Unit 4 - Structural Geology Flashcards
(65 cards)
1
Q
Why is structural geology important for a geologist?
A
- Many traps are formed by structural deformation in the past → important to determine sequence of events that took place to evaluate chance of petroleum being trapped
- Structural features (folds, faults or fractures) in reservoir will affect fluid flow and production of reservoir
- Rock mechanics and the analysis of structural features help predict or avoid problems that may occur during drilling or production
2
Q
What is a fold?
A
A flexure of layers
3
Q
How can flexures (folds) be produced ?
A
by faulting, buckling, diapirism and by differential compaction of underlying rock layers
4
Q
Why is tilting of rock layers to non horizontal orientations a very important part in structural geology
A
- tilting of rocks is almost essential to let HC migrate from source rock to traps
- a tilted carrier bed ( a rock layer with similar characteristics to reservoir rocks) can provide a pathway
- tilting is a major way to create change in elevation ( anticline , fault block) → reservoir usually exist at top of structures (exception stratigraphic type)
5
Q
What are the three methods used to illustrate structural features?
A
- block diagrams
- cross sections
- maps
6
Q
What is a stratigraphic trap?
A
- they occur if HC accumulate in a reservoir rock unit at point where that rock unit interfingers with other, non-reservoir rocks
7
Q
What is a little problem with a block diagram?
A
- It is partly imaginary since the sides of the block diagram are interpretations of the subsurface arrangement of rock units
8
Q
We define the orientation of a line by using a trend and a plunge - show it in the picture
A
9
Q
How do we call the trend of a line if we only need to record the compass direction?
A
Bearing or Azimuth
10
Q
What is a fault?
A
- a more or less planar discontinuity surface or zone, within a rockmass, across which the rocks on either side have moved by shear displacement (displacement parallel to the fault surface)
11
Q
What is important to consider something a fault?
A
- movement → fractures and joints don’t have movements
12
Q
Classifying faults by angle of dip
A
- vertical fault has a dip of 90°
- very shallow (10-30°)
- moderate (40-60°)
- steep (70-89°)
13
Q
Definition of Fault Slip
A
14
Q
Types of Fault defined by displacement along the fault plane?
A
15
Q
What is a horse block?
A
A large intact block of rock, which is found in a fault zone
16
Q
What is synthetic dip?
A
Similar dip
17
Q
What is antithetic dip
A
Opposite dip
18
Q
Name the different normal faults in this assembly ?
A
19
Q
What is actually causing the movements along the faults?
A
Large Scale forces, acting in the crust
20
Q
What is a listric fault?
A
- a curved fault, that is steeply dipping at shallow structural levels and gently dipping at deeper levels
21
Q
What is a horse?
A
a block of rock, that is completely surrounded by faults
22
Q
Why is the grain size along the fault plane sometimes way smaller than in the other layers?
A
- Grinding action of fault movement
23
Q
What is dilatant fracturing?
A
Fault movement creats open fractures or surfaces within a fault zone
24
Q
Why are faults important for HC?
A
Often used for migration of HC but impermeable faults can also act as traps
25
Do faults have a constant displacement?
- No, towards the ends they go to 0 and in the middle they are larger
26
How do you call a diagram, where footwall and hanging wall blocks are projected onto a map of the fault surface?
- Allen diagram → almost impossible to create by hand, requires computer software
27
What happens to the permeability of a fault if muddy sediments are smeared along the fault plane?
Shale, or mudstone (both clay rich), is often much weaker than other rocks → potential to deform rapidly and easily can be smeared along fault plane
→ leading to a reduction of permeability.
28
What is an approach on prediction the sealing or non-sealing properties of a fault zone?
- Create estimate of total shale material in fault zone
29
What does SGR stand for and what is it used for?
- shale gouge ratio
- represents proportion of shale in the rocks that have moved past any point on fault → expressed as percentage of total displacement on the fault
30
What is the inflection line?
boundary between convex-up and convex-down curvature of the surface
31
What is the culmination?
highest point of a crest or trough line
32
What is a depression?
lowest point of a crest or trough line
33
Description of folds - just a graphic with expressions
34
Hinges and Limbs on a graphic?
