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

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

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

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

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

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

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

Sealing Mechanisms: Mica Orientation Processes Shearing Fluid flow

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

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

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

Shale Gouge Example

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.

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.

Clay Smearing on Microscopic Scale

Effectiveness of Fault Sealing Mechanisms

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: 30-70 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: 0.2 - 0.4 g/cm3) g = acceleration due to gravity

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

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

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

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)

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)

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

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

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

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 - 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

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.

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

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

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.

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.

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 90-73781-32-9, 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