Getting to Know your Faults

An article by Stuart Buck to accompany Task's PESGB advert for June 2002.

If you would like more information, or to comment on this article, please contact Stuart at Task Geoscience.

 


Figure 1: Cataclastic fracturing associated with a major fault in the Permian Penrith Sandstone at George Gill, Vale of Eden, UK. Grid reference NY 718 188. View Map.

In the last decade, 3-D seismic techniques have completely revolutionised the structural and stratigraphic modelling of reservoirs and have led to a remarkable resurgence, world-wide, in the discovery of new fields. Some fields previously interpreted as lacking any significant structural control have for the first time been shown by 3-D surveys to contain important fault and fracture trends, such as the super-giant Ghawar Field of Saudi Arabia. However, most faulting within reservoirs occurs at a sub-seismic scale and the complexities of fault zones are often not considered. The details of these faults and fractures are best revealed by a combination of borehole images and cores. This article discusses the occurrence of fault damage zones in which cataclastic fracturing has a profound influence upon reservoir properties. Examples are given of modern techniques of borehole imaging and core analysis to characterise the importance of such fracture systems.

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Damaged rocks


Figure 2: Cataclastic fractures in the Triassic Burghead Beds, Moray, UK (NJ 126 691). Two types of fractures are evident, composite cluster of fractures. View Map.
Field observations (Figures 1 and 2) provide evidence that large scale faults in porous sandstone reservoirs are usually surrounded by damage zones in which the rocks are affected by a combination of cataclastic fractures (granulation seams and deformation bands) and small faults (slip surfaces). Each fracture and fault is a band of very low permeability relative to the main reservoir sandstones and therefore severely restricts productivity from these parts of the reservoir. The intensity of damage decreases away from large-scale faults and therefore reservoir models need to address these lateral changes in productivity. In addition, damage zones widen the influence of restricted flow approaching faults and therefore strongly increase the ability of faults to act as effective seals.

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Size that matters

The size of the damage zone adjacent to major faults has been found to be generally proportional to the throw of the fault (Knott 1994, Beach et al. 1998, Shipton & Cowie 2001). The bigger the fault, the wider and more complex the surrounding zone of cataclastic fracturing. In a recent publication (Shipton & Cowie 2001) the damage zone width is reported as approximately equal to 2.5 times the total fault throw. A typical field bounding fault of throw of 100 m will therefore be surrounded by a damage zone of approximately 250 m on either side. Clearly a substantial reservoir rock volume can be impacted by fault influence. The throw of large-scale faults is readily determined from seismic data and the presence of associated damage zones can therefore be empirically introduced to reservoir models. However, many more faults are sub-seismic in scale and can only be identified from the gathering of core and borehole image data.

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A definition of cataclasis

The process of fracturing and breaking apart of rock particles and crystal components, together with the rotation and mechanical mixing of particles. The product may be incoherent clastic materials, breccia or gouge, or form cohesive rocks: cataclasites. (Ramsey & Huber 1997).




Figure 3. Cataclastic fracturing in the Triassic Burghead Beds, Moray, Scotland (grid reference NJ 126 691). The upper photograph shows ramp (left) and eye (right) structures at the connections between segments of the individual cataclastic fractures. The lower photograph shows the complex clustering of cataclastic fractures forming zones of deformation. View Map.

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Shaping up with fractures

Individual cataclastic fractures tend to occur in segments of dm-m scale having arcuate to sinuous morphology. They link with neighbouring fractures by “eye” and “ramp” structures to form essentially linear but anastomosing trains of cataclastic fractures (Figure 3). Commonly, trains of fractures are closely clustered and merge to make a composite band of greater linearity. In many cases, these are bound on one side by a slip surface forming a distinct small fault (Antonellini & Aydin 1994, 1995).

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What’s in the name?

Cataclastic fractures, like most geological features, have been described under a variety of names and this will probably continue for the foreseeable future due to local preferences in terminology within the scientific and engineering communities.


