Faults as Fluid Flow Faults as Fluid Flow Barriers and Their Role in Barriers and Their Role in Trapping HydrocarbonsTrapping Hydrocarbons

Suzanne CooganSuzanne Coogan

Richard Nice Richard Nice

Ayeni GboyegaAyeni Gboyega

Kate Carter-WalfordKate Carter-Walford

IntroductionIntroduction

Fault seal mechanismsFault seal mechanisms

Influence on hydrocarbon fieldsInfluence on hydrocarbon fields

Modelling and Flow PropertiesModelling and Flow Properties

Case StudiesCase Studies

How Can Faults Create Barriers How Can Faults Create Barriers to Fluid Flow?to Fluid Flow?

•JuxtapositionJuxtaposition

•CataclasisCataclasis

•CementationCementation

•Clay SmearingClay Smearing

JuxtapositionJuxtaposition

JuxtapositionJuxtaposition of lithologies with different permeabilities across the fault of lithologies with different permeabilities across the fault

JuxtapositionJuxtaposition

Coarse grained material

(light colour)

Fine grained material

(dark colour)

Analogous to a reservoir

being juxtaposed against a sealing

lithology

Cataclasis Cataclasis CataclasticCataclastic grain-size reduction results from abrasion during grain-size reduction results from abrasion during deformation; smaller grains fill pore space and reduce deformation; smaller grains fill pore space and reduce porosityporosity

Stress concentration during deformation:

a) undeformed sediment

b) stress concentrated on the centre grain

c) centre grain fractured

(after Antonellini et al., 1994b)

Stress concentration during deformation:

a) undeformed sediment

b) stress concentrated on the centre grain

c) centre grain fractured

(after Antonellini et al., 1994b)

•Effectiveness depends on:Effectiveness depends on:

The hardness of the intact rockThe hardness of the intact rockThe magnitude of displacementThe magnitude of displacementInitial normal stress on the fault surface prior to Initial normal stress on the fault surface prior to

movementmovement

Grain Size Reduction andGrain Size Reduction andMica OrientationMica Orientation

Processes

Shearing

Mica Orientation

Clay formation

Fluid flow

CementationCementation

Minerals carried in solution in Minerals carried in solution in water under high pressurewater under high pressure

As fault opens, pressure As fault opens, pressure release occurs, water flows release occurs, water flows through fault and minerals through fault and minerals precipitate out of solutionprecipitate out of solution

Crystallisation of cements in Crystallisation of cements in pore spaces reduces pore spaces reduces permeabilitypermeability

Cement

Reduced grain size

Clay SmearingClay Smearing

Layers of shale contained within sequence are drawn into fault as movement along plane progresses

Impermeable layer formed along fault depending on Shale Gouge Ratio (SGR) – ratio of sand to shale

Shale Gouge RatioShale Gouge Ratio

This measure is useful for predicting sealing ability of fault. 18 – 30% indicates high probability of a seal

The SGR is the percentage of shale within a part of the sequence which has moved past a point on the fault surface

CSP & smear factor definition

Yielding et al. (1997)

• SSF <3 SSF <3

– Continuous smears.Continuous smears.

• SSF 3 -10 SSF 3 -10

– 60% Continuous.60% Continuous.

• SSF >10 SSF >10

– 70% Discontinuous.70% Discontinuous.

Shale Gouge ExampleShale Gouge Example

Shale Gouge RatioShale Gouge RatioAs SGR increases, sealing ability improves. The clay has a small pore throat size and therefore high capillary entry pressure

With smaller gouge ratios, brittle fracture and therefore cataclasis dominates. Sealing is less effective than clay smear

Examples of Clay SmearingExamples of Clay Smearing

These examples show 3 faults in outcrop that range from sand-prone  to shale-prone gouge and an intermediate sand/shale ratio gouge.  These faults demonstrate a spectrum of gouge composition and of seal behaviour

Clay Smearing on Microscopic Clay Smearing on Microscopic ScaleScale

Effectiveness of Fault Sealing Effectiveness of Fault Sealing Mechanisms Mechanisms

