Fluids in rocks and sed processes and resources A2

Question 1

Porosity

Answer 1

volume of pore spaces (volume occupied by spaces between grains)

Question 2

porosity equation

Answer 2

(total vol of pore spaces/ total vol of rock)
x 100

Question 3

factors effecting porosity

Answer 3

only sed rocks as interlocking crystals= no porosity

degree of sorting-
poorly sorted= lower porosity as smaller grains fill spaces

degree of diagenesis-
more compaction/pressure + more diagenesis= less pore spaces
pressure dissolution= liquid fills in pore spaces

packing of grains-
fit well= reduced porosity
compact= pack closer
angular grains fit better= lower porosity

Question 4

secondary porosity

Answer 4

fractures/ joints/ cleavage in rocks allow for secondary porosity.
may form due to dissolving/ altering of grains, cement or both

Question 5

porosity examples

Answer 5

high-clay, chalk and sand
low- shale and granite

Question 6

permeability

Answer 6

rate at which fluid flows through a rock

Question 7

equation for permeability

Answer 7

distance moved/ time taken

Question 8

factors effecting permeability

Answer 8

porosity-
not porous= not permeable
has to be good connectivity of pore spaces

temp-
temp increase, less viscous, more permeable
hot= rock expands = decrease pore spaces

grain size- coarse= higher permeability as less resistance to flow

Question 9

secondary permeability

Answer 9

fractures/joints/voids/crevassesformed after to form secondary permeability

Question 10

examples of diff permeabilities

Answer 10

high- sandstone, chalk, limestone
low-granite, shale, clay

Question 11

capillary pressure

Answer 11

pressure between 2 immiscible fluids in pore spaces.
results from interactions of forces between fluid and solid
fluids attracted to surfaces and each other
diff fluids= diff levels of attraction to surface/ itself
therefore in rock sometimes fluids can displace eachother

Question 12

hydrostatic pressure

Answer 12

pressure at a point due to mass of overlying water column

Question 13

hydrostatic head

Answer 13

height of overlying column of water

Question 14

hydraulic gradient + equation

Answer 14

diff in H pressure between 2 points / by distance between them

vert diff/ horizontal diff

Question 15

Water table BG

Answer 15

line that separates saturated rock from unsaturated

Question 16

Aquifers

Answer 16

confined- rock is directly open at surface of ground ( exposed to atmosphere)

unconfined- thick deposit overlays aquifer blocking it from atmosphere

perched aquifer- occurs at lensed shaped area of impermeable rock (above WT)

Question 17

aquiclude

Answer 17

impermeable rock that doesn’t transmit H20

Question 18

aquitard

Answer 18

Rock with low permeability allows transmission of water at slow rates

Question 19

live vs fossil aquifer

Answer 19

live- currently replenished by rain water via recharge zone

fossil- no longer replenished + relic of past climate

Question 20

artisan basin and well

Answer 20

basin- A large, synclinal confined aquifer where groundwater is stored under pressure

well- brings ground water to surface without pumping due to high pressure

Question 21

piezometric surface

Answer 21

imaginary surface at which G water rises under H pressure to produce spring

Question 22

Springs

Answer 22

occur where water table intersect land surface + G water flows out onto surface

often see junction between permeable and impermable rocks resulting in spring line

Question 23

lithological springs

Answer 23

In a valley

next to an intrusion- impermeable intrusion rock causes spring to move

Question 24

spring at fault

Answer 24

moves the impermable rock so position of spring changes

Question 25

spring at unconformity

Answer 25

unconformity between permeable and impermeable rock, spring appears at unconformity boundary

Question 26

seep

Answer 26

Where hydrocarbons reach the surface. because oil in pores is displaced by H2O and moved to the surface

e.g. Kimmeridge bay
natural oil seep through Jurassic shale
oil well here produces 65 barrels a day and 300 at peak

