock or sediment. This does not neces-
sarily imply silica-rich, but rather particles of sand size, 0.625 to 2

mm, according to the Udden-Wentworth scale.

Answer 233

arenaceous

Question 234

[BaSO4] A dense sulfate mineral that can occur in a variety of

rocks, including limestone and sandstone, with a range of acces-
sory minerals, such as quartz, chert, dolomite, calcite, siderite

and metal sulfides. Barite is commonly used to add weight to
drilling fluid. Barite is of significance to petrophysicists because
excess barite can require a correction factor in some well log
measurements.

Answer 234

barite

Question 235

A layer of sediment or sedimentary rock, or stratum. This is the

smallest stratigraphic unit, generally a centimeter or more in thick-
ness. To be labeled a bed, the stratum must be distinguishable

from adjacent beds.

Answer 235

bed

Question 236

Pertaining to the environment and conditions of organisms

Answer 236

benthic

Question 237

The application of plant and animal fossils to date and correlate
strata in order to elucidate Earth history, combining the principles
of paleontology and stratigraphy. In the petroleum industry, this
often denotes the use of terrestrial (pollen and spores) and marine
(diatoms, foraminifera, nannofossils) microfossils to determine the

absolute or relative age and depositional environment of a partic-
ular formation, source rock or reservoir of interest.

Answer 237

biostratigraphy

Question 238

A map on which primary data and interpretations can be plotted.
It typically includes locations of lease or concession boundaries,

wells, seismic survey points and other cultural data such as build-
ings and roads, with a geographic reference such as latitude

and longitude or Universal Transverse Mercator (UTM) grid in-
formation. Geologists use topographic maps as base maps for

construction of surface geologic maps. Geophysicists typically use
shot point maps, which show the orientations of seismic lines and
the specific points at which seismic data were acquired, to display
interpretations of seismic data. In the field, geologists can use a
plane table and alidade to construct a base map.

Answer 238

base map

Question 239

The thickness of a layer or stratum of sedimentary rock measured
perpendicular to its lateral extent, presuming deposition on a
horizontal surface. Because sediment deposition can occur on
inclined surfaces, apparent or measured bed thickness might differ
from true bed thickness. The thickness of a given bed often varies
along its extent.

Answer 239

bed thickness

Question 240

Organisms that live at the bottom of a body of water.

Answer 240

benthos

Question 241

Naturally-occurring, inflammable organic matter formed from
kerogen in the process of petroleum generation that is soluble in
carbon bisulfide. This includes hydrocarbons such as asphalt and
mineral wax. Typically solid or nearly so, brown or black, bitumen
has a distinctive petroliferous odor. Laboratory dissolution with
organic solvents allows determination of the amount of bitumen
in samples, an assessment of source rock richness.

Answer 241

bitumen

Question 242

The rock layer below which economic hydrocarbon reservoirs are
not expected to be found, sometimes preceded by economic. This
is usually older, deformed igneous or metamorphic rocks, which
seldom develops the porosity and permeability necessary to serve
as a hydrocarbon reservoir, and below which sedimentary rocks
are not common. Basement rocks typically have different density,
acoustic velocity, and magnetic properties from overlying rocks.

Answer 242

basement

Question 243

Solid rock either exposed at the surface or situated below surface

soil, unconsolidated sediments and weathered rock.

Answer 243

bedrock

Question 244

A material composed of clay minerals, predominantly montmoril-
lonite with minor amounts of other smectite group minerals, com-
monly used in drilling mud. Bentonite swells considerably when

exposed to water, making it ideal for protecting formations from
invasion by drilling fluids. Montmorillonite forms when basic rocks
such as volcanic ash in marine basins are altered.

Answer 244

bentonite

Question 245

A characteristic sequence of sedimentary structures occurring in

sedimentary rocks deposited in areas of deep water sedimenta-
tion by turbidity currents, which form deposits called turbidites. In

theory, a complete Bouma sequence comprises sediments that
fine upwards, consisting of a lowermost layer of coarse, chaotic
clastic sediments deposited under conditions of high depositional
energy overlain by successively finer grained and better stratified

sediments like sands and muds deposited under calmer condi-
tions that are labeled as Units A though E. In practice, however,

the chaotic, high-energy nature of turbidite deposition can alter
or remove underlying sediments so that incomplete sequences of
sediments typically remain preserved.

Answer 245

bouma sequence

Question 246

A depression in the crust of the Earth, caused by plate tectonic ac-
tivity and subsidence, in which sediments accumulate. Sedimen-
tary basins vary from bowl-shaped to elongated troughs. Basins

can be bounded by faults. Rift basins are commonly symmetrical;
basins along continental margins tend to be asymmetrical. If rich
hydrocarbon source rocks occur in combination with appropriate
depth and duration of burial, then a petroleum system can develop
within the basin. Most basins contain some amount of shale, thus
providing opportunities for shale gas exploration and production.

Answer 246

basin

Question 247

A zone of the upper mantle in which earthquakes occur when a
lithospheric plate is subducted, named in honor of seismologists
Kiyoo Wadati and Hugo Benioff. The dip of this zone, also referred

to as the Wadati-Benioff zone, coincides with the dip of the sub-
ducting plate. The Wadati-Benioff zone extends to a depth of about

700 km [435 miles] from the Earth’s surface.

Answer 247

benioff zone

Question 248

Pertaining to biostratigraphy, the application of plant and animal
fossils to date and correlate strata in order to elucidate Earth
history, combining the principles of paleontology and stratigraphy.
In the petroleum industry, biostratigraphy often denotes the use of
terrestrial (pollen and spores) and marine (diatoms, foraminifera,
nannofossils) microfossils to determine the absolute or relative
age and depositional environment of a particular formation, source
rock or reservoir of interest.

Answer 248

biostratigraphic

Question 249

Water containing more dissolved inorganic salt than typical sea-
water.

Answer 249

brine

Question 250

Pertaining to the environment of deposition and the organisms
of the ocean between depths of 200 m [656 ft], the edge of the
continental shelf, and 2000 m [6560 ft]. The bathyal environment
is intermediate between the neritic environment and the abyss.

Answer 250

bathyal

Question 251

The crystalline form of calcium carbonate and chief constituent of
limestone and chalk. Calcite reacts readily with dilute hydrochloric
acid [HCl], so the presence of calcite can be tested by simply
placing a drop of acid on a rock specimen.

Answer 251

calcite

Question 252

The study of the ages of strata. The comparison, or correlation,
of separated strata can include study of their relative or absolute
ages.

Answer 252

chronostratigraphy

Question 253

The physical process by which sediments are consolidated, re-
sulting in the reduction of pore space as grains are packed closer

together. As layers of sediment accumulate, the ever increasing

overburden pressure during burial causes compaction of the sed-
iments, loss of pore fluids and formation of rock as grains are

welded or cemented together.

Answer 253

compaction

Question 254

A reservoir in which buoyant forces keep hydrocarbons in place
below a sealing caprock. Reservoir and fluid characteristics of
conventional reservoirs typically permit oil or natural gas to flow
readily into wellbores. The term is used to make a distinction from
shale and other unconventional reservoirs, in which gas might
be distributed throughout the reservoir at the basin scale, and in
which buoyant forces or the influence of a water column on the
location of hydrocarbons within the reservoir are not significant.

Answer 254

conventional reservoir

Question 255

A crust of coarse sediments or weathered soil rich in calcium car-
bonate. It forms when lime-rich groundwater rises to the surface by

capillary action and evaporates into a crumbly powder, forming a
tough, indurated sheet called calcrete. Caliche typically occurs in
desert or semi-arid areas. Of particular concern to geophysicists is
the difficulty in acquiring good seismic data when shooting through
a layer of caliche. A deposit of sodium nitrate that is mined and
used for fertilizer in parts of South America.

Answer 255

caliche

Question 256

The value of the separation between two adjacent contours. A net
pay isopach map might have a contour interval of 10 feet [3 m],
whereas a structure contour map might have a contour interval of
1000 feet [300 m]. Contour intervals are chosen according to the
map scale and the amount and distribution of control points.

Answer 256

Contour Intervals (CI)

Question 257

Describes a bed that maintains its original thickness during defor-
mation. Often pertains to relatively brittle, solid strata that deform

by faulting, fracturing or folding, rather than flowing under stress.
Incompetent beds are more ductile and tend to flow under stress,
so their bed thickness changes more readily during deformation.

Answer 257

competent

Question 258

The movement of tectonic plates toward each other, generating
compressional forces and ultimately resulting in collision, and in
some cases subduction, of tectonic plates. The boundary where
tectonic plates converge is called a convergent margin.

Answer 258

convergence

Question 259

A relatively impermeable rock, commonly shale, anhydrite or salt,
that forms a barrier or seal above and around reservoir rock so that
fluids cannot migrate beyond the reservoir. It is often found atop
a salt dome. The permeability of a cap rock capable of retaining
fluids through geologic time is ~ 10^-6-10^-8 darcies.

Answer 259

cap rock

Question 260

Structures formed by sediment injection. Because they resemble
intrusive and extrusive igneous features, much of the vocabulary
for describing clastic intrusions, or injectites, comes from igneous
geology. Sills are emplaced parallel to bedding, whereas dikes
cut through bedding. The strata containing the intrusion are called
host strata and the layers that feed the intrusion are the parent
beds. Sand-injection features exhibit size scales from millimeters
to kilometers, and have been seen in cores, borehole image
logs, seismic sections, outcrops, aerial photographs and satellite
images.

Answer 260

clastic intrusion

Question 261

The deformation of rock layers in which the thickness of each
layer, measured perpendicular to initial undeformed layering, is
maintained after the rock layers have been folded.

