13. Control of P&P by Defm

7/30/2019 13. Control of P&P by Defm

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13. EFFECT OF

DEFORMATION ONRESERVOIR POROSITY

AND PERMEABILITY

Topics

• Factors Controlling Porosity and Permeability

• Deformation Mechanisms That Usually Enhance

Porosity and Permeability• Deformation Mechanisms That Usually Reduce

Porosity and Permeability

• Effects of Faults

• Fractured Reservoirs

1. Deformation mechanisms

Rock type

Initial porosity

Environment of deformation

2. Mechanics and kinematics of the structure

3. Amount of deformation

Factors Controlling Porosity and Permeability

Deformation Mechanisms that Usually

Enhance Porosity and Permeability

1. Joints

2. Other tension fractures

3. Fault brecciation

Tension fractureTypically open

Joints

occur as sets of parallel fractures



plumose markings

Fault-related veins

Shear fracture / faultslickensides on fault surface

Opening-fault veins

favored by

low differential stress,

high fluid pressure

Fault Shape Effects

neutral overlap

contractional overlap,

restraining bend

extensional overlap,

releasing bend



Pull aparts (releasing bends) and tip fractures

Fault breccia

Deformation Mechanisms that Usually

Reduce Porosity and Permeability

1. Crystal-plastic deformation of grains

2. Pressure solution

3. Cataclasis

Low -Temperature Crystal Plasticity

Twin glide in calcite

Translation glide in quartzFossiliferous limestone

Before

After



Pressure Solution

Precipitation

in pore space

Stylolite formation

Residuum

Stylolite geometry

Wavy stylolites parallel to bedding

Sutured stylolite

perpendicular to bedding

b e d d i n

g

Low porosity

adjacent to

stylolites indicates

local cementation

Dukhan limestone

Dukhan field, Qatar

Rarely, dissolution along a styloliteCataclasis



Identify deformation mechanism(s) and stress directions

ST 13-1

Effect of Faults

on Fluid

Migration

Conduit

Barrier

Filter

Juxtaposition

Fault brecciaFault Conduit

Fault Conduit

Chalk: the only

permeability is

along faults

Barrier



Trap At Fault Barrier

Mecca Hills

Barrier

Fault barriers:

fault blocks have different productivities

3-D side view

Fault blocks have different productivities

Filter: blocks oil, transmits gas

juxtaposition trap

Juxtaposition

juxtaposition conduit



Barrier or conduit due to

juxtaposition of lithologies

and entrainment

Sealing vs. Nonsealing Faults

Parts of the same sandstone juxtaposed: Nonsealing

Sandstone against shale: Sealing

Sandstone of different ages juxtaposed: Commonly sealing

Section with high % sand: Commonly nonsealing

Allan diagram = Juxtaposition diagram Constructing fault cutoff maps (Allan diagrams)

SGR = Σ HiSi / Tn

i = 1

SGR = Shale Gouge Ratio

H = unit thickness

S = % shale in unitT = fault thrown = number of layers

Allan diagram for a normal fault

Shale gouge ratios

sealing

FW

HW

sand bed



Predict fault traps with Allan diagram

ST 13-2

Predict traps

Fractured Reservoirs

Regionally jointed

Compressional folds

Extensional folds

Ductile

Brittle



Regionally jointed reservoirs Regional orthogonal joint pattern

Open joint

Formerly

open vein

Regional

orthogonal

joint

pattern:

planar

systematic

and cross

joints

σ1

Joints endat bedding

planes

Maximum

spacing

approximately

equal to bed

thickness

Compressional Folds



Fold Strain

Controls locationand mechanisms

of deformation

almost pure bend

pure bend

buckle

Hinge

Collapse

by

Faulting

Hinge

Collapse

by

Pressure

Solution

Outer-Arc

Extension by

Veins

Layer-parallel shortening throughout compressional foldsHinge collapse faults

Bed-normal stylolites

strain

concentrated

in steep limb

Small outer-arc

extension in an oil-

field scale bucklefold



Compressional Fractured Reservoir

Outcrop example: Oil Mt. anticline,

Wyoming

Oil Mt. anticline, B-B’

Joint set orientation and distribution

J1 = regional,

spacing ≈ bed

thickness

J2 = anticlinal,

normal to crest

J3 = anticlinal,oblique

Observed fracture intensity

Predicting fracture intensity

observed intensity correlates

best to rate of dip change

high degree of local variability in

intensity

Extensional Folds

Large

outer-arc

extension



Vertical shortening on stylolites

causes reservoir segmentation

Pressure solution is inhibited by presence of oil or gas

Dukhan field, Qatar

Extension required tomaintain vertical pin line

before

Extension required tomaintain vertical pin line

after

Constant BLT Flexural Slip

Extension required in HW rollover

Gilbertown field

Extensional Fractured Reservoir

Chalk reservoir



THINK LIKE A ROCK

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13. Control of P&P by Defm