Figure 4. Cataclastic microfault in sandstones. The fault is characterised by minor bedding offset (of a few cm). The fault is complex in nature with splays and anastomosing structure. Clair Field, UKCS.
Figure 5. Cataclastic granulation seams in sandstones. These fractures have no inherent resistivity contrast with the host rock. Clair Field, UKCS.

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Crushing times

Microscopic studies of cataclastic fractures in sandstones show that the detrital grains and any crystalline cements are subjected to rotation, fracturing and crushing resulting in the mechanical mixing of particles along the fracture (Figure 6).


Figure 6: Photomosaic generated from large format thin section showing variations in porosity (blue) between the host sandstone and the deformation band associated with a cataclastic fracture. (From Conybeare et al 1996).

Within single cataclastic fractures of mm scale and no appreciable displacement, many of the grains remain as coherent particles, but show rotation and alignment associated with tighter packing and reduction of porosity and permeability. However, in some cases a zone of fabric dilation occurs immediately adjacent to the fracture resulting in enhanced porosity and permeability (Figure 7) (Fowles & Burley 1994).

Where mm-scale displacements are apparent, many grains have been subject to crushing and fabric reorganisation resulting in visible particle size reduction, greater grain packing, loss of visible porosity and greatly reduced permeability. When ductile grains are present, such as micas and mudstone clasts, these are severely affected becoming comminuted within the matrix.

Clustered cataclastic fractures show the collective aggregation of the crushed fabrics of individual fractures as if each fracture once formed is strain hardened and that subsequent failure forms a new fracture adjacent to the earlier formed fractures. Aydin and Johnson (1978) suggested that each fracture is a discrete element and that the collective zone of fractures has a fabric no more severely crushed than each individual fracture.

Minor faults are characterised by slickenside surfaces indicating significant shearing along that surface.


Figure 7. Permeability imaging of cataclastic fractures in sandstone core: a) core photograph showing bedding dipping to RHS and fracture to LHS, b) permeability image showing reduced permeability along fracture and enhanced permeability in adjacent sandstones. Bruce Field, UKCS. (From Conybeare et al. 1996).

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Cementing the problem

Diagenesis, either coeval with, or post-dating faulting, appears to have been favoured by the permeability barrier presented by the fractures such that cements are more commonly developed within and adjacent to the zone of cataclasis and thereby aggravating the reservoir problems posed by the fractures.

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Permeability - No flow like slow flow

Cataclastic fractures commonly result in a localised porosity reduction by an order of magnitude and permeability reduction by one-four orders of magnitude (Antonellini & Aydin 1994, 1995). Associated small faults show almost zero porosity and greater than seven orders of magnitude reduction in permeability, but permeability parallel to a slip plane can be enhanced (dilation?). (Antonellini & Aydin 1994, 1995). A 2.5-3.5 order of magnitude permeability reduction was determined by experiment (Crawford, 1998).

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Going places

The dominant strike orientation of cataclastic fracturing is sub-parallel to the associated major fault. In vertical section the fractures often divide into two conjugate sets. A major set that is antithetic the main fault and generally includes most composite fracture bands and small faults, and a subordinate set which is parallel to the major faults and includes fewer composite bands and small faults. Where two composite bands occur in close proximity, the intervening rock is intensely fractured by antithetically oriented fractures running between composite bands (Antonellini & Aydin 1994, 1995). Cross-cutting relationships between fractures and displacement of older fractures by younger fractures indicates that fracturing occurred incrementally over an extended period.


Figure 8. Composite bands of clustered cataclastic fractures connected by an antithetic subordinate set of arcuate discontinuous cataclastic fractures. Triassic Burghead Sandstone, Hopeman, Moray, Scotland (grid reference NJ 126 691). View Map

Figure 9. Cataclastic fractures in a cross-bedded aeolian sandstone from the Permian Penrith Sandstone. Normal displacements of mm-scale are evident from both foreset laminae and older fractures cut by fractures. George Gill, Vale of Eden, England (grid reference NY 718 188). View Map.