Answer 261

concentric fold

Question 262

Sediment consisting of broken fragments derived from preex-
isting rocks and transported elsewhere and redeposited before

forming another rock. Examples of common clastic sedimentary
rocks include siliciclastic rocks such as conglomerate, sandstone,
siltstone and shale. Carbonate rocks can also be broken and
reworked to form clastic sedimentary rocks.

Answer 262

clastic sediment

Question 263

A low-density, high-API gravity liquid hydrocarbon phase that gen-
erally occurs in association with natural gas. Its presence as a

liquid phase depends on temperature and pressure conditions in

the reservoir allowing condensation of liquid from vapor. The pro-
duction of condensate reservoirs can be complicated because of

the pressure sensitivity of some condensates: During production,
there is a risk of the condensate changing from gas to liquid if the
reservoir pressure drops below the dew point during production.
Reservoir pressure can be maintained by fluid injection if gas
production is preferable to liquid production. Gas produced in
association with condensate is called wet gas. The API gravity of
condensate is typically 50 degrees to 120 degrees.

Answer 263

condensate

Question 264

Innermost layer of the Earth. Studies of compressional and shear
waves indicate that the core makes up nearly 3500 km [2170
miles] of the Earth’s radius of 6370 km [3950 miles]. Such studies
also demonstrate that because shear waves do not pass through
the outer part of the core (2250 km [1400 miles] thick), it is liquid
(only solids can shear). The inner core is solid and 1220 km [750
miles] thick. The core’s iron and nickel composition was inferred

through studies of the Earth’s gravitational field and average den-
sity. The relatively low density of the outer layers of the Earth

suggests a dense inner layer.

Answer 264

core

Question 265

A class of sedimentary rock whose chief mineral constituents
(95% or more) are calcite and aragonite (both CaCo3) and
dolomite [CaMg(CO3)2], a mineral that can replace calcite during
the process of dolomitization. Limestone, dolostone or dolomite,
and chalk are carbonate rocks. Although carbonate rocks can
be clastic in origin, they are more commonly formed through
processes of precipitation or the activity of organisms such as
coral and algae. Carbonates form in shallow and deep marine
settings, evaporitic basins, lakes and windy deserts. Carbonate
rocks can serve as hydrocarbon reservoir rocks, particularly if
their porosity has been enhanced through dissolution. They rely
on fractures for permeability.

Answer 265

carbonate

Question 266

A group of minerals found mostly in limestone and dolostone
that includes aragonite, calcite and dolomite. Calcite is the most
abundant and important of the carbonate minerals.

Answer 266

carbonate

Question 267

An unusual occurrence of hydrocarbon in which molecules of
natural gas, typically methane, are trapped in ice molecules. More
generally, hydrates are compounds in which gas molecules are
trapped within a crystal structure. Hydrates form in cold climates,
such as permafrost zones and in deep water. To date, economic
liberation of hydrocarbon gases from hydrates has not occurred,
but hydrates contain quantities of hydrocarbons that could be of
great economic significance. Hydrates can affect seismic data by
creating a reflection or multiple.

Answer 267

clathrate

Question 268

In sequence stratigraphy, a section of fine-grained sedimentary

rocks that accumulated slowly, thereby representing a consider-
able span of time by only a thin layer. In condensed sections,

fossils and organic, phosphatic and glauconitic material tend to
be concentrated compared with rapidly deposited sections that

contain few fossils. Condensed sections are most commonly de-
posited during transgressions. In such cases they are associated

with “maximum flooding surfaces” and form important sequence
stratigraphic markers.

Answer 268

condensed section

Question 269

To seek a comparison or equivalence. Scientists attempt to com-
pare or match up well log signatures, chemical signatures, seismic

signatures, fossils and rock samples across wide areas to deter-
mine the equivalence, extent, thickness, quality, relative age or

other properties of stratigraphic units and rock bodies.

Answer 269

correlate

Question 270

A type of metamorphic rock with shearing and granulation of min-
erals caused by high mechanical stress during faulting or dynamic

metamorphism, typically during episodes of plate tectonic activity.

Answer 270

cataclasite

Question 271

Fine-grained sediments less than 0.0039 mm in size.

Answer 271

clay

Question 272

1. Parallel strata that have undergone a similar geologic history,

deposited in succession without interruption.

Answer 272

conformable

Question 273

2. The nature of the contact between strata deposited in continu-
   ous succession.

Answer 273

conformable

Question 274

A connection of points from well to well in which the data suggest

that the points were deposited at the same time (chronostrati-
graphic) or have similar and related characteristics.

Answer 274

correlation

Question 275

Pertaining to a type of metamorphic rock with shearing and gran-
ulation of minerals caused by high mechanical stress during fault-
ing or dynamic metamorphism, typically during episodes of plate

tectonic activity.

Answer 275

cataclastic

Question 276

Pertaining to a sedimentary rock, such as sandstone or limestone,

that contains only minimal amounts of clay minerals. These reser-
voir rocks typically have better porosity and permeability than dirty

rocks whose pores are clogged with fine clay particles. Clean and
dirty are qualitative, descriptive terms.

Answer 276

clean

Question 277

A bedding surface separating younger from older strata, along
which there is no evidence of subaerial or submarine erosion
or of nondeposition, and along which there is no evidence of
a significant hiatus. Unconformities (sequence boundaries) and
flooding surfaces (parasequence boundaries) pass laterally into
correlative conformities, or correlative surfaces.

Answer 277

conformity

Question 278

A stable area of continental crust that has not undergone much
plate tectonic or orogenic activity for a long period. A craton
includes a crystalline basement of commonly Precambrian rock
called a shield, and a platform in which flat-lying or nearly flat-lying
sediments or sedimentary rock surround the shield. A commonly
cited example of a craton is the Canadian Shield.

Answer 278

craton

Question 279

The physical and chemical alteration of sediments and pore fluids
at temperatures and pressures higher than those of diagenesis.
Catagenesis involves heating in the range of 50° to 150°C [122°
to 302°F]. At these temperatures, chemical bonds break down in
kerogen and clays within shale, generating liquid hydrocarbons. At
the high end of this temperature range, secondary cracking of oil
molecules can generate gas molecules.

Answer 279

catagenesis

Question 280

Water containing more dissolved inorganic salt than typical sea-
water.

Answer 280

clear brine

Question 281

Water trapped in the pores of a rock during formation of the
rock. The chemistry of connate water can change in composition
throughout the history of the rock. Connate water can be dense
and saline compared with seawater. Formation water, or interstitial
water, in contrast, is simply water found in the pore spaces of a
rock, and might not have been present when the rock was formed.
Connate water is also described as fossil water.

Answer 281

connate water

Question 282

The highest point of a wave, mountain or geologic structure.

Answer 282

crest

Question 283

Natural gas, predominantly
methane [CH4], generated during coal formation and adsorbed in
coal. Natural gas adsorbs to the surfaces of matrix pores within
the coal and natural fractures, or cleats, as reservoir pressure
increases.

Answer 283

CBM

Question 284

CBM stands for

Answer 284

coal-bed methane

Question 285

Production of natural gas from coal requires decreasing the pore
pressure below the coal’s desorption pressure so that methane
will desorb from surfaces, diffuse through the coal matrix and
become free gas. Because the diffusivity and permeability of the
coal matrix are ultralow, coal must have an extensive cleat system
to ensure adequate permeability and flow of methane to wellbores
at economic production rates.

Answer 285

CBM

Question 286

Coal seams are typically saturated with water. Consequently, the
coal must be dewatered for efficient gas production. Dewatering
reduces the hydrostatic pressure and promotes gas desorption

from coal. As dewatering progresses, gas production often in-
creases at a rate governed by how quickly gas desorbs from coal,

the permeability of the cleat and the relative permeability of the
gas-water system in the cleat. Eventually, the rate and amount of
gas desorption decreases as the coal seam is depleted of its gas,
and production declines.
Coal seams with no water (dry coal) have been discovered and
commercially exploited. In these reservoirs, the adsorbed gas
is held in place by free gas in the cleats. Consequently, gas
production consists of both free gas from the cleat system and
desorbed gas from the matrix.

Answer 286

CBM

Question 287

1. The vertical distance from the apex of a structure to the lowest
   structural contour that contains the structure. Measurements of
   both the areal closure and the distance from the apex to the lowest closing contour are typically incorporated in calculations of the
   estimated hydrocarbon content of a trap.

Answer 287

closure

Question 288

2. The area, or areal closure, included in the lowest closing con-
   tour of a trap. Measurements of both the areal closure and the

distance from the apex to the lowest closing contour are typically
incorporated in calculations of the estimated hydrocarbon content
of a trap.

Answer 288

closure

Question 289

Pertaining to sediments that have been compacted and cemented
to the degree that they become coherent, relatively solid rock.
Typical consequences of consolidation include an increase in
density and acoustic velocity, and a decrease in porosity.

Answer 289

consolidated

Question 290

The time of maximum depth of burial of a hydrocarbon source rock.

This is the time of highest probability of entrapment and preser-
vation of hydrocarbons in a petroleum system-after traps form and

hydrocarbons migrate into a reservoir and accumulate-and marks
the beginning of preservation in a viable petroleum system.

Answer 290

critical moment

Question 291

The binding material in sedimentary rocks that precipitates be-
tween grains from pore fluids. Calcite and quartz are common

cement-forming minerals.

Answer 291

cement

Question 292

A carbon-rich sedimentary rock that forms from the remains of

plants deposited as peat in swampy environments. Burial and in-
crease in temperature bring about physical and chemical changes

called coalification. Because of the organic origin of coal, it cannot
be classified as a mineral. The main types of coal, anthracite,
bituminous coal and lignite, can be distinguished by their hardness
and energy content, which are affected by their organic content
as well as their conditions of formation. Natural gas associated
with coal, called coal gas or coalbed methane, can be produced
economically from coal beds in some areas. In some basins coals
form source rocks.