Sealing Capacity of FaultsSealing Capacity of FaultsHHdd = = 22hh(r(rtt

-1-1-r-rpp-1-1)/g)/g((ww--hh))

rrtt = pore throat radius in the seal = pore throat radius in the seal

rrpp = pore throat radius in the reservoir = pore throat radius in the reservoir

hh = hydrocarbon-formation water interface tension = hydrocarbon-formation water interface tension (Oil: 5-35 dynes/cm Gas: 30-70 dynes/cm).(Oil: 5-35 dynes/cm Gas: 30-70 dynes/cm).

ww = density of the formation water (1 – 1.2 gm/cm = density of the formation water (1 – 1.2 gm/cm33))

hh = density of the hydrocarbon phase = density of the hydrocarbon phase (Oil: 0.5 –(Oil: 0.5 –1.0 gm/cm1.0 gm/cm3 3 Gas: 0.2 - 0.4 g/cmGas: 0.2 - 0.4 g/cm33))

gg = acceleration due to gravity = acceleration due to gravity

ModellingModelling •Empirical methods for risking the sealing potential of faults have Empirical methods for risking the sealing potential of faults have been devised in combination with outcrop and laboratory studiesbeen devised in combination with outcrop and laboratory studies

•Estimates the sealing potential of a fault offsetting a particular Estimates the sealing potential of a fault offsetting a particular sequence and therefore the entry pressuresequence and therefore the entry pressure

•For a detailed modelFor a detailed model identify where a fault is sealed and where leakage may identify where a fault is sealed and where leakage may occuroccur

establish where significant pressure differences are likely establish where significant pressure differences are likely to beto be

supported across a fault surface and their magnitudesupported across a fault surface and their magnitude

understanding of the migration pathway and the column understanding of the migration pathway and the column heightheight

•Can use models to predict the flow and flow restrictions of Can use models to predict the flow and flow restrictions of hydrocarbons due to fault properties using a programs such as hydrocarbons due to fault properties using a programs such as the SEMI migration model or TransGenthe SEMI migration model or TransGen

•Such modelling can only be achieved by 3-D analysis of the Such modelling can only be achieved by 3-D analysis of the geometry and sealing characteristics of faultsgeometry and sealing characteristics of faults

Calculating fault Calculating fault propertiesproperties

•Calculate the Shale Gauge Ratio on Calculate the Shale Gauge Ratio on the fault surface and convert to fault the fault surface and convert to fault seal potentialseal potential

•Calculate the reservoir elevation at Calculate the reservoir elevation at fault traces fault traces

•Construct a sequence/throw Construct a sequence/throw juxtaposition diagram form log and juxtaposition diagram form log and lithological information lithological information

•Input sequence containing shale Input sequence containing shale values and reservoir offset values and reservoir offset

•In this example, throw is between In this example, throw is between zero and thickness of sequence - zero and thickness of sequence - triangular plot triangular plot

•Plot is annotated according to SGRPlot is annotated according to SGR

What Controls Seal What Controls Seal Effectiveness and Fluid Effectiveness and Fluid

TransmissibilityTransmissibility• Hydrostatic / CapillaryHydrostatic / Capillary

– Buoyancy controlBuoyancy control

• Hydrodynamic / Capillary:Hydrodynamic / Capillary:– Fluid pressure gradient Fluid pressure gradient

controlcontrol

• Hydrodynamic / Open Hydrodynamic / Open FracturesFractures

– Network Geometry, Network Geometry, aperture and pressure aperture and pressure gradient controlgradient control

h

h

Leak

Column HeightsColumn Heights•Two distinct geometries have to be considered Two distinct geometries have to be considered when estimating oil column heightswhen estimating oil column heights

•When fault throw is less than the thickness When fault throw is less than the thickness of the reservoir and the reservoir is self of the reservoir and the reservoir is self juxtaposed. Column height is determined by juxtaposed. Column height is determined by the threshold pressurethe threshold pressure

•When the fault throw exceeds the thickness When the fault throw exceeds the thickness of the carrier interval, oil leakage occurs of the carrier interval, oil leakage occurs along the fault. Leakage can occur along fault along the fault. Leakage can occur along fault surface if fault is not sealingsurface if fault is not sealing

•Column Height is determined by the depth Column Height is determined by the depth (Z), density of oil and water (po, pw) and (Z), density of oil and water (po, pw) and gravity (g)gravity (g)

-6 -5 -2-3-4

10

1

.1

100

1000

Pore throat radius (log cm)

Oil

colu

mn

hei

ght

s

Cataclasites

Fine sandstone

Coarse sandstone

Shale smear / cementation

Potential Oil Column Heights

MigratioMigrationn

Po Pw

Po

Migration is the movement of hydrocarbons though rock Migration is the movement of hydrocarbons though rock pores and fault networkspores and fault networks

Migration is driven by buoyancy and resisted by capillary Migration is driven by buoyancy and resisted by capillary pressurepressure

Leakage occurs when the entry pressure (Pe) equals the Leakage occurs when the entry pressure (Pe) equals the pressure of oil and water (Po, Pw)pressure of oil and water (Po, Pw)

Flow Model Flow Model PredictionsPredictions

•It is possible to predict fault properties and attach them to flow models, eg. TransGen (Mansocchi et al 1999)

•Attach a transmissibility multiplier – a unique property attached to the face of a grid block

•Transmissibility is assessed using the length of the block (Lg), the permeability (k) and the fault thickness (tf)

•Transmissibility of 0 is sealing, 1 is for unimpeded cross-flow

Applications of Modelling Fault Applications of Modelling Fault SealingSealing

•All indicate that faults and fault sealing play a key role in hydrocarbon distribution and migration

•Different fault seal properties result in different hydrocarbon distributions and migration pathways

•Using different fault sealing properties to Using different fault sealing properties to perform migration modelling perform migration modelling

•In this example the following conditions In this example the following conditions exist exist

reservoir contains known hydrocarbon reservoir contains known hydrocarbon accumulations accumulations different hydrocarbon-water contact different hydrocarbon-water contact levels in adjacent fault blocks levels in adjacent fault blocks a dry fault-bounded structural high a dry fault-bounded structural high existsexists

All faults are open with no fault sealing – small

accumulation in east and spills to the south

Larger accumulation of hydrocarbons due to sealing

capacity of bounding faults with little excess spilling to east

Ninian Field - JuxtapositionNinian Field - Juxtaposition

• Lower Jurassic marine shale, Dunlin Group, overlain by Middle Lower Jurassic marine shale, Dunlin Group, overlain by Middle Jurassic Brent Group, a prime reservoir in the areaJurassic Brent Group, a prime reservoir in the area

• The Kimmeridge Clay acts as a cap to the formation and is an The Kimmeridge Clay acts as a cap to the formation and is an excellent hydrocarbon sourceexcellent hydrocarbon source

• Several faults place the Brent Group against the older Dunlin GroupSeveral faults place the Brent Group against the older Dunlin Group

• In the horst block the Brent Group is faulted against the In the horst block the Brent Group is faulted against the Kimmeridge clayKimmeridge clay

Moab Fault - JuxtapositionMoab Fault - Juxtaposition

• Due to exposure at surface Due to exposure at surface lithologies can be defined lithologies can be defined using field and published using field and published data.data.

• The lithological descriptions The lithological descriptions can be used to model the can be used to model the geometry of the fault yielding geometry of the fault yielding juxtaposed seal analysis.juxtaposed seal analysis.

• Sand units in the hangingwall Sand units in the hangingwall and footwall are seen sealed and footwall are seen sealed due to the fault.due to the fault.

• Areas with less displacement Areas with less displacement represent leak points along represent leak points along the fault.the fault.

Moab Fault - SGRMoab Fault - SGR

• Modeling of shale gouge ratios Modeling of shale gouge ratios along the fault are consistent along the fault are consistent with field observations.with field observations.

• Greatest SGR where Greatest SGR where displacement is greatestdisplacement is greatest

• Juxtaposition Juxtaposition predicted predicted pathways in the pathways in the north remainnorth remain

• Those in the Those in the central region central region are sealed by are sealed by clay smearclay smear

Entrada Sandstone & Entrada Sandstone & CementationCementation

• Hydrocarbon bearing reducing fluidsHydrocarbon bearing reducing fluids

• Cementation occurred post faulting Cementation occurred post faulting and was coeval with but not related and was coeval with but not related to hydrocarbon migrationto hydrocarbon migration

• Calcite cementation occurred Calcite cementation occurred around faults but the faults were not around faults but the faults were not conduitsconduits

• Cements related to the faults acted Cements related to the faults acted as ephemeral seals causing fluid as ephemeral seals causing fluid pressure fluctuationspressure fluctuations

• Increased pressure due to ponding Increased pressure due to ponding of hydrocarbons caused dissolution of hydrocarbons caused dissolution of earlier calcite deposits…of earlier calcite deposits…

• ……subsequent pressure release subsequent pressure release resulted in exsolution of gaseous resulted in exsolution of gaseous CO2, forming calcit structuresCO2, forming calcit structures

Artemis Field, North SeaArtemis Field, North Sea• The Artemis field is much smaller The Artemis field is much smaller

scale than the Moab Fault regionscale than the Moab Fault region

• Accumulations of gas migrate Accumulations of gas migrate through the reservoir toward through the reservoir toward high south eastern corner…high south eastern corner…

• …….however gas also accumulates .however gas also accumulates in the footwall and hangingwall in the footwall and hangingwall of faultsof faults

• No one fault has produced a seal No one fault has produced a seal for hydrocarbons but the for hydrocarbons but the combined result of the many combined result of the many faults has created many local faults has created many local trapping geometries.trapping geometries.

ConclusionConclusion

Faults as Fluid Flow Barriers and Faults as Fluid Flow Barriers and Their Role in Trapping HydrocarbonsTheir Role in Trapping Hydrocarbons

In this presentation we have briefly shown In this presentation we have briefly shown how faults serve as fluid flow barriers by how faults serve as fluid flow barriers by forming low transmissivity membranes, forming low transmissivity membranes, and their further role in trapping and their further role in trapping hydrocarbons by juxtaposing lithologies of hydrocarbons by juxtaposing lithologies of differing permeabilities. differing permeabilities.

ReferencesReferences

• The Moab Fault, Utah, U.S.A. - A Three-Dimensional Approach to Fault Seal The Moab Fault, Utah, U.S.A. - A Three-Dimensional Approach to Fault Seal and Hydrocarbon Flow Pathway Modelling - S.M. Clarke, S.D. Burley & G.D. and Hydrocarbon Flow Pathway Modelling - S.M. Clarke, S.D. Burley & G.D. WilliamsWilliams

• The 3D fault segmentation development: A conceptual model. Implications The 3D fault segmentation development: A conceptual model. Implications on fault sealing A. BENEDICTO1, T. RIVES2 AND R. SOLIVA1- on fault sealing A. BENEDICTO1, T. RIVES2 AND R. SOLIVA1- EAGE, In : EAGE, In : Proceedings Fault and Top Seals, Extended Abstracts volume, ISBN 90-Proceedings Fault and Top Seals, Extended Abstracts volume, ISBN 90-73781-32-9, Montpellier, September 200373781-32-9, Montpellier, September 2003

• A Method for Including The Capillary Properties of Faults in Hydrocarbon A Method for Including The Capillary Properties of Faults in Hydrocarbon Migration Models O Sylta, C Childs, S Moriya, JJ Walsh, T ManzocchiMigration Models O Sylta, C Childs, S Moriya, JJ Walsh, T Manzocchi

• An Exhumed Paleo-Hydrocarbon Migration Fairway In a Faulted Carrier An Exhumed Paleo-Hydrocarbon Migration Fairway In a Faulted Carrier System, Entrada Sandstone of SE Utah, USA – Garden, Guscott, Burley, System, Entrada Sandstone of SE Utah, USA – Garden, Guscott, Burley, Foxford, Walsh and MarshallFoxford, Walsh and Marshall

• Knipe, R.J., Jones, G., and 1998 Fisher, Q.J. Faulting fault sealing and fluid Knipe, R.J., Jones, G., and 1998 Fisher, Q.J. Faulting fault sealing and fluid

flow in hydrocarbon reservoirs: An introduction. In: Faulting Fault Sealing flow in hydrocarbon reservoirs: An introduction. In: Faulting Fault Sealing and Fluid Flow in Hydrocarbon Reservoirs, edited by Jones, G., 1998 Fisher, and Fluid Flow in Hydrocarbon Reservoirs, edited by Jones, G., 1998 Fisher, Q.J andKnipe, R.J. Geological Society of London Special Publication 147, p 7-Q.J andKnipe, R.J. Geological Society of London Special Publication 147, p 7-2121