Riley's geology Flashcards
What does SP and GR stand for?
Gamma Ray
spontaneous potential
well log annotations are
Baselines; Shapes; Abrupt breaks; Unusual values Trends.
What is a baseline
Vertical constant log value ideally
lithological and stratigraphical significance.
‘base value’.
can be recorded over tens or hundred meters.
true or false, baselines can aid in characterizing shale sections defined by 3 kinds of well logs including gamma ray, neutron, density, wavetrolls.
true except for wavetrolls
How is the idea that well logs indicate always the sedimentary paleoenvironments in siliciclastic rocks?
too simplistic
When are baselines typically present on gamma ray logs ?
in which the content of shale is extremely low or shales are completely absent.
Typical log shapes ?
bell, funnel, cylinder ( often complex pattern)
What are abrupt breaks ?
Probably it is the most important facies annotation.
what causes abrupt breaks ?
lithological nature;
structural breaks;
a succession of facies;
presence of certain fluids in the rock pores.
Abrupt breaks - classification
Lithologically related - surfaces are ?
Surfaces of erosion;
Surfaces of flooding
Abrupt breaks - classification
Non-lithologically-related
Unconformities;
Faults
What are unconformities ?
additional data from dipmeter,
seismic,
sequence stratigraphy
biostrat.
What are Unusual values
- anomalous values
- excessively high or excessively low peaks
- can have stratigraphical value.
What causes unusual values ?
Concentration of rare minerals, organic matter or phosphates in the proximity of the unconformities or other surfaces related to sequence stratigraphy can result in unusually high gamma ray peaks.
what does Sequence stratigraphy represent ?
represents the study of rock relationships within a chronostratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities.
what does Sequence stratigraphy postulate?
Sequence stratigraphy postulates that sedimentation patterns, sediment architecture and sediment thickness are all highly dependent of the sea-level changes. *
What is the Exxon Model ?
Exxon Model: a sequence is composed of 3 or 4 units (systems tracts: transgressive, highstand and lowstand), which are separated by key surfaces (transgressive surface, maximum flooding surface and sequence boundary).
T-R model?
T-R system: a sequence is formed only of 2 units (transgressive and regressive) separated by 2 key surfaces: transgressive surface and maximum flooding surface.
describe the transgressive systems tract
A transgressive systems tract is bounded at the base by a transgressive surface and at the top by a maximum flooding surface.
Where do the sediments accumulate in the highstand systems tract?
in the period with the highest sea-level
A typical lowstand systems tract consists of
a basin floor fan;
a slope fan;
a lowstand wedge.
The stacking patterns of the parasequence sets refer to the
basinward and/or landward migration of the sedimentary facies in a stratigraphic succession
Retrogradation;
Progradation;
Aggradation.
Retrogradation is when?
Rate of deposition is smaller than the rate or accommodation.
Retrogradation occurs when?
It occurs during periods of increase of the sea level.
Progradation is when?
Rate of deposition is much higher than the rate of accommodation.
Practically there is not enough accommodation space to take all of the sediment transported in the sedimentation area.
Aggradation is when ?
In this situation the rate of deposition is equal or extremely close to the rate of accommodation.
The sediment infill balances the new formed space in the area of sedimentation.
TST?
transgressive systems tract. retrogradational pattern
HST?
Highstands system tracts. progradational and/or aggradational.
what are the 4 systems tract ?
FST, HST, TST, LST
failing-stage
highstand
transgressive
lowstand
Parasequence stacking patterns are strongly controlled by ?
the terrigenous input and rate of rise/fall of the sea-level.
Crude oil classification 1 ?
quantities of various hrdrocarbons present in the crude oil
Crude oil classification 2 ?
ratio between 3 large groups of crude oil compounds which are ?
Paraffins
naphthenes
sum of aromatics and heterccompounds
Hydrocarbon Formation
According to the general amounts of dissolved oxygen and its distribution within the sea water column two settings can be recognized they are ?
oxic
anoxic
what is Diagenesis
pT conditions are close to normal conditions;
main transformations are the biogenic decay and abiogenic reactions
Simple chemical compounds (such as CH4, CO2, H2O, etc) interact to form a complex hydrocarbon –KEROGEN
GENERAL RESULT: reduction of oxygen content while the H/C ratio remains almost unchanged.