Answer 292

coal

Question 293

Compaction and cementation of sediments to the degree that they
become coherent, relatively solid rock. Typical consequences of
consolidation include an increase in density and acoustic velocity,
and a decrease in porosity.

Answer 293

consolidation

Question 294

In the case of a body of rock, to be exposed at the surface of
the Earth. Construction of highways and other man-made facilities
and resultant removal of soil and rock has created spectacular
outcrops in some regions.

Answer 294

crop out

Question 295

The process of precipitation of cement between mineral or rock
grains and forming solid clastic sedimentary rock, one phase of
lithification.

Answer 295

cementation

Question 296

Natural gas, predominantly methane [CH4], generated during coal

formation and adsorbed in coal. Natural gas adsorbs to the sur-
faces of matrix pores within the coal and natural fractures, or

cleats, as reservoir pressure increases.

Answer 296

coal bed methane

Question 297

The interface, also called fluid contact, that separates fluids of
different densities in a reservoir. Horizontal contacts are usually
assumed, although tilted contacts occur in some reservoirs. The

contact between fluids is usually gradual rather than sharp, form-
ing a transition zone of mixed fluid. A mixed-fluid reservoir will

stratify according to fluid density, with gas at the top, oil in the
middle, and water below. Production of fluids often perturbs the
fluid contacts in a reservoir.

Answer 297

contact

Question 298

A diagram of a vertical section through a volume, as opposed to
the surface, “bird’s eye,” or plan view of a map. Cross sections
are useful for displaying the types and orientations of subsurface
structures and formations.

Answer 298

cross section

Question 299

A porous marine limestone composed of fine-grained remains of
microorganisms with calcite shells, coccolithophores, such as the
White Cliffs of Dover (UK). The Austin Chalk of the US Gulf coast is
a prolific, fractured oil reservoir that spurred widespread horizontal
drilling activity.

Answer 299

chalk

Question 300

Natural gas, predominantly methane [CH4], generated during coal

formation and adsorbed in coal. Natural gas adsorbs to the sur-
faces of matrix pores within the coal and natural fractures, or

cleats, as reservoir pressure increases.

Answer 300

coal seam gas

Question 301

The area at the edges of a continent from the shoreline to a depth
of 200 m [660 ft], where the continental slope begins. The shelf is
commonly a wide, flat area with a slight seaward slope. The term
is sometimes used as a for platform.

Answer 301

continental shelf

Question 302

Pertaining to a diagram of a vertical section through a volume,
as opposed to the surface, “bird’s eye,” or plan view of a map.
Cross sections are useful for displaying the types and orientations
of subsurface structures and formations.

Answer 302

cross-sectional

Question 303

A linear, commonly concave-based depression through which wa-
ter and sediment flow and into which sediment can be deposited in

distinctive, often elongated bodies. Channels can occur in a variety
of morphologies, e.g., straight, meandering or braided. In some
areas, coarse sediments can fill channels of streams or rivers that
cut through finer grained sediments or rocks. The close proximity
of coarse-grained and fine-grained sediments can ultimately lead
to the formation of stratigraphic hydrocarbon traps.

Answer 303

channel

Question 304

Natural gas, predominantly methane [CH4], generated during coal

formation and adsorbed in coal. Natural gas adsorbs to the sur-
faces of matrix pores within the coal and natural fractures, or

cleats, as reservoir pressure increases.

Answer 304

coal-bed methane

Question 305

A line on a map that represents a constant value of the parameter

being mapped. This line includes points of equal value and sep-
arates points of higher value from points of lower value. Contours

are commonly drawn on maps to portray the structural configu-
ration of the Earth’s surface or formations in the subsurface. For

example, structure maps contain contours of constant elevation
with respect to a datum (such as sea level). Contours are also
used to interpret subsurface configurations of rock bodies in areas
of limited control, such as drawing contours of the thickness of a
common rock unit in several widely separated wells to extrapolate
its thickness in a nearby undrilled location.

Answer 305

contour

Question 306

A general term for unrefined petroleum or liquid petroleum.

Answer 306

crude oil

Question 307

A sedimentary rock and a variety of quartz made of extremely
fine-grained, or cryptocrystalline, silica, also called chalcedony.
The silica might be of organic origin, such as from the internal
structures of sponges called spicules, or inorganic origin, such
as precipitation from solution. The latter results in the formation
of flint. Chert can form beds, but is more common as nodules in
carbonate rocks.

coal-bed methane

Natural gas, predominantly

Answer 307

chert

Question 308

Natural gas, predominantly methane [CH4], generated during coal

formation and adsorbed in coal. Natural gas adsorbs to the sur-
faces of matrix pores within the coal and natural fractures, or

cleats, as reservoir pressure increases.

Answer 308

coal-bed methane

Question 309

The value of the separation between two adjacent contours. A net
pay isopach map might have a contour interval of 10 feet [3 m],
whereas a structure contour map might have a contour interval of
1000 feet [300 m]. Contour intervals are chosen according to the
map scale and the amount and distribution of control points.

Answer 309

contour interval

Question 310

The thin, outermost shell of the Earth that is typically 5 km to 75 km
thick [3 to 46 miles]. The continental crust comprises rocks similar
in composition to granite and basalt (i.e., quartz, feldspar, biotite,
amphibole and pyroxene) whereas the composition of oceanic
crust is basaltic (pyroxene and feldspar). The crust overlies the
more dense rock of the mantle, which consists of rocks composed
of minerals like pyroxene and olivine, and the iron and nickel core
of the Earth. The Mohorovicic discontinuity abruptly separates
the crust from the mantle; the velocity of compressional waves
is significantly higher below the discontinuity. The crust, mantle
and core of the Earth are distinguished from the lithosphere and
asthenosphere on the basis of their composition and not their
mechanical behavior.

Answer 310

crust

Question 311

Containing chert, a sedimentary rock and a variety of quartz made
of extremely fine-grained, or cryptocrystalline, silica, also called
chalcedony. The silica might be of organic origin, such as from
the internal structures of sponges called spicules, or inorganic
origin, such as precipitation from solution. The latter results in the
formation of flint. Chert can form beds, but is more common as
nodules in carbonate rocks.

Answer 311

cherty

Question 312

Natural gas, predominantly methane [CH4], generated during coal

formation and adsorbed in coal. Natural gas adsorbs to the sur-
faces of matrix pores within the coal and natural fractures, or

cleats, as reservoir pressure increases.

Answer 312

coal seam gas

Question 313

A map displaying lines that include points of equal value and sep-
arate points of higher value from points of lower value. Common

types of contour maps include topographic contour maps, which
show the elevation of the Earth’s surface; structure contour maps,
which show the elevation or depth of a formation; and gross or
net sand or pay maps, which show variations in the thickness of a
stratigraphic unit, also called isopachs.

Answer 313

contour map

Question 314

Abbreviation for coal seam gas. Natural gas, predominantly
methane [CH4], generated during coal formation and adsorbed in
coal. Natural gas adsorbs to the surfaces of matrix pores within
the coal and natural fractures, or cleats, as reservoir pressure
increases.

Answer 314

CSG

Question 315

[(Mg,Al,Fe) 12(Si,Al) 8O20(OH) 16] A platy, pale green mineral of
the mica group of sheet silicates, also considered to be a type
of clay mineral, found in sedimentary and low-grade metamorphic
rocks. Chlorite is a common authigenic mineral lining the pores of
sandstones. In some cases, the presence of authigenic chlorite on
sand grains can inhibit the growth of pore-filling cements during

diagenesis and preserve pore space for occupation by hydrocar-
bons.

Answer 315

chlorite

Question 316

An interaction of lithospheric plates that can result in the formation
of mountain belts and subduction zones. The collision of two
plates of continental lithosphere, known as an A-type collision, can
produce high mountains as rocks are folded, faulted and uplifted
to accommodate the converging plates, as observed in the Alps
and the Himalayas. B-type collisions, in which oceanic lithospheric
plates collide with continental lithospheric plates, typically produce

a subduction zone where the relatively denser oceanic plate de-
scends below the relatively lighter continental plate, as seen on

the Pacific coast of South America.

Answer 316

collision

Question 317

The density- and heat-driven cycling, transfer or circulation of
energy through which material initially warms up and becomes
relatively less dense, then rises, cools and becomes relatively
more dense, and finally sinks. As a consequence of convection,
material can turn over repeatedly in a convection cell. Within the
Earth, radiogenic heating results in convection appearing in the mantle and might drive plate tectonic motions. Convection also
occurs in the ocean waters and in the Earth’s atmosphere.

Answer 317

convection

Question 318

The arrangement in space of uniform spheres (atoms and mole-
cules in mineral crystals, or grains in clastic sedimentary rocks)

that results in a cubic material structure. Cubic packing is mechan-
ically unstable, but it is the most porous packing arrangement, with

about 47% porosity in the ideal situation. Most sediments are not
uniform spheres of the same size, nor can they be arranged in a
cubic structure naturally, so most sediments have much less than
47% porosity.

Answer 318

cubic packing

Question 319

A graphic display, with geologic time along the vertical axis and
distance along the horizontal axis, to demonstrate the relative
ages and geographic extent of strata or stratigraphic units in a

given area, also known as a Wheeler diagram. In addition, in-
formation from seismic data, well logs and rock samples, and

biostratigraphic and lithostratigraphic information can be shown
within each chronostratigraphic unit. A chronostratigraphic chart
can concisely illustrate sequence stratigraphic interpretations.