35
What is the crest?
the locally highes point
36
What is a hinge ?
The point of the greatest curvature
37
Description of different interlimb angles of a fold !
38
What is a thrust fault?
Reverse fault, which dips with less than 45 degrees
39
Terms to describe fold orientations
40
How are buckle folds created?
By pushing rock layer along its length
41
What do you call the transverse direction of a fold?
Perpendicular direction across the fold → mostly used when drawing folds
42
Bend folds?
43
What is a key mechanical characteristic, that fold can be created ?
The fact that they are layered → slippage between the layers
44
What is defining for an asymmetric fold?
They have limbs of different lengths
45
What is a compaction band ?
Area, where grains are crushed together → porosity reduction
46
Fractures associated with a normal fault?
47
What is a fractured reservoir?
A reservoir in which natural fractures have a significant effect on the producing characteristics of the reservoir, usually through increased permeability
48
What is a diapir?
Body of flowable rock, which has migrated upwards and maybe sideways due to the lower density (as compared to surrounding rocks) and relatively low strength → is often salt
49
What is cleavage?
planar fabric associated with strong preferred alignment of clay-size particles
50
What is an orogenic system ?
- Refers to process involved in the construction of mountain belts during plate convergence
51
Describe a little Fold and Thrust belts!
- very long 100's - 1000's of km
- curvi-linear zones of deformation with immense horizontal shortening - maybe 100 km or more
- occur in zones where major plates converge
52
Name some key characteristics for Fold and Thrust belts!
- Asymmetric folds and buckle-folds
- detachment-style → thrust/reverse faults
- Good strike continuity of structures, moderate dip and depth continuity.
- normally forms a belt bowed towards the movement direction of the thrusts
- often a narrow deformed zone located at a acutal or former continental margin
- structure becomes more complex from foreland to hinterland
53
Describe a wrench province!
- in continental terrains
- can be very broad (1000`s of km in each direction)
- produced by lateral movements of major plates
54
Key characteristics of wrench provinces?
- Intermixing of folding and faulting
- Fault sense and fold shapes change rapidly
- Vertical and horizontal predictability is poor
- Often strong localization of deformation, with intervening regions of little or no damage
- Contains examples of structural forms of all types.
55
What is rifting and extension?
Lateral movement of crust, caused by its own weight when lateral constrains are removed
56
Where do rifting and extension often occur?
- often occur at the sites of previous orogenic shortening when these later spread sideways and return to normal crustal thicknesses
57
Key characteristics of rifting and shortening?
- tilted fault blocks
- Syn-tectonic deposition of sediments
- possibility of major and/or rapid changes in depositional setting (non-marine to deep marine, and possibly back to non-marine)
- Listric faults and smaller-scale faults
- Volcanics
58
Where are diapirs and growth structures often found together?
- in passive margin settings
59
Where do diapirs and growth structures usually exist?
- typically present in passive margins
- depositional thickness changes
- synthetic and antithetic fault sets
- growth faults are often on flanks of diapirs
60
How are diapirs activated?
both salt and mud diapirs are activated because of density differences
61
What is the cause for drape folds or block faulting?
- high-angle faults which sit very deep (in basement rocks, or in deeper portions of the basin) cause bending of the overlying layered rocks at shallow structural levels
62
What are key characteristics for drape fold and block faulting?
- have a wide range of fault orientations
- Key element is unit of ductile/weak rock between 'basement' and layered rocks ('cover')
- style is probably present throughout the central 'stable' regions of the continents, as intra-continental basins form and evolve.
63
Name some geodynamic processes, that basin formation is associated with?
- crustal stretching
- flexural loading
- density changes
- tectonism
64
How are listric faults produced?
- at margins of rapidly-subsiding basins
- recently-deposited rocks are weak, and slide towards the deeper portion of the basin
→ as this motion takes place, rocks are stretched, and large, listric normal faults are produced
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