Week 18 Research Example

Question 1

Location

Answer 1

Lewisian gneisses, Canisp, NW Scotland

West of Shetland

Question 2

Why is it a fracture reservoir?

Answer 2

Crystalline basement rocks (Lewisian gneiss) = low porosity and permeability

Oil stored within fracture systems only

Migrated from adjacent sedimentary rock

Question 3

Type of rock

Answer 3

Lewissian gneisses

High grade granulite facies metamorphic rock

Question 4

Components of rock

Answer 4

Grey granodioritic gneiss 55%

Pink granitic gneiss 15%

Foliated porphyritic granite 15%

Green mafic gneiss 10%

Grey dioritic gneiss 5%

~20% interlayed on cm to mm scale

Question 5

Rona ridge =

Answer 5

series of uplifted footwall blocks of fractured basement bounded Mesozoic normal faults

Clair field = multi billion barrels, potential field life >25 years

Question 6

Evidence for transient over pressuring episodes

Answer 6

i.e. seismogenically-driven fluid flow

Microbreccias and veins associated with:

• sediment filled open fractures/injections
• larger shear fractures (often poor preservation in core)
• muggy microbreccia fractures/veins full of all

Question 7

Evidence for shear failure rather than tensile

Answer 7

Dilation pull aparts

Question 8

Standout features

Answer 8

1) primary sedimentary fracture fills

2) way-up criteria (grading/geopetal fills); youngs up-core in vertical wells, across core in horizontal

Question 9

Processes

Answer 9

SEISMIC PUMPING
- inter seismic period = fluids/sediment/oil sucked in laterally

SEDIMENTATION
- traps oil

SEISMOGENIC FAULTING

• drove fluid flow; injection and migration
• slip along fault = snaps fractures closed = fluids expelled along fault and into basement system