Answer 319

chronostratigraphic chart

Question 320

Oil at sufficiently low pressure that it contains no dissolved gas or
a relatively thick oil or residue that has lost its volatile components.

Answer 320

dead oil

Question 321

The three-dimensional array of sediments or lithofacies that fills
a basin. Depositional systems vary according to the types of
sediments available for deposition as well as the depositional
processes and environments in which they are deposited. The
dominant depositional systems are alluvial, fluvial, deltaic, marine,
lacustrine and eolian systems.

Answer 321

depositional system

Question 322

A soft, silica-rich sedimentary rock comprising diatom remains that
forms most commonly in lakes and deep marine areas. Diatomite
can form an excellent reservoir rock. The Belridge diatomite in
the San Joaquin basin, California, USA, is a prolific oil-producing
formation.

Answer 322

diatomite

Question 323

A rock composed chiefly (> 90%) of dolomite. The rock is some-
times called dolomite, but dolostone is preferable to avoid ambi-
guity between the mineral and rock names. Replacement dolomite

that forms soon after deposition is typically fine-grained and pre-
serves original sedimentary structures. Recrystallization late in

diagenesis produces coarser grained dolomite, destroys sedi-
mentary structures and results in higher porosity.

Answer 323

dolostone

Question 324

A fault surface parallel to a mechanically weak horizon or lay-
er, or parallel to bedding, that detaches or separates deformed

rocks above from undeformed or differently deformed rocks below.
Decollements, or decollement surfaces, are typical of regions of
thrust faulting such as the Alps.

Answer 324

decollement

Question 325

Pertaining to particles of rock derived from the mechanical break-
down of preexisting rocks by weathering and erosion. Detrital frag-
ments can be transported to recombine and, through the process

of lithification, become sedimentary rocks. Detrital is usually used
synonymously with clastic, although a few authors differentiate
between weathering of particles, which forms detrital sediments,
and mechanical breakage, which produces clastic sediments.

Answer 325

detrital

Question 326

A phenomenon that occurs after the deposition of some sedi-
ments such that different parts of the sedimentary accumulation

develop different degrees of porosity or settle unevenly during
burial beneath successive layers of sediment. This can result from
location on an uneven surface, such as near and over a reef
structure, or near a growth fault, or from different susceptibility
to compaction. The porosity in a formation that has experienced
differential compaction can vary considerably from one area to
another.

Answer 326

differential compaction

Question 327

A type of anticline that is circular or elliptical rather than elongate.
The upward migration of salt diapirs can form domes, called salt
domes.

Answer 327

dome

Question 328

Exploration activity located in offshore areas where water depths

exceed approximately 600 feet [200 m], the approximate wa-
ter depth at the edge of the continental shelf. While deep-water

reservoir targets are geologically similar to reservoirs drilled both
in shallower present-day water depths as well as onshore, the
logistics of producing hydrocarbons from reservoirs located below
such water depths presents a considerable technical challenge.

Answer 328

deepwater play

Question 329

Particles of rock derived from the mechanical breakdown of preex-
isting rocks by weathering and erosion. Detrital fragments can be

transported to recombine and, through the process of lithification,
become sedimentary rocks. Detrital is usually used synonymously

with clastic, although a few authors differentiate between weath-
ering of particles, which forms detrital sediments, and mechanical

breakage, which produces clastic sediments.

Answer 329

detritus

Question 330

An intrusive rock that invades preexisting rocks, commonly in a

tabular shape that cuts vertically or nearly vertically across pre-
existing layers. Dikes form from igneous and sedimentary rocks.

Answer 330

dike

Question 331

Located down the slope of a dipping plane or surface. In a dipping
(not flat-lying) hydrocarbon reservoir that contains gas, oil and
water, the gas is updip, the gas-oil contact is downdip from the
gas, and the oil-water contact is still farther downdip.

Answer 331

down dip

Question 332

The phase of petroleum operations that occurs after exploration
has proven successful, and before full-scale production. The newly
discovered oil or gas field is assessed during an appraisal phase,
a plan to fully and efficiently exploit it is created, and additional
wells are usually drilled.

Answer 332

development

Question 333

The magnitude of the inclination of a plane from horizontal. True,
or maximum, dip is measured perpendicular to strike. Apparent
dip is measured in a direction other than perpendicular to strike.

Answer 333

dip

Question 334

The termination of more steeply dipping overlying strata against a
surface or underlying strata that have lower apparent dips; a term
used to describe a particular geometry of reflections in seismic
data in sequence stratigraphy.

Answer 334

down lap

Question 335

An area of deposition or the deposit formed by a flowing sedi-
ment-laden current as it enters an open or standing body of water,

such as a river spilling into a gulf. As a river enters a body of water,
its velocity drops and its ability to carry sediment diminishes,
leading to deposition. The term has origins in Greek because the
shape of deltas in map view can be similar to the Greek letter
delta. The shapes of deltas are subsequently modified by rivers,
tides and waves. There is a characteristic coarsening upward
of sediments in a delta. The three main classes of deltas are
river-dominated (Mississippi River), wave-dominated (Nile River),
and tide-dominated (Ganges River). Ancient deltas contain some
of the largest and most productive petroleum systems.

Answer 335

delta

Question 336

Pertaining to a strike-slip fault or right-lateral fault in which the
block across the fault moves to the right. If it moves left, the relative motion is described as sinistral. Clockwise rotation or spiraling is
also described as dextral.

Answer 336

dextral

Question 337

A layer of rock or sediment that is not horizontal.

Answer 337

dipping bed

Question 338

Located down the slope of a dipping plane or surface. In a dipping
(not flat-lying) hydrocarbon reservoir that contains gas, oil and
water, the gas is updip, the gas-oil contact is downdip from the
gas, and the oil-water contact is still farther downdip.

Answer 338

downdip

Question 339

Pertaining to an area of deposition or the deposit formed by a
flowing sediment-laden current as it enters an open or standing
body of water, such as a river spilling into a gulf. As a river enters
a body of water, its velocity drops and its ability to carry sediment
diminishes, leading to deposition. The term has origins in Greek
because the shape of deltas in map view can be similar to the
Greek letter delta. The shapes of deltas are subsequently modified

by rivers, tides and waves. There is a characteristic coarsening up-
ward of sediments in a delta. The three main classes of deltas are

river-dominated (Mississippi River), wave-dominated (Nile River),
and tide-dominated (Ganges River). Ancient deltas contain some
of the largest and most productive petroleum systems.

Answer 339

deltaic

Question 340

The physical, chemical or biological alteration of sediments into
sedimentary rock at relatively low temperatures and pressures
that can result in changes to the rock’s original mineralogy and
texture. After deposition, sediments are compacted as they are
buried beneath successive layers of sediment and cemented by
minerals that precipitate from solution. Grains of sediment, rock
fragments and fossils can be replaced by other minerals during
diagenesis. Porosity usually decreases during diagenesis, except
in rare cases such as dissolution of minerals and dolomitization.
Diagenesis does not include weathering processes. Hydrocarbon

generation begins during diagenesis. There is not a clear, accept-
ed distinction between diagenesis and metamorphism, although

metamorphism occurs at pressures and temperatures higher than
those of the outer crust, where diagenesis occurs.

Answer 340

diagenesis

Question 341

Describing sedimentary rock that contains clay minerals. Even
small amounts of clay minerals in pores can drastically reduce

porosity and permeability. Dirty and clean are qualitative, descrip-
tive terms to describe the relative amount of clay minerals in a

rock.

Answer 341

dirty

Question 342

The termination of more steeply dipping overlying strata against a
surface or underlying strata that have lower apparent dips; a term
used to describe a particular geometry of reflections in seismic
data in sequence stratigraphy.

Answer 342

downlap

Question 343

Mass per unit of volume. Density is typically reported in g/cm3 (for
example, rocks) or pounds per barrel (drilling mud) in the oil field.

Answer 343

density

Question 344

Pertaining to diagenesis, which is the physical, chemical or bio-
logical alteration of sediments into sedimentary rock at relatively

low temperatures and pressures that can result in changes to the
rock’s original mineralogy and texture. After deposition, sediments
are compacted as they are buried beneath successive layers of
sediment and cemented by minerals that precipitate from solution.
Grains of sediment, rock fragments and fossils can be replaced
by other minerals during diagenesis. Porosity usually decreases
during diagenesis, except in rare cases such as dissolution of

minerals and dolomitization. Diagenesis does not include weath-
ering processes. Hydrocarbon generation begins during diagene-
sis. There is not a clear, accepted distinction between diagenesis

and metamorphism, although metamorphism occurs at pressures
and temperatures higher than those of the outer crust, where
diagenesis occurs

Answer 344

diagenetic

Question 345

A geologic surface that separates younger strata from older strata
and represents a time of nondeposition, possibly combined with
erosion. Some disconformities are highly irregular whereas others
have no relief and can be difficult to distinguish within a series of
parallel strata.

Answer 345

disconformity

Question 346

This test is one in which the pore fluid in the sample is able to
flow and equilibrate to imposed pore pressure conditions; the fluid
mass and volume will vary but its pressure will be constant. A
drained test could be on a dry sample.

Answer 346

drained test

Question 347

An influx of rapidly moving, sediment-laden water down a slope
into a larger body of water; the suspended sediment causes the
current to have a higher density than the clearer water into which
it flows, hence the name. Such currents can occur in lakes and
oceans, in some cases as by-products of earthquakes or mass
movements such as slumps. The sedimentary deposits that form
as the current loses energy are called turbidites and can be

preserved as Bouma sequences. Density currents are character-
istic of trench slopes of convergent plate margins and continental

slopes of passive margins.