Catagenesis occurs when?
after diagenesis as temperature and pressure increase. Hydrocarbons are released from kerogen in the following order: first oil and then the gas (first wet, then dry). H/C ratio decreases while O/C ratio is almost constant.
describe Metagenesis
During this phase the pressure and temperature increase towards values common to the incipient metamorphism. The last hydrocarbons are expelled during this phase (largely methane). H/C ratio decreases until only carbon is left as graphite. Porosity and permeability are now negligible.
true or false Kerogen is a macromolecular complex with a polymer-like structure.
True. It is that fraction of the organic matter that does not dissolve in organic solvents.
Where does Kerogen form ?
It forms in the upper few hundred meters of sediments from organic precursors that have been modified or resulted during diagenesis.
Kerogen maturation depends on ?
temperatures, time and pressure. (oil from 60-120)
what happens after gas generation of kerogen?
becomes as inert as graphite
What is primary migration ?
Petroleum migration is a general term which defines the hydrocarbon movement from the source rock toward the reservoir and within the reservoir rocks.
why are fractures of importance ??
substantial increase in permeability parameters
When are you better off analyzing fractures from geographical well logs instead of cores only ?
very large fractures
Fractures type of occurrences?
compressional folding’
adjacent to faults
beneath unconformities
Definition of a trap?
In a general sense ‘a trap is a place where oil and gas are barred from further movement’
better yet…
‘A trap is geological structure or setting able to trap subsurface hydrocarbons for a longer period of time’.*
The nomenclature comprises elements of both ?
geometrical and economical nature
IMPORTANT GEOMETRICAL TERMS FOR TRAP
Crest (culmination) – highest point of a trap;
Spill – lowest point of a trap; no hydrocarbons can be met below the spill point; spill point defines the spill plane on contour maps;
Closure – vertical distance between crest and spill plane; the closure differs from the structural relief if the anticline is tilted;
Bottom water – water just below OW or GW contact; the other water in a trap is referred as lateral (edge) water.
traps economical terms
Pay – the productive reservoir; this definition is in a general sense;
Gross pay – represents the vertical distance from the trap crest to the HW contact;
Net pay – represents the interval that produces hydrocarbons effectively.
What causes tilted fluid contacts ?
Hydrodynamic flow;
Trap tilting;
Lateral change of facies;
production from other wells in the adjacent
connected reservoirs or oil-fields.
The existence of a tilted contact due to the hydrodynamic flow requires
an active aquifer and the absence of tar mats in the proximity of the HWC.
Tilted contacts due to the trap tilting occurs when ?
This situation requires the existence of a tar mat at the HWC and an active tectonical regime.
Tilted contacts due to facies change
These tilted contacts occur due to the impermeable rocks (shales and, rarely, limestones) that alternate with siliciclastic ones in various sedimentary systems
Most effective cap rocks?
evaporites
Most common seals?
shale ( high capillary forces preventing fluid flow due to their very fine grain size )
Trap Classifications
(i) structural traps (faults and folds related traps);
(ii) diapiric traps (salt or mud related traps);
(iii) stratigraphic traps;
(iv) hydrodynamic traps (caused by water flow);
(v) combination traps (combination between two or more between the above mentioned);
(vi) subtle traps.
Structuaral traps formed by ?
postdepositional tectonic modification of a reservoir rock (faulting and/or folding).
Fold traps commonly found ?
areas of plate collision and subduction . mostly present in the oilfields situated in the proximity of the mountain chains.
Compressional anticlines
Example of folded oil bearing structures in western Iran (Taurus Mts).
Compactional anticlines
formed from crustal tension
horsts and grabens formed during initiation stage
initial deposition infills this irregular topography
North sea is an example of a
compactional anticline draped over an old basement rock;
How many theoretical geometries of the fault traps? how many being used ?
8, 6 being used
Diapiric traps produced by
the upward movement of the plastic sediments presenting lower densities when compared with the overlaying rocks.
Wrench fault related traps commonly formed
in the strike slip basins, associated with wrench faults.
Salt diapirs-mechanism
based on the upward movement capabilities of the salt sediments
What point will the Salt diapirs present a trend to flow upward
critical depth is reached 800-1200m) when the salt becomes less dense than the overlaying clay stones or sandstones
stratigraphic trap defined as
‘One of the chief trap-making element is some variation in the stratigraphy, or lithology, or both, of the reservoir rock, such as a facies change, variable local porosity and permeability, or an up-structure termination of the reservoir rock, irrespective of the cause’. *
stratigraphic trap classifications?
related to unconformities
unrelated to unconformities