Question 27

BIFS

Answer 27

Banded iron formations

units of sedimentary rock from Archean and Paleoproterozoic

repeated layers of iron oxide

often haematite or magnetite

began to form as O2 was produced by photosynthesises

Question 28

Oxygenation event

Answer 28

cyanobacteria and photoferrotropes began to produce O2

O2 reacted with Fe in sea to produce FeO which settled to produce banded iron formations

o2 enter atmosphere
changed atmosphere
reacted with methane
Photosynthesizers removed CO2 from atmosphere
temp drop
mass glaciation (Huronia)
mass extinction

produce ozone layer

Question 29

photoferrotropes

Answer 29

photosynthetic bacteria oxidised Fe2+ ions to produce Fe3+ ions

using sunlight and hydroxides

Fe3+ then oxidised to form minerals e.g. magnetite

Question 30

Walther’s Law of facies

Answer 30

a vert succession of facies will be the product of a series of dep enviros that lays laterally adjacent to each other.

e.g. regression and transgression

Question 31

connate water

Answer 31

H20 included in sed when dep and then trapped in pore spaces
brines may also be rich in silica and bicarb

expelled during diagenesis, some solute precip into pores
reducing porosity and permeability
less suitable as reservoir/ aquifer

Question 32

facies

Answer 32

include all characteristics of sed rocks that are produced by the envrio of dep and allow it to be distinguished from rocks deposited in adjacent environments

Question 33

problems caused by abstraction

Answer 33

subsidence-
H20 extracted
empty pores
collapse
causes downward movement of ground
biggest impact on buildings + cant be undone

salt H2O encroachment-
extracted
near sea
pull in saltwater
once contaminated no longer drinkable
high residence time so long contaminations

lowering WT-
loss of springs
loss of surface water
habitat loss

well problems
not deep enough
water shortage

cones of depression-
caused by WT lowering by wells
when wells to close cones overlap
lowers water table to lower completely

Question 34

Turbidite current

Answer 34

a high velocity current, flows down a gentle gradient, sed dispersed within it makes it denser than sea water.
triggered by earthquakes/instability

Question 35

turbidite formation

Answer 35

river brings sed to the sea
cumulates on cont shelf
sed on edge becomes unstable
gravity=moves down slope
submarine gravity flows from turbidite current
fining up sequence

Question 36

Bouma sequence

Answer 36

E-
shale
parallel lamination in mud
no current + suspension settling
D-
fine sandstone and silt
parallel laminations
low e
suspension settling
alternation coarse and fine
C-
sandstone
crossbedding
climbing ripples
saltation
dep exceeds rate of migration leading to C ripples
insufficient energy to erode
B-
coarse then med sandstone
flat bed
graded bedding
sole marks= tool marks and flute clasts
sole marks on base form pits and act as mould fill to produce clasts

A-
coarse bed of pebbles (conglom)
unconformity
graded bedding
rip up clasts
erosional base
high e
rip up shale to form rip up clasts

Question 37

biogenic deposition

Answer 37

background dep when no current

comp=comp of planktonic organism

silica dissolves at slow rate in sea
ooze accumulates when rate of dep is higher than rate solution

planktonic organism dies and test sinks and is preserved as a micro fossil

diatoms near poles

Radiolaria near equator

sed rate on abyssal plain est 1mm-3mm per 1000 yrs

Question 38

Threats to ground water

Answer 38

pollution-
Sources= nitrates, pesticides and microbes from agri run off and sewage
H carbons and solvents from factories
toxic fluids from landfill
acid mine drainage = water containing toxic chemicals e.g. Lead

Long residence time and vert impossible to get rid of

unconfined aquifers are more at risk as Porous rock exposed

Over pumping-
if too much GW is extracted may not be enough left to provide reliable public water supply

EXTRA
Ground water filtration-
sometimes GW percolate through pore spaces and is naturally filter removing impurities so doesn’t require treatment and some of the minerals in sol give taste and health benefits e.g. Hard water has lots of Ca 2+ or F- to reduce tooth decay

some have naturally high arsenic e.g. India