Answer 347

density current

Question 348

A type of secondary porosity created during diagenesis, common-
ly through dissolution or dolomitization or both. Diagenesis usually

destroys porosity, so diagenetic porosity is rare.

Answer 348

diagenetic porosity

Question 349

Pertaining to structures in which the shapes of adjacent layers
differ or do not conform to one another. Folds of rock layers that
have different mechanical properties or competence tend to be
disharmonic, with a change in fold shape, symmetry or wavelength
from one layer to the next.

Answer 349

disharmonic

Question 350

A configuration of layers of rock that has the appearance of a fold,
but might form simply through sagging or differential compaction
of layers around a preexisting structure (such as a reef) or on an
uneven surface.

Answer 350

drape

Question 351

The area of thickest deposition in a basin.

Answer 351

depocenter

Question 352

A relatively mobile mass that intrudes into preexisting rocks. Di-
apirs commonly intrude vertically through more dense rocks be-
cause of buoyancy forces associated with relatively low-densi-
ty rock types, such as salt, shale and hot magma, which form

diapirs. The process is known as diapirism. By pushing upward
and piercing overlying rock layers, diapirs can form anticlines, salt
domes and other structures capable of trapping hydrocarbons.
Igneous intrusions are typically too hot to allow the preservation
of preexisting hydrocarbons.

Answer 352

diapir

Question 353

The offset of segments or points that were once continuous or
adjacent. Layers of rock that have been moved by the action of
faults show displacement on either side of the fault surface.

Answer 353

displacement

Question 354

Natural gas that occurs in the absence of condensate or liquid

hydrocarbons, or gas that has had condensable hydrocarbons re-
moved. Dry gas typically has a gas-to-oil ratio exceeding 100,000

scf/STB.

Answer 354

dry gas

Question 355

Sediments that have accumulated, usually after being moved by

wind, water or ice.

Answer 355

deposit

Question 356

A microscopic, single-celled, freshwater or saltwater algae that
has a silica-rich cell wall called a frustule. Diatoms are so abundant

that they can form thick layers of sediment composed of the frus-
tules of the organisms that died and sank to the bottom. Frustules

have been an important component of deep-sea deposits since
Cretaceous time. Diatomite is the sedimentary rock that forms
from diatom frustules.

Answer 356

diatom

Question 357

A widely-distributed carbonate mineral and chief constituent of
dolostone. The name given to dolomitized limestone.

Answer 357

dolomite

Question 358

A subsurface rock that lacks contact with aquifers or meteoric

water within the Earth.

Answer 358

dry rock

Question 359

The relative kinetic energy of the environment. A high-energy en-
vironment might consist of a rapidly flowing stream that is capable

of carrying coarse-grained sediments, such as gravel and sand.
Sedimentation in a low-energy environment, such as an abyssal
plain, usually involves very fine-grained clay or mud. Depositional
energy is not simply velocity. For example, although glaciers do
not move quickly, they are capable of carrying large boulders.

Answer 359

depositional energy

Question 360

Pertaining to a diatom, which is a microscopic, single-celled,
freshwater or saltwater algae that has a silica-rich cell wall called a
frustule. Diatoms are so abundant that they can form thick layers of
sediment composed of the frustules of the organisms that died and
sank to the bottom. Frustules have been an important component
of deep-sea deposits since Cretaceous time. Diatomite is the
sedimentary rock that forms from diatom frustules.

Answer 360

diatomaceous

Question 361

The geochemical process in supratidal sabkha areas where mag-
nesium [Mg] ions from the evaporation of seawater replace calci-
um [Ca] ions in calcite, forming the mineral dolomite. The volume

of dolomite is less than that of calcite, so the replacement of calcite
by dolomite in a rock increases the pore space in the rock by 13%
and forms an important reservoir rock. Dolomitization can occur
during deep burial diagenesis.

Answer 361

dolomitization

Question 362

An intrusive rock that invades preexisting rocks, commonly in a

tabular shape that cuts vertically or nearly vertically across pre-
existing layers. Dikes form from igneous and sedimentary rocks.

Answer 362

dyke

Question 363

The area in which and physical conditions under which sediments
are deposited, including sediment source; depositional processes
such as deposition by wind, water or ice; and location and climate,
such as desert, swamp or river.

Answer 363

depositional environment

Question 364

The sudden release of accumulated stress in the Earth by move-
ment or shaking. Earthquakes are caused by tectonic activity,

volcanoes and human activity (such as explosions). Earthquakes
occur in the outer 720 km [445 miles] of the Earth, where rocks
tend to break rather than flow under stress. The magnitude of
earthquakes is determined according to the logarithmic Richter

scale. An earthquake of magnitude 4.5 can cause damage, al-
though humans can feel earthquakes as weak as magnitude 2.0.

The San Francisco earthquake of 1906 measured 8.25 on the
Richter scale, and the largest ever recorded were 8.9 magnitude
earthquakes in Colombia and Ecuador (1906) and Japan (1933),
and 9.5 in Chile (1960).

Answer 364

earthquake

Question 365

The yield point, or the point at which a material can no longer
deform elastically. When the elastic limit is exceeded by an applied
stress, permanent deformation occurs.

Answer 365

elastic limit

Question 366

The process of denudation of rocks, including physical, chemical

and biological breakdown and transportation.

Answer 366

erosion

Question 367

The process by which material weathered from rocks is transport-
ed by wind, water, ice, or abrasive solid particles, or by mass-wast-
ing, as in rock falls and landslides.

Answer 367

erosion

Question 368

A class of sedimentary minerals and sedimentary rocks that form

by precipitation from evaporating aqueous fluid. Common evap-
orite minerals are halite, gypsum and anhydrite, which can form

as seawater evaporates, and the rocks limestone and dolostone.
Certain evaporite minerals, particularly halite, can form excellent

cap rocks or seals for hydrocarbon traps because they have minimal porosity and they tend to deform plastically (as opposed to
brittle fracturing that would facilitate leakage).

Answer 368

evaporite

Question 369

The ability to preferentially flow or transmit a particular fluid when
other immiscible fluids are present in the reservoir (e.g., effective

permeability of gas in a gas-water reservoir). The relative satu-
rations of the fluids as well as the nature of the reservoir affect

the effective permeability. In contrast, absolute permeability is the
measurement of the permeability conducted when a single fluid or
phase is present in the rock.

Answer 369

effective permeability

Question 370

Ability of a material to undergo stress, deform, and then recover
and return to its original shape after the stress ceases. Once stress
exceeds the yield stress or elastic limit of a material, permanent
deformation occurs and the material will not return to its original
shape when the stress is removed. In some materials, including

rocks, elastic behavior depends on the temperature and the du-
ration of the stress as well as its intensity.

Answer 370

elasticity

Question 371

Pertaining to an estuary, a semi-enclosed coastal environment of
deposition in which a river mouth permits freshwater to contact
and mix with seawater.

Answer 371

estuarine

Question 372

Pertaining to evaporite, a class of sedimentary minerals and
sedimentary rocks that form by precipitation from evaporating
aqueous fluid. Common evaporite minerals are halite, gypsum and
anhydrite, which can form as seawater evaporates, and the rocks
limestone and dolostone. Certain evaporite minerals, particularly
halite, can form excellent cap rocks or seals for hydrocarbon traps
because they have minimal porosity and they tend to deform
plastically (as opposed to brittle fracturing that would facilitate
leakage).

Answer 372

evaporitic

Question 373

The interconnected pore volume or void space in a rock that
contributes to fluid flow or permeability in a reservoir. Effective
porosity excludes isolated pores and pore volume occupied by
water adsorbed on clay minerals or other grains. Total porosity is
the total void space in the rock whether or not it contributes to fluid
flow. Effective porosity is typically less than total porosity.

Answer 373

effective porosity

Question 374

Describing parallel or subparallel, closely-spaced, overlapping or
step-like minor structural features in rock, such as faults and
tension fractures, that are oblique to the overall structural trend.

Answer 374

en echelon

Question 375

A semi-enclosed coastal environment of deposition in which a river
mouth permits freshwater to contact and mix with seawater.

Answer 375

estuary

Question 376

The initial phase in petroleum operations that includes generation
of a prospect or play or both, and drilling of an exploration well.
Appraisal, development and production phases follow successful
exploration.

Answer 376

exploration

Question 377

Pertaining to a material that can undergo stress, deform, and then
recover and return to its original shape after the stress ceases.
Once stress exceeds the yield stress or elastic limit of a material,
permanent deformation occurs and the material will not return to
its original shape once the stress is removed. In some materials,
including rocks, elastic behavior depends on the temperature and
the duration of the stress as well as its intensity.

Answer 377

elastic

Question 378

Pertaining to the environment of deposition of sediments by wind,

such as the sand dunes in a desert. Because fine-grained sedi-
ments such as clays are removed easily from wind-blown deposits,

eolian sandstones are typically clean and well-sorted.

Answer 378

eolian

Question 379

Global sea level variations. Changes in sea level can result from
movement of tectonic plates altering the volume of ocean basins,
or when changes in climate affect the volume of water stored in

glaciers and in polar icecaps. Eustasy affects positions of shore- lines and processes of sedimentation, so interpretation of eustasy
is an important aspect of sequence stratigraphy.

Answer 379

eustasy

Question 380

A conceptual model for a style of hydrocarbon accumulation used
by explorationists to develop prospects in a basin, region or trend
and used by development personnel to continue exploiting a given
trend. A play (or a group of interrelated plays) generally occurs in
a single petroleum system.

Answer 380

exploration play

Question 381

The deformation that can be recovered when an applied stress
has been removed. When the elastic limit of a material has been
exceeded, nonrecoverable, permanent deformation occurs.

