Anatomy of growth fault zones in poorly lithified sandstones and shales: implications for reservoir studies and seismic interpretation: part 1, outcrop study M. Burhannudinnur 1,2 and C. K. Morley 1 1 Department of Petroleum Geosciences, University of Brunei Darussalam Bandar Seri Begawan, 2028, Negara Brunei Darussalam 2 Present address: Jurusan Teknki Geologi, Fakultas Teknologi Mineral, Universitas Trisakti, Jl. Kyai Tapa No 1, Grogol, Jakarta Barat, Indonesia ABSTRACT: Some normal faults developed in poorly lithified sediments in Miocene Pliocene deposits of NW Borneo in the vicinity of Brunei display regular zonations of deformation bands. For fault displacements of a few metres the zones of deformation bands extend up to about 10 m into both the hanging wall and footwall. They range from closely spaced anastomosing seams within or adjacent to the main slip planes, to more widely spaced sub-parallel and parallel seams passing away from the fault zone. They reduce porosity and permeability, and if the faults are closely spaced, are likely to impact reservoir production characteristics and reserve estimates. In cross-section and map view fault zones are commonly composed of several important gouge and cataclasis zones, which branch and join, display listric detachments and various types of hard and soft linkage. Some of these geometries have been described as common characteristics of faults, others are comparitively rare. They have significant implications for the interpretation of seismic data. KEYWORDS: sealing faults, deformation bands, cataclasis, porosity, permeability INTRODUCTION Recently it has been recognized that shale smears or sheared zones and cataclasis zones play an important role in making fault zones act as seals to hydrocarbon migration and entrapment (Weber et al. 1978; Bouvier et al. 1989; Gibson 1994; Berg & Avery 1995). Despite the generally poor exposure of growth faults and poor recovery of fault zones from cores (Berg & Avery, 1995) details about the complexity of fault zones have begun to emerge. In particular the study of faults from the Arches National Park by Antonellini & Aydin (1994) demonstrated the reduction of both porosity and permeability due to deformation bands in broad areas around fault zones. This study describes the details of growth fault geometries in outcrop from the Baram delta province of NW Borneo, centered around Brunei Darussalem (Fig. 1). The significance of the growth fault geometry described from outcrop is discussed with respect to reservoir modelling and seismic interpretation in Part 2 (Morley & Burhannudinnur 1997). Seismic studies provide much better information on the entire fault system than outcrop studies but cannot resolve the detailed geometry of a fault zone. Both types of studies can be used together to build a more complete picture of growth fault geometry and evolution. GEOLOGICAL SETTING Faults were examined in a number of outcrops of the Miri Formation, which is found near the coast of Brunei Darussalem from the Jerudong area in Brunei Darussalam to the Lambir Hill area south of Miri in neighbouring Sarawak (Fig. 1). In Petroleum Geoscience, Vol. 3 1997, pp. 211 224 particular the Jerudong area contains numerous exposures of normal faults. The formation is a sequence of sandstones and shales which, based on benthonic foraminifera assemblages (Liechti 1960; Wilford 1960), were deposited in a marine environment during the late Miocene Pliocene. The Miri Formation was deposited in the Baram province or Baram Belait depocenter. This depocenter, of middle Miocene to Present day age, formed after south southeastward oblique subduction of the China Sea Plate under Borneo ceased ( James 1984). Magnetic and gravity surveys indicate up to 15 km subsidence has occurred in the depocenter (James 1984). It was gravitational instability of the young, thick deltaic sediments, overlying a thick, mobile, overpressured marine shale substratum that caused the development of the growth faults examined in this study. The following descriptions are of growth fault zones developed in a deltaic setting. Fault geometries in outcrop A number of studies have described deformation band zonation in faulted sandstones (e.g. Aydin 1978; Aydin & Johnson 1978, 1983; Jamison & Stearns 1982). The term deformation band (Aydin 1978) is used to describe small planar features, about 1 mm thick that accommodate small offsets with displacements ranging up to a few centimetres. Recently, detailed studies have shown that deformation band development consists of initial dilatancy followed by compaction and crushing of grains (Antonellini et al. 1994). Variations in porosity and confining pressure affect whether the seams are dilatant with no cataclasis or undergo compaction with cataclasis (Antonellini et al. 1994; Antonellini & Pollard 1995). 1354-0793/97/$10.00 1997 EAGE/Geological Society, London