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Rocks between fractures

Between cataclastic fractures the host rock commonly appears un-deformed and may show excellent preservation of sedimentary fabrics such as cross-bedding or delicate trace fossils. However porosity measurements of these host rocks show a statistically lower porosity and permeability compared to similar host rocks distant to the deformation zone (Aydin and Johnson 1978, Mair 1997). Porosity loss has probably been caused by enhanced compaction and the re-organisation of grains by rotation. Some evidence of enhanced grain point contact solution and stylolitisation is seen in these rocks.

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Dividing the damage zone - slow recovery

The net effect of cataclastic fracturing around major faults is to divide the reservoir into small polygonal reservoir compartments bound by partial fault seals. Although a large amount of hydrocarbons may reside within fault damage zone, the successful extraction of that oil is fraught with problems. Well planning therefore becomes of uppermost importance in calculating and designing the methods for the recovery of oil from fault damage zones.

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Characterising reservoir damage zones


Figure 10. Contoured poles to planes (upper hemisphere) and strike histogram for raw cataclastic fracture data (uncorrected for borehole bias) gathered in an inclined (50 degrees to vertical) borehole drilled towards the south-west.

Figure 11. Fracture data corrected for borehole bias. The corrected data shows that fractures dipping to the north-east are more important than those dipping to the south west (the converse to that shown by the raw data-set above). In addition, an east-west striking fracture population has become prominent and the overall fracture population has increased in number.

Cataclastic fractures in reservoir sandstones are commonly recovered in cores and identified in borehole images and these provide the best method for the characterisation of fault damage zones. These data, especially when combined, enable determination of the abundance, orientation and spacing of fractures, although care must be taken in evaluating fracture sampling to correct for any bias introduced by the borehole orientation (Figure 10) (Terzaghi 1965, Priest 1993).

The reduction of porosity and permeability associated with cataclasis and later diagenetic cementation results in fractures being well preserved in cores and well imaged by both resistivity and acoustic borehole imaging tools. The high resolution of the resistivity borehole image tools (Figures 4 and 5), especially those designed for water-based mud systems, provides the best images for identifying fractures. In most reservoirs the quality of the borehole images is directly governed by the borehole conditions, such that poor results may be experienced in intervals of excessive rugosity, washout or mudcake build-up. Where resistivity or acoustic contrast is poor between the host rock and fracture, or when the fractures are extremely thin, then the fractures may not be resolved by borehole imaging tools. Calibration of the analysis of borehole images against core is always recommended as providing a bench mark for the quantification of fracture studies from borehole images in uncored intervals.

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Acknowledgements

We gratefully acknowledge permission to present image data from the Clair (BP, Amerada Hess, Conoco, ChevronTexaco and Enterprise Oil) and Bruce Fields (BP).

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References - Selected Bibliography

Adams, J.T. & Dart, C.J., 1999. Potential sealing faults on borehole images. In: Jones, G., Fisher, Q.J. & Knipe, R.J. Faulting, Fault Sealing and Fluid Flow in Hydrocarbon Reservoirs. Geological Society London Special Publication, 147, 71-86.

Aydin, A. & Johnson, 1983. Analysis of faulting on porous sandstones. Journal of Structural Geology, 5, 19-31.

Antonellini, M.A. and Aydin, A., 1994. Effect of faulting on fluid flow in porous sandstones: petrophysical properties. Bull. American Association of Petroleum Geologists, 78, 355-377.

Antonelli, M.A. and Aydin, A., 1995. Effect of faulting on fluid flow in porous sandstones: geometry and spatial distribution. Bull. American Association of Petroleum Geologists, 79, 642-671.