Answer 381

elastic deformation

Question 382

To cause or undergo erosion, the process of denudation of rocks,

including physical, chemical and biological breakdown and trans-
portation. The material from the rocks can be transported by wind,

water, ice, or abrasive solid particles, or by mass-wasting, as in
rock falls and landslides.

Answer 382

erode

Question 383

a term for global sea level and its varia-
tions. Changes in sea level can result from movement of tectonic

plates altering the volume of ocean basins, or when changes in
climate affect the volume of water stored in glaciers and in polar
icecaps. Eustasy affects positions of shorelines and processes of
sedimentation, so interpretation of eustasy is an important aspect
of sequence stratigraphy.

Answer 383

eustatic

Question 384

Pertaining to exploration, the initial phase in petroleum operations
that includes generation of a prospect or play or both, and drilling
of an exploration well. Appraisal, development and production
phases follow successful exploration.

Answer 384

exploratory

Question 385

The overall characteristics of a rock unit that reflect its origin
and differentiate the unit from others around it. Mineralogy and
sedimentary source, fossil content, sedimentary structures and
texture distinguish one facies from another.

Answer 385

facies

Question 386

A graphical display of three-dimensional data and interpretations
in two-dimensional perspective view. Geologic cross sections can
be displayed in a network to form a fence diagram. Stratigraphic
changes can be displayed clearly in fence diagrams.

Answer 386

fence diagram

Question 387

The fundamental unit of lithostratigraphy. A body of rock that is
sufficiently distinctive and continuous that it can be mapped. In
stratigraphy, a formation is a body of strata of predominantly one
type or combination of types; multiple formations form groups, and
subdivisions of formations are members.

Answer 387

formation

Question 388

That portion of a dual-porosity reservoirs permeability that is
associated with the secondary porosity created by open, natural
fractures. In many of these reservoirs, fracture permeability can
be the major controlling factor of the flow of fluids.

Answer 388

fracture permeability

Question 389

The trend along which a particular geological feature is likely, such

as a sand fairway or a hydrocarbon fairway. Prediction of con-
ceptual fairways helps explorationists develop prospects. Along a

sand fairway, for example, sand was transported and, presum-
ably, was deposited, allowing an interpretation of the presence of

reservoir rock in the fairway.

Answer 389

fairway

Question 390

An accumulation, pool, or group of pools of hydrocarbons or other
mineral resources in the subsurface. A hydrocarbon field consists
of a reservoir in a shape that will trap hydrocarbons and that
is covered by an impermeable, sealing rock. Typically, the term
implies an economic size.

Answer 390

field

Question 391

The pressure of fluids within the pores of a reservoir, usually
hydrostatic pressure, or the pressure exerted by a column of water
from the formation’s depth to sea level. When impermeable rocks
such as shales form as sediments are compacted, their pore fluids cannot always escape and must then support the total overlying
rock column, leading to anomalously high formation pressures.
Because reservoir pressure changes as fluids are produced from
a reservoir, the pressure should be described as measured at a
specific time, such as initial reservoir pressure.

Answer 391

formation pressure

Question 392

A type of secondary porosity produced by the tectonic fracturing
of rock. Fractures themselves typically do not have much volume,

but by joining preexisting pores, they enhance permeability sig-
nificantly. In exceedingly rare cases, nonreservoir rocks such as

granite can become reservoir rocks if sufficient fracturing occurs.

Answer 392

fracture porosity

Question 393

A break or planar surface in brittle rock across which there is
observable displacement. Depending on the relative direction of
displacement between the rocks, or fault blocks, on either side
of the fault, its movement is described as normal, reverse or

strike-slip. According to terminology derived from the mining in-
dustry, the fault block above the fault surface is called the hanging

wall, while the fault block below the fault is the footwall. Given
the geological complexity of some faulted rocks and rocks that
have undergone more than one episode of deformation, it can be
difficult to distinguish between the various types of faults. Also,
areas deformed more than once or that have undergone continual
deformation might have fault surfaces that are rotated from their
original orientations, so interpretation is not straightforward. In a
normal fault, the hanging wall moves down relative to the footwall
along the dip of the fault surface, which is steep, from 45o to
90o. A growth fault is a type of normal fault that forms during
sedimentation and typically has thicker strata on the downthrown
hanging wall than the footwall. A reverse fault forms when the
hanging wall moves up relative to the footwall parallel to the dip of
the fault surface. A thrust fault, sometimes called an overthrust, is
a reverse fault in which the fault plane has a shallow dip, typically
much less than 45o. Movement of normal and reverse faults can
also be oblique as opposed to purely parallel to the dip direction
of the fault plane. The motion along a strike-slip fault, also known
as a transcurrent or wrench fault, is parallel to the strike of the
fault surface, and the fault blocks move sideways past each other.
The fault surfaces of strike-slip faults are usually nearly vertical. A
strike-slip fault in which the block across the fault moves to the right
is described as a dextral strike-slip fault. If it moves left, the relative
motion is described as sinistral. A transform fault is a particular
type of strike-slip fault that is a boundary of an oceanic tectonic
plate. The actual movement of a transform fault is opposite to its
apparent displacement. The presence of a fault can be detected
by observing characteristics of rocks such as changes in lithology
from one fault block to the next, breaks and offsets between strata
or seismic events, and changes in formation pressure in wells
that penetrate both sides of a fault. Some fault surfaces contain
relatively coarse rubble that can act as a conduit for migrating
oil or gas, whereas the surfaces of other faults are smeared with
impermeable clays or broken grains that can act as a fault seal.

Answer 393

fault

Question 394

Folded structures associated with strike-slip faults. In areas where
strike-slip faults occur in converging crust, or transpression, rocks
are faulted upward in a positive flower structure. In areas of
strike-slip faulting in diverging crust, or transtension, rocks drop
down to form a negative flower structure. Flower structures can
form hydrocarbon traps. The term “flower structure” reflects the
resemblance of the structure to the petals of a flower in cross
section.

Answer 394

flower structure

Question 395

Water that occurs naturally within the pores of rock. Water from

fluids introduced to a formation through drilling or other interfer-
ence, such as mud and seawater, does not constitute formation water. Formation water, or interstitial water, might not have been
the water present when the rock originally formed. In contrast,
connate water is the water trapped in the pores of a rock during
its formation, and may be called fossil water.

Answer 395

formation water

Question 396

The gaseous phase present in a reservoir or other contained area.
Gas may be found either dissolved in reservoir fluids or as free gas
that tends to form a gas cap beneath the top seal on the reservoir
trap. Both free gas and dissolved gas play important roles in the
reservoir-drive mechanism.

Answer 396

free gas

Question 397

A type of structural hydrocarbon trap in which closure is controlled

by the presence of at least one fault surface.

Answer 397

fault trap

Question 398

The interface that separates fluids of different densities in a
reservoir. Horizontal contacts are usually assumed, although tilted
contacts occur in some reservoirs. The contact between fluids is
usually gradual rather than sharp, forming a transition zone of
mixed fluid. A mixed-fluid reservoir will stratify according to fluid
density, with gas at the top, oil in the middle, and water below.
Production of fluids often perturbs the fluid contacts in a reservoir.

Answer 398

fluid contact

Question 399

Preserved remnants of plants or animals, such as skeletons,
shells, casts or molds, tracks or borings, and feces.

Answer 399

fossil

Question 400

Water that is mobile, available to flow, and not bound to surfaces

of grains or minerals in rock.

Answer 400

free water

Question 401

A group of rock-forming silicate minerals that are essential
constituents of igneous rocks and are common in sandstones.
Feldspar can weather to form clay minerals. Feldspar can occur
in all three major rock types and forms approximately 60% of the
crust of the Earth.

Answer 401

feldspar

Question 402

Pertaining to an environment of deposition by a river or running

water. Fluvial deposits tend to be well sorted, especially in com-
parison with alluvial deposits, because of the relatively steady

transport provided by rivers.

Answer 402

fluvial

Question 403

A crack or surface of breakage within rock not related to foliation
or cleavage in metamorphic rock along which there has been no
movement. A fracture along which there has been displacement
is a fault. When walls of a fracture have moved only normal to
each other, the fracture is called a joint. Fractures can enhance
permeability of rocks greatly by connecting pores together, and for

that reason, fractures are induced mechanically in some reser-
voirs in order to boost hydrocarbon flow. Fractures may also be

referred to as natural fractures to distinguish them from fractures
induced as part of a reservoir stimulation or drilling operation. In
some shale reservoirs, natural fractures improve production by
enhancing effective permeability. In other cases, natural fractures
can complicate reservoir stimulation.

Answer 403

fracture

Question 404

Water that is low in dissolved salt (< 2000 ppm).

Answer 404

fresh water

Question 405

Pertaining to minerals or igneous rocks composed of minerals
such as quartz and feldspar that are relatively light in color and

density. The word comes from the terms feldspar and silica. Gran-
ite is a felsic igneous rock. (Compare with mafic.)

Answer 405

felsic

Question 406

A wave-like geologic structure that forms when rocks deform by

bending instead of breaking under compressional stress. Anti-
clines are arch-shaped folds in which rock layers are upwardly

convex. The oldest rock layers form the core of the fold, and out-
ward from the core progressively younger rocks occur. A syncline

is the opposite type of fold, having downwardly convex layers with
young rocks in the core. Folds typically occur in anticline-syncline
pairs. The hinge is the point of maximum curvature in a fold. The
limbs occur on either side of the fold hinge. The imaginary surface bisecting the limbs of the fold is called the axial surface. The axial

surface is called the axial plane in cases where the fold is sym-
metrical and the lines containing the points of maximum curvature

of the folded layers, or hinge lines, are coplanar. Concentric folding
preserves the thickness of each bed as measured perpendicular
to original bedding. Similar folds have the same wave shape, but
bed thickness changes throughout each layer, with thicker hinges
and thinner limbs.