212 M. Burhannudinnur and C. K. Morley Fig. 1. Location map of Brunei within Borneo showing the extent of the Baram delta province, and the main areas where the Miri Formation is present in outcrop (based on James 1984). The exact conditions of lithification of the Miri Formation at the time of deformation are unknown, but cores from wells and many outcrops are poorly lithified at present, hence the same is likely to be true for the Miri Formation in the past. The high pore fluid pressures and low degree of lithification were probably conducive for granular flow as well as cataclastic flow during extension. Most of the outcrops that contain growth faults display some degree of diffuse zones of deformation bands extending up to several metres beyond the main fault zone. In some areas remarkably regular arrays of seams of different intensity could be distinguished, that lay sub-parallel to the fault zone. Such zones can be many metres wide for faults with only a few metres displacement. Commonly the zones are characterized by complex linkage of multiple intense cataclasis bands, fault gouge zones and shale smears set in a zone of less intense deformation bands. Where the faults displayed regular zonation of deformation bands three main types could be identified and mapped in both the hanging wall and the footwall (Fig. 2a, b, c). Their characteristics are as follows (Fig. 3): (1) Parallel seams. These occur in the most external portion of the fault zone. They are characterized by light coloured, parallel deformation bands 2 40 cm long, 0.1 0.5 cm wide and spaced 5 60 cm apart. They represent very low strain deformation and throw on individual seams is generally less than 1 mm. The belt of parallel deformation bands is up to 10 m wide in map view, for faults with throws of several metres (Fig. 2). Seams in some areas exhibit dilatant behaviour since, in comparison to the surrounding rock, they are preferentially stained and filled by late forming iron oxide cements. Such examples are relatively rare. In most cases the opposite is found, the seams stand out as light coloured bands, and the surrounding rock is stained by iron oxides. This indicates that the seams are compactional and acted as barriers to fluid migration. These visual observations are confirmed by examination of porosity changes in thin section. The two types of behaviour occur in adjacent fault zones, in similar sandstones. Hence subtle variations in strain rather than variations in porosity or confining pressure (e.g. Antonellini et al. 1994) are probably responsible for the differences. (2) Sub-parallel seams occur between, and are transitional to, the anastomosing zones and parallel zones. They are characterized by light coloured sub-parallel deformation bands 2 60 cm long, 0.1 0.5 cm wide and spaced 2 20 cm apart. The sub-parallel seams are generally parallel with some oblique low-angle and wavy connections. Displacement on individual seams is about 1 mm. In map view, for fault displacements of several metres, the belt of sub-parallel deformation bands is up to 4 m wide. (3) Anastomosing seams occur in the area nearest to or within the narrow zones of largest displacement. They are characterized by either black or white anastomosing seams. In both cases the seams are closely spaced, 0.1 0.8 cm wide and individually range in length from about 2 cm to 1 m, however they link to form systems almost as long as the fault length. For fault displacements of several metres the belt of anastomosing deformation bands are up to 75 cm wide. In detail anastomosing seams can be divided into two types. Type 1 represents the highest intensity deformation. They are usually dark grey to black in colour and form in the principal displacement zones of the fault zone. Consequently they are commonly associated with shale smears. The black colour is primarily due to fine grained cataclastic material and smeared shale, not iron oxide staining. They form zones 1 cm to 15 cm wide in map view. Type 2 are characterized by light coloured seams that in places are coloured dark red or black due to precipitation of iron oxides. They are spaced 0.5 mm to
Growth fault zones, outcrop study 213 Fig. 2a.
214 M. Burhannudinnur and C. K. Morley Fig. 2b. Fig. 2. Location cross-section and map view examples of fault zones in small normal faults (4 5 m throw) illustrating the zonation of different types of deformation band or cataclasis seam around the fault zone.