Beach, A., Welbon, A.I., Brockbank, P.J. and McCallum, J.E., 1999. Reservoir damage around faults: outcrop examples from the Suez rift. Petroleum Geoscience, 5, 109-116.

Burhannudinnur, M. & Morley, C.C., 1997. Anatomy of growth fault zones in poorly lithified sandstones and shales: implications for reservoir studies and seismic interpretation: part 1, outcrop study. Petroleum Geoscience, 3, 211-224.

Conybeare, D.M., Buck, S., Tribe, I., McKeever, M.E., Aplin, G.F., Walker, D., Dixon, R. & Buck, S. (1996). Microstructure and permeability variations associated with deformation bands in North Sea Jurassic sandstone reservoirs. Conference Abstract - Faulting, Fault Sealing and Fluid Flow in Hydrocarbon Reservoirs, Leeds University.

Crawford, B.R. 1998. Experimental Fault sealing: shear band permeability dependency on fault gouge characteristics. In: Coward, M.P., Daltaban, T.S. and Johnson, H. (eds), Structural Geology in Reservoir Characterisation. Geological Society London Special Publication, 127, 27-47.

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Edwards, H.E., Becker, A.D. & Howell, J.A., 1994. Compartmentalisation of an aeolian sandstone by structural heterogeneities: Permo-Triassic Hopeman Sandstone, Moray Firth, Scotland. In: North, C.P. & Prosser, D.J. (eds). Characterisation of fluvial and aeolian reservoirs. Geological Society, London, Special Publication, 73, 339-365.

Fowles, J. & Burley, S., 1994. Textural and permeability characteristics of faulted, high porosity sandstones. Marine and Petroleum Geology, 11, 608-623.

Gibson, R.G., 1998. Physical character and fluid flow properties of sandstone-derived fault zones. In: Coward, M.P., Daltaban, T.S. and Johnson, H. (eds), Structural Geology in Reservoir Characterisation. Geological Society Special Publication, 127, 83-97.

Jamison, W.R. & Stearns, D.W., 1982. Tectonic deformation of Wingate Sandstone, Colorado National Monument. Bull. American Association Petroleum Geologists, 66, 2584-2608.

Knott, S.D. 1994. Fault zone thickness versus displacement in the Permo-Triassic sandstones of NW England. Journ. Geological Society London, 151, 17-25.

Knott S.D., Beach, A., Brockbank, P.J., Brown, J.L., McCallum, J.E. and Welbon, A.I., 1996. Spatial and mechanical controls on normal fault populations. Journal of Structural Geology 18, 359-372.

Mair, K., Main, I. and Elphick, S., 2000. Sequential growth of deformation bands in the laboratory. Journal of Structural Geology 22, 25-42.

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Manzocchi, T., Ringrose, P.S. and Underhill, J.R. 1998. Flow through fault systems in high-porosity sandstones. In: Coward, M.P., Daltaban, T.S. and Johnson, H. (eds), Structural Geology in Reservoir Characterisation. Geological Society Special Publication, 127, 65-82.

Pittman, E.D., 1981. Effect of fault-related granulation on porosity and permeability of quartz sandstones, Simpson Group (Ordivician), Oklahoma. Bull. American Association Petroleum Geologists, 65, 2381-2387.

Preist, S.D. 1993. Discontinuity Analysis for Rock Engineering. Chapman & Hall, London, 473 pp.

Ramsey, J.G. & Huber, M.I., 1997. The Techniques of Modern Structural Geology. Volume 2: Folds and Fractures. Academic Press, London, 700 pp.

Shipton, Z.K. and Cowie, P.A., 2001. Damage zone and slip face evolution over um to km scales in high-porosity Navajo Sandstone, Utah. Journal of Structural Geology, 23, 1825-1844.

Terzaghi, R.D., 1965. Sources of error in joint surveys. Geotechnique, 15, 287-304.

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© Task Geoscience 2002