Answer 406

fold

Question 407

The pressure required to induce fractures in rock at a given depth.

Answer 407

fracture gradient

Question 408

An unusual occurrence of hydrocarbon in which molecules of
natural gas, typically methane, are trapped in ice molecules. More
generally, hydrates are compounds in which gas molecules are
trapped within a crystal structure. Hydrates form in cold climates,
such as permafrost zones and in deep water. To date, economic
liberation of hydrocarbon gases from hydrates has not occurred,
but hydrates contain quantities of hydrocarbons that could be of
great economic significance. Hydrates can affect seismic data by
creating a reflection or multiple.

Answer 408

gas hydrate

Question 409

The study of the chemistry of the Earth and within solid bodies
of the solar system, including the distribution, circulation and
abundance of elements (and their ions and isotopes), molecules,

minerals, rocks and fluids. For geochemists in the petroleum in-
dustry, source rock geochemistry is a major focus. Geochemical

techniques can determine whether a given source rock is rich
enough in organic matter to generate hydrocarbons, whether the

source rock has generated hydrocarbons, and whether a particu-
lar oil sample was generated by a given source rock.

Answer 409

geochemistry

Question 410

The pressure within the Earth, or formation pressure. The common
oilfield usage, however, is to indicate anomalous subsurface pore

pressure that is higher or lower than the normal, predicted hydro-
static pressure for a given depth, or the pressure exerted per unit

area by a column of fresh water from sea level to a given depth.
Abnormally low pore pressure might occur in areas where fluids

have been drained, such as a depleted hydrocarbon reservoir. Ab-
normally high pore pressure might occur in areas where burial of

water-filled sediments by an impermeable sediment such as clay
was so rapid that fluids could not escape and the pore pressure
increased with deeper burial.

Answer 410

geopressure

Question 411

A system of numerous Earth-orbiting satellites that can be used
to determine the location (latitude, longitude and elevation) of a
receiver or station on the Earth within about 2 m [6 ft]. Fixed
receivers on Earth can be used to determine the relative motions
of fault blocks and lithospheric plates. Hand-held receivers can be
used for producing accurate geologic maps, acquiring navigation
data for 3D seismic surveys, and positioning wells in the field.

Answer 411

GPS

Question 412

Gas that is dissolved in a liquid, such as water or oil.

Answer 412

gas in solution

Question 413

The study of the relative or absolute age of rocks, minerals and
fossils. Absolute age is the measurement of age in years, but
“absolute” ages typically have some amount of error and are
inexact. Relative age, in contrast, is the approximate age of rocks,
fossils or minerals made by determining the age of the material
relative to other surrounding material.

Answer 413

geochronology

Question 414

The change in pore pressure per unit depth, typically in units of
pounds per square inch per foot (psi/ft) or kilopascals per meter
(kPa/m). The geopressure gradient might be described as high or
low if it deviates from the normal hydrostatic pressure gradient of
0.433 psi/ft [9.8 kPa/m].

Answer 414

geopressure gradient

Question 415

A record of the onset and duration of the multitude of episodes of
reversal of the Earth’s magnetic polarity, or geomagnetic polarity
reversals. The GPTS was developed by thorough study of rocks
from around the world, during which it was observed that rocks
from specific time periods contained magnetic minerals whose
orientation was opposite to that of the current magnetic field. By
comparing the patterns of magnetic reversals with those of rocks
of known age, the approximate ages of rocks can be established.
This is particularly useful for basalts of the oceanic crust, which
record the Earth’s magnetic field as they solidify from molten
lava symmetrically about the midocean ridges. The time scale has
been accurately extended back to the Upper Jurassic, the age of
oldest existing oceanic crust.

Answer 415

GPTS (geomagnetic polarity time scale)

Question 416

A bounding surface in a reservoir above which predominantly gas
occurs and below which predominantly oil occurs. Gas and oil are
miscible, so the contact between gas and oil is transitional, forming
a zone containing a mix of gas and oil.

Answer 416

gas oil contact

Question 417

The study of the Earth-its history, structure, composition, life forms

and the processes that continue to change it.

Answer 417

geologic

Question 418

Subject to the pressure within the Earth, or formation pressure.
The common oilfield usage, however, is to indicate anomalous
subsurface pore pressure that is higher or lower than the normal,
predicted hydrostatic pressure for a given depth, or the pressure
exerted per unit area by a column of fresh water from sea level
to a given depth. Abnormally low pore pressure might occur in

areas where fluids have been drained, such as a depleted hydro-
carbon reservoir. Abnormally high pore pressure might occur in

areas where burial of water-filled sediments by an impermeable
sediment such as clay was so rapid that fluids could not escape
and the pore pressure increased with deeper burial.

Answer 418

geopressured

Question 419

A relatively low-standing fault block bounded by opposing normal
faults. This (used as both singular and plural) can form in areas
of rifting or extension, where normal faults are the most common
type of fault. Between graben are relatively high-standing blocks
called horsts. A half-graben is a downdropped block bounded by
a normal fault on only one side.

Answer 419

graben

Question 420

The quality of a source rock that makes it more likely to generate
gas than oil. The nature of the organic matter or kerogen in source

rocks varies from coaly, plant-like material commonly found in ter-
restrial source rocks to algal or other marine material that makes

up marine source rocks. Terrestrial source rocks are commonly
gas-prone.

Answer 420

gas prone

Question 421

A map showing the type and spatial distribution of rocks at the

surface of the Earth. Rock formations are color-coded and sym-
bols for geological structures are annotated, so age relationships

are evident. Topographic contours and cultural features can also
appear on geologic maps.

Answer 421

geologic map

Question 422

The pressure of the weight of overburden, or overlying rock, on a

formation; also called lithostatic pressure.

Answer 422

geostatic pressure

Question 423

The density of a rock or mineral with no porosity, also known as

matrix density, commonly in units of g/cm3.

Answer 423

grain density

Question 424

A porous sand layer or sand body charged with natural gas.

Answer 424

gas sand

Question 425

A chronological chart of the stages and ages of events in the
history of the Earth, from its initial formation to present, that has
been constructed on the basis of the rock record. As is the typical
natural position of rocks, the oldest event is at the bottom of the
chart and the youngest is at the top. Both absolute and relative
ages of rocks and fossils supplement interpretations from rocks. The vastness of geologic time and the slowness of geological
processes are difficult to capture in a simple chart.

Answer 425

geologic time scale

Question 426

The rate of increase in temperature per unit depth in the Earth.
Although this varies from place to place, it averages 25 to 30
°C/km [15 °F/1000 ft]. Temperature gradients sometimes increase
dramatically around volcanic areas. It is particularly important for
drilling fluids engineers to know the geothermal gradient in an area
when they are designing a deep well. The downhole temperature

can be calculated by adding the surface temperature to the prod-
uct of the depth and the geothermal gradient.

Answer 426

geothermal gradient

Question 427

A coarse-grained, plutonic or intrusive igneous rock of felsic com-
position having large crystals of quartz, feldspar and mica. In the

oil field, “granite” is sometimes used incorrectly to indicate any
type of hard rock.

Answer 427

granite

Question 428

A bounding surface in a reservoir above which predominantly gas
occurs and below which predominantly water occurs. Gas and
water are somewhat miscible, so the contact between gas and
water is not necessarily sharp and there is typically a transition
zone between 100% gas and 100% water in reservoirs.

Answer 428

gas water contact

Question 429

Pertaining to geology, the study of the Earth-its history, structure,
composition, life forms and the processes that continue to change
it.

Answer 429

geological

Question 430

Pertaining to the environment of deposition by glaciers.

Answer 430

glacial

Question 431

Water in the subsurface below the water table. This is held in the
pores of rocks, and can be connate, from meteoric sources, or
associated with igneous intrusions.

Answer 431

groundwater

Question 432

A scientist trained in the study of the Earth. In the petroleum in-
dustry, geologists perform a wide variety of functions, but typically

generate prospects and interpret data such as maps, well logs,
outcrops, cuttings, core samples and seismic data.

Answer 432

geologist

Question 433

A green silicate mineral found in sedimentary rocks and formed
on continental shelves characterized by slow sedimentation and

organic matter, such as fecal pellets, present in an oxidizing envi-
ronment. In sufficient quantity, it can form a sandy, green deposit

such as the Cretaceous greensands of the US and UK.

Answer 433

glauconite

Question 434

A type of normal fault that develops and continues to move during
sedimentation and typically has thicker strata on the downthrown,
hanging wall side of the fault than in the footwall. Growth faults are
common in the Gulf of Mexico and in other areas where the crust
is subsiding rapidly or being pulled apart.

Answer 434

growth fault

Question 435

The study of the Earth-its history, structure, composition, life forms

and the processes that continue to change it.

Answer 435

geology

Question 436

The periodic switching of the magnetic north and south poles of
the Earth throughout time, probably as a result of movement of
fluid within the Earth’s core. The onset and duration of the many

episodes of reversed polarity have been documented by examin-
ing the polarity of magnetic minerals within rocks of different ages

from around the world, particularly in basalts or igneous rocks of
the oceanic crust. Oceanic basalts record the Earth’s magnetic
field as they solidify from molten lava symmetrically on each side
of the midoceanic ridges. These data have been compiled to
create a time scale known as the geomagnetic polarity time scale
(GPTS). In the oil field, borehole recordings allow direct correlation
to GPTS and well-to-well correlations.

Answer 436

geomagnetic polarity reversal

Question 437

[CaSO4 72H2O] A highly insoluble sulfate mineral that is the first
to precipitate from evaporating seawater. Dehydration of gypsum
can produce anhydrite. Fine-grained gypsum is called alabaster.

Answer 437

gypsum

Question 438

The formation of hydrocarbons from a source rock as bitumen
forms from kerogen and accumulates as oil or gas. This depends
on three main factors: the presence of organic matter rich enough
to yield hydrocarbons, adequate temperature, and sufficient time
to bring the source rock to maturity. Pressure and the presence
of bacteria and catalysts also affect generation. Generation is a
critical phase in the development of a petroleum system.

Answer 438

generation

Question 439

A record of the onset and duration of the multitude of episodes of
reversal of the Earth’s magnetic polarity, or geomagnetic polarity
reversals. The GPTS was developed by thorough study of rocks
from around the world, during which it was observed that rocks
from specific time periods contained magnetic minerals whose
orientation was opposite to that of the current magnetic field. By
comparing the patterns of magnetic reversals with those of rocks
of known age, the approximate ages of rocks can be established.
This is particularly useful for basalts of the oceanic crust, which
record the Earth’s magnetic field as they solidify from molten
lava symmetrically about the midocean ridges. The time scale has
been accurately extended back to the Upper Jurassic, the age of
oldest existing oceanic crust.

Answer 439

geomagnetic polarity time scale

Question 440

[NaCl] A soft, soluble evaporite mineral commonly known as salt
or rock salt. Because salt is less dense than many sedimentary
rocks, it is relatively buoyant and can form salt domes, pillars or
curtains by flowing and breaking through or piercing overlying
sediments, as seen in the Gulf of Mexico and the Zagros fold
belt. Halite can be critical in forming hydrocarbon traps and seals
because it tends to flow rather than fracture during deformation,
thus preventing hydrocarbons from leaking out of a trap even
during and after some types of deformation.

Answer 440

halite

Question 441

Pertaining to a hiatus, a cessation in deposition of sediments
during which no strata form or an erosional surface forms on the
underlying strata; a gap in the rock record. This period might be
marked by development of a lithified sediment (hardground) or
burrowed surface characteristic of periods when sea level was
relatively low. A disconformity can result from a hiatus.

Answer 441

hiatal

Question 442

A relatively high-standing area formed by the movement of nor-
mal faults that dip away from each other. Horsts occur between

low-standing fault blocks called graben. Horsts can form in areas
of rifting or extension, where normal faults are the most abundant
variety of fault.

Answer 442

horst

Question 443

An area of the subsurface where source rock has reached ap-
propriate conditions of pressure and temperature to generate

hydrocarbons; also known as source kitchen, oil kitchen or gas
kitchen.

Answer 443

hydrocarbon kitchen

Question 444

A term applied to igneous and metamorphic rocks that are distin-
guished from sedimentary rocks because they are typically more

difficult to disaggregate. Well cemented sedimentary rocks are
sometimes described as being hard, but are usually called soft
rock. The term can be used to differentiate between rocks of
interest to the petroleum industry (soft rocks) and rocks of interest
to the mining industry (hard rocks).

Answer 444

hard rock

Question 445

A cessation in deposition of sediments during which no strata form
or an erosional surface forms on the underlying strata; a gap in
the rock record. This period might be marked by development of a
lithified sediment (hardground) or burrowed surface characteristic
of periods when sea level was relatively low. A disconformity can
result from a hiatus.

Answer 445

hiatus

Question 446

An unusual occurrence of hydrocarbon in which molecules of
natural gas, typically methane, are trapped in ice molecules. More
generally, hydrates are compounds in which gas molecules are
trapped within a crystal structure. Hydrates form in cold climates,
such as permafrost zones and in deep water. To date, economic
liberation of hydrocarbon gases from hydrates has not occurred,
but hydrates contain quantities of hydrocarbons that could be of
great economic significance. Hydrates can affect seismic data by
creating a reflection or multiple.

Answer 446

hydrate

Question 447

The height of a column of freshwater that exerts pressure at
a given depth. Some authors use the term synonymously with
hydrostatic pressure.

Answer 447

hydrostatic head

Question 448

A horizon cemented by precipitation of calcite just below the
sea floor. Local concretions form first in a hardground and can
be surrounded by burrows of organisms until the cement is well
developed.

Answer 448

hardground

Question 449

The quality of uniformity of a material. If irregularities are distrib-
uted evenly in a mixture of material, the material is homogeneous.

(Compare with heterogeneity.)

Answer 449

homogeneity

Question 450

Incorporation of water into the atomic structure of a mineral, i.e.,

the chemical combination of water and another substance. Gyp-
sum is a hydrate mineral. Its anhydrous equivalent is anhydrite.

Answer 450

hydration

Question 451

The normal, predicted pressure for a given depth, or the pressure
exerted per unit area by a column of freshwater from sea level
to a given depth. Abnormally low pressure might occur in areas
where fluids have been drained, such as a depleted hydrocarbon
reservoir. Abnormally high pressure might occur in areas where
burial of water-filled sediments by an impermeable sediment such
as clay was so rapid that fluids could not escape and the pore
pressure increased with deeper burial.

Answer 451

hydrostatic pressure

Question 452

Pertaining to structures in which the shapes of adjacent layers
resemble or conform to one another. Folds of rock layers that have
similar mechanical properties or competence tend to be harmonic,
with little change in fold shape, symmetry or wavelength from one
layer to the next.

Answer 452

harmonic

Question 453

Possessing the quality of uniformity. If irregularities are distributed
evenly in a material, the material is homogeneous. (Compare with
heterogeneous.)

Answer 453

homogeneous

Question 454

The force per unit area exerted by a column of liquid at a height
above a depth (and pressure) of interest. Fluids flow down a
hydraulic gradient, from points of higher to lower hydraulic head.
The term is sometimes used synonymously with hydrostatic head.

Answer 454

hydraulic head

Question 455

Pertaining to hot fluids, particularly hot water, or the activity of hot
water, or precipitates thereof. Hydrothermal alteration can change
the mineralogy of rock, producing different minerals, including
quartz, calcite and chlorite. Hydrothermal activity is commonly
associated with hot water that accompanies, or is heated by,
magma.

Answer 455

hydrothermal

Question 456

The quality of variation in rock properties with location in a
reservoir or formation. Shale gas reservoirs are heterogeneous
formations whose mineralogy, organic content, natural fractures,
and other properties vary from place to place. This heterogeneity
makes petroleum system modeling, formation evaluation, and
reservoir simulation critical to maximizing production from shale
reservoirs.

Answer 456

heterogeneity

Question 457

An informal term used to denote a surface in or of rock, or a
distinctive layer of rock that might be represented by a reflection in seismic data. The term is often used incorrectly to describe a
zone from which hydrocarbons are produced.

Answer 457

horizon

Question 458

A naturally occurring organic compound comprising hydrogen and
carbon. Hydrocarbons can be as simple as methane [CH4], but
many are highly complex molecules, and can occur as gases,
liquids or solids. The molecules can have the shape of chains,
branching chains, rings or other structures. Petroleum is a complex
mixture of hydrocarbons. The most common hydrocarbons are
natural gas, oil and coal.

Answer 458

hydrocarbon

Question 459

A change of preexisting rocks or minerals caused by the activity of
hot solutions, such as fluids accompanying or heated by magma.

Quartz, serpentine and chlorite are minerals commonly associ-
ated with hydrothermal alteration. Ore deposits, such as lead (as

the mineral galena), zinc (sphalerite), and copper (malachite), can
occur in areas of hydrothermal alteration.

Answer 459

hydrothermal alteration

Question 460

Possessing the quality of variation in rock properties with loca-
tion in a reservoir or formation. Shale gas reservoirs are hetero-
geneous formations whose mineralogy, organic content, natural

fractures, and other properties vary from place to place. This

heterogeneity makes petroleum system modeling, formation eval-
uation, and reservoir simulation critical to maximizing production

from shale reservoirs.

Answer 460

heterogeneous

Question 461

Pertaining to one of three main classes of rocks. These rocks
crystallize from molten rock, or magma, with interlocking mineral
crystals. Igneous rocks that crystallize slowly, typically below the
surface of the Earth, are plutonic igneous rocks and have large

crystals (large enough to see with the naked eye). Volcanic ig-
neous rocks crystallize quickly at the Earth’s surface and have

small crystals (usually too small to see without magnification).

Common examples include granite (plutonic) and rhyolite (vol-
canic), diorite (plutonic) and andesite (volcanic), and gabbro (plu-
tonic) and basalt (volcanic). Igneous rocks typically comprise the

minerals quartz, mica, feldspar, amphibole, pyroxene and olivine.

Answer 461

igneous

Question 462

In the original location or position, such as a large outcrop that has
not been disturbed by faults or landslides. Tests can be performed
in situ in a reservoir to determine its pressure and temperature
and fluid properties.

Answer 462

in situ

Question 463

Water that occurs naturally within the pores of rock. Water from flu-
ids introduced to a formation through drilling or other interference,

such as mud and seawater, does not constitute interstitial water.
Interstitial water, or formation water, might not have been the water
present when the rock originally formed. In contrast, connate water
is the water trapped in the pores of a rock during its formation, also
called fossil water.

Answer 463

interstitial water

Question 464

The state of gravitational equilibrium between the lithosphere and
the asthenosphere of the Earth such that lithospheric plates “float”
at a given elevation depending on their thickness. The balance

between the elevation of the lithospheric plates and the asthenos-
phere is achieved by the flowage of the denser asthenosphere.

Various hypotheses about isostasy take into account density (Pratt
hypothesis), thickness (Airy hypothesis), and pressure variations
to explain topographic variations among lithospheric plates. The
current model consists of several layers of different density.

Answer 464

isostasy