Growth fault zones, outcrop study 215 Fig. 3. Schematic sketches of the main types of deformation band geometry in map view based upon map view exposures of fault zones (see Fig. 2 above). 5 cm apart and displacement on individual seams is generally around 1 5 mm. They represent relatively high strain zones, but lower strain than the type 1 zones. Different types of lateral variation of fault geometry were described based on the pattern of principal shear planes in map view (Fig. 2). The lateral variations of fault geometry illustrate how faults develop; link and die out. Six types were recognized in the study area (Fig. 4). (1) Relay structures, this type of transfer zone can be located anywhere within a fault zone, as one set of type 1 anastomosing seams is offset from another set, and displacement is transferred between the two sets. (2) Horses are usually defined by narrow type 1 anastomosing cataclastic bands or fault gouge. (3) Splays are most frequently found at the termination of a fault or at a transfer zone within the fault zone. The main fault branches into a number of smaller faults. (4) Linking cross-faults form another type of transfer zone involving oblique slip on a system of minor faults that lie between two major displacement strands of the fault zone, but do not extend beyond the two strands. The major strands transfer displacement to one another via these small faults. (5) Conjugate fault zones are regions where numerous minor conjugate minor faults, fractures and deformation bands are present. The two sets of faults not only display different dip directions, but also intersect in strike view at an acute angle. The spacing of minor cross-cutting faults is around 2 30 cm. Secondary minerals, in particular iron oxides, commonly preferentially stain the cross-cutting fault zones (but not adjacent zones), suggesting they are regions of high porosity and permeability. The conjugate fault zones tend to form between splays in type 1 anastomosing seams or fault gouge zones near the fault tip, in particular they are usually associated with loss of displacement on a fault in sandstone. Despite normal Fig. 4. Summary cartoons of the main map view variations in fault geometry observed from faults in the Miri Formation. These linking or cross-cutting features can be seen in Fig. 2. Mostly they enable transfer of displacement from one fault gouge zone or anastamosing cataclasis zone to another, or they occur at the tips of these zones where the fault zone dies out. dip-slip indicators in nearby areas, the conjugate fault zones display horizontal or low-angled oblique slickenslides. This suggest material is being moved laterally away from the fault zone at the unconstrained margin of the fault. (6) Fault gouge and type 1 anastomosing zones are commonly offset by minor oblique faults. Such faults extend beyond the boundaries of the faults they offset and may be due to independent antithetic faults or second order tear faults within the fault zone. CROSS-SECTION CHARACTERISTIC OF GROWTH FAULTS IN OUTCROP Faults in cross-sectional view are commonly developed as zones, not discrete planes. One control on the width of the fault zone appears to be lithology. A fault zone is composed of bundles of high-strain cataclasis zones, none of which extends throughout the width of the fault. The high strain zones pass into others laterally and vertically via a variety of transfer zone types which are usually characterized by a high intensity of cataclastic deformation; cross-cutting minor faults and small-scale pull-apart structures. The fault Hn-A is a small growth fault that has up to 4 m displacement in syn-tectonic sediments. The sandstone was not drag folded, but shales as interbeds or lenses in the sandstone were strongly drag folded and incorporated within the main cataclasis zones as shale smears (Fig. 5). The parallel and sub-parallel cataclasis zones that are clearly visible in map view are harder to observe in cross-section, particularly in sections through interbedded sandstones and shales. Anastomosing seams are the easiest to distinguish, they merge with, and lay
216 M. Burhannudinnur and C. K. Morley Fig. 5. Cross-section of fault Hn-A from outcrop. This fault zone displays many typical features of growth faults in outcrop from the Baram Delta province. There is expansion of the hanging wall section into the fault, suggesting it was a growth fault. The fault plane is kinked, in places it forms a single fault zone, but in other places it is composed of a number of faults, towards the top it splays, and towards the bottom are small displacement high-angle listric faults. Along the fault zone there are numerous examples of shale drag and sandstone lenses intensely deformed by cataclasis seams. sub-parallel to the main fault gouge zone. Where clearly visible the parallel and sub-parallel deformation band zones described from map view vary depending upon whether they are located in the foot- or hanging-walls (Fig. 6). In the hanging wall the parallel seams tend to have high-angle (70 45 ) dips, antithetic to the main fault zone, though some seams may have synthetic dips. The sub-parallel seams tend to be a mixture of high angled synthetic and antithetic dips, with synthetic dips becoming more dominant approaching the fault zone. In the footwall a similar pattern is produced except that the dominant parallel cataclasis dip direction is synthetic to the major fault zone. This pattern of cataclasis orientation should produce a predictable pattern in cores, and indicate the location of important fault zones, even if they are zones of poor recovery (Fig. 6). The parallel seams in particular seem to be forming not directly in response to the shear within the fault zone, but as a
Growth fault zones, outcrop study 217 (a) (b) Fig. 6. (a) Schematic block diagram illustrating the type of deformation band and fault gouge geometries found around fault zones in the Miri Formation. PC, parallel cataclasis; SPC, subparallel cataclasis. (b) Schematic section illustrating the likely arrangement of structