1. Faults as Fluid Flow Barriers and Their Role in Trapping Hydrocarbons
Suzanne Coogan
Richard Nice
Ayeni Gboyega
Kate Carter-Walford

2. Introduction Fault seal mechanisms Influence on hydrocarbon fields
Modelling and Flow Properties
Case Studies

3. How Can Faults Become Barriers to Fluid Flow?
Juxtaposition of lithologies with different permeabilities across the fault
Smearing of impermeable/semi-impermeable fault rocks in the fault zones
Cataclastic grain-size reduction results from abrasion during deformation; smaller grains fill pore space and reduce porosity
Cementation of damage zone by precipitation of solutes
A complex fault zone will exhibit varying transmissibility values in three dimensions and these will change with displacement through time

4. Juxtaposition
Need definnation
Juxtaposition of lithologies with different permeabilities across the fault

5. Juxtaposition Coarse grained material (light colour)
Analogous to a reservoir being juxtaposed against a sealing lithology
Fine grained material (dark colour)

6. Cataclasis
Cataclastic grain-size reduction results from abrasion during deformation; smaller grains fill pore space and reduce porosity
Effectiveness depends on:
The hardness of the intact rock
The magnitude of displacement
Initial normal stress on the fault surface prior to movement

7. Grain Size and Sorting Reduction
Feldspar Alteration
Clay Mineral Formation
Cataclasis
Shale smearing

8. Sealing Mechanisms: Mica Orientation
Processes
Shearing
Fluid flow

9. Cementation Minerals carried in solution in water under high pressure
As fault opens, pressure release occurs, water flows through fault and minerals precipitate out of solution
Crystallisation of cements in pore spaces reduces permeability
Cement
Reduced grain size

10. Clay 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

11. Shale 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

12. Shale Gouge Example

13. Shale Gouge Ratio
As 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
Shale forms an extremely effective capillary seal and permeability barrier due to the every small size of the pore throats and pore spaces.
A high SGR value for a particular region of a fault would indicate that rock units with a high percentage of shale have slipped past that part of the fault. It has been determined that the onset of static seal over geological time occurs when the fault gouge reaches 18 – 20 % SGR.
The pressure drop of the (hydrocarbon column height) that the fault gouge can maintain increases as the calculated clay content of the fault increases.

14. Examples 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
The left-most fault displaces Tertiary sandstone and the gouge consists solely of sand.  The center fault displaces Jurassic shale and limestone and the gouge consists of both shale and limestone.  The right-most fault displaces a Jurassic limestone-shale sequence and the gouge consists of a 1 m thick layer of shale.  Only small amounts of the more brittle limestones are incorporated in the gouge.

15. Clay Smearing on Microscopic Scale

16. Effectiveness of Fault Sealing Mechanisms

17. Sealing Capacity of Faults
Hd = 2gh(rt-1-rp-1)/g(rw-rh)
rt = pore throat radius in the seal
rp = pore throat radius in the reservoir
gh = hydrocarbon-formation water interface tension
(Oil: 5-35 dynes/cm Gas: dynes/cm).
rw = density of the formation water (1 – 1.2 gm/cm3)
rh = density of the hydrocarbon phase (Oil: 0.5 –1.0 gm/cm3 Gas: g/cm3)
g = acceleration due to gravity

18. Modelling
Empirical methods for risking the sealing potential of faults have been devised in combination with outcrop and laboratory studies
Estimates the sealing potential of a fault offsetting a particular sequence and therefore the entry pressure
For a detailed model
identify where a fault is sealed and where leakage may occur
establish where significant pressure differences are likely to be
supported across a fault surface and their magnitude
understanding of the migration pathway and the column height
Can use models to predict the flow and flow restrictions of hydrocarbons due to fault properties using a programs such as the SEMI migration model or TransGen
Such modelling can only be achieved by 3-D analysis of the geometry and sealing characteristics of faults

19. Calculating fault properties
Calculate the Shale Gauge Ratio on the fault surface and convert to fault seal potential
Calculate the reservoir elevation at fault traces
Construct a sequence/throw juxtaposition diagram form log and lithological information
Input sequence containing shale values and reservoir offset
In this example, throw is between zero and thickness of sequence - triangular plot
Plot is annotated according to SGR

20. What Controls Seal Effectiveness and Fluid Transmissibility
Hydrostatic / Capillary
Buoyancy control
Hydrodynamic / Capillary:
Fluid pressure gradient control
Hydrodynamic / Open Fractures
Network Geometry, aperture and pressure gradient control h
Leak

21. Column Heights
Two distinct geometries have to be considered when estimating oil column heights
When fault throw is less than the thickness of the reservoir and the reservoir is self juxtaposed. Column height is determined by the threshold pressure
When the fault throw exceeds the thickness of the carrier interval, oil leakage occurs along the fault. Leakage can occur along fault surface if fault is not sealing
Column Height is determined by the depth (Z), density of oil and water (po, pw) and gravity (g)

22. Potential Oil Column Heights
1000
Oil column heights
100
Shale smear / cementation
Cataclasites 10
Fine sandstone 1
Coarse sandstone .1 -6 -5 -4 -3 -2
Pore throat radius (log cm)

23. Migration
Migration is the movement of hydrocarbons though rock pores and fault networks
Migration is driven by buoyancy and resisted by capillary pressure
Leakage occurs when the entry pressure (Pe) equals the pressure of oil and water (Po, Pw) Po Po Pw

24. Flow Model Predictions
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

25. Applications of Modelling Fault Sealing
Using different fault sealing properties to perform migration modelling
In this example the following conditions exist
reservoir contains known hydrocarbon accumulations
different hydrocarbon-water contact levels in adjacent fault blocks
a dry fault-bounded structural high exists
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

26. 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

27. Ninian Field - Juxtaposition
Lower Jurassic marine shale, Dunlin Group, overlain by Middle Jurassic Brent Group, a prime reservoir in the area
The Kimmeridge Clay acts as a cap to the formation and is an excellent hydrocarbon source
Several faults place the Brent Group against the older Dunlin Group
In the horst block the Brent Group is faulted against the Kimmeridge clay

28. Moab Fault - Juxtaposition
Due to exposure at surface lithologies can be defined using field and published data.
The lithological descriptions can be used to model the geometry of the fault yielding juxtaposed seal analysis.
Sand units in the hangingwall and footwall are seen sealed due to the fault.
Areas with less displacement represent leak points along the fault.

29. Moab Fault - SGR
Modeling of shale gouge ratios along the fault are consistent with field observations.
Greatest SGR where displacement is greatest
Juxtaposition predicted pathways in the north remain
Those in the central region are sealed by clay smear

30. Entrada Sandstone & Cementation
Hydrocarbon bearing reducing fluids
Cementation occurred post faulting and was coeval with but not related to hydrocarbon migration
Calcite cementation occurred around faults but the faults were not conduits
Cements related to the faults acted as ephemeral seals causing fluid pressure fluctuations
Increased pressure due to ponding of hydrocarbons caused dissolution of earlier calcite deposits…
…subsequent pressure release resulted in exsolution of gaseous CO2, forming calcit structures

31. Artemis Field, North Sea
The Artemis field is much smaller scale than the Moab Fault region
Accumulations of gas migrate through the reservoir toward high south eastern corner…
….however gas also accumulates in the footwall and hangingwall of faults
No one fault has produced a seal for hydrocarbons but the combined result of the many faults has created many local trapping geometries.

32. Conclusion
Faults as Fluid Flow Barriers and Their Role in Trapping Hydrocarbons
In this presentation we have briefly shown how faults serve as fluid flow barriers by forming low transmissivity membranes, and their further role in trapping hydrocarbons by juxtaposing lithologies of differing permeabilities.

33. References
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. Williams
The 3D fault segmentation development: A conceptual model. Implications on fault sealing A. BENEDICTO1, T. RIVES2 AND R. SOLIVA1- EAGE, In : Proceedings Fault and Top Seals, Extended Abstracts volume, ISBN , Montpellier, September 2003
A Method for Including The Capillary Properties of Faults in Hydrocarbon Migration Models O Sylta, C Childs, S Moriya, JJ Walsh, T Manzocchi
An Exhumed Paleo-Hydrocarbon Migration Fairway In a Faulted Carrier System, Entrada Sandstone of SE Utah, USA – Garden, Guscott, Burley, Foxford, Walsh and Marshall
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 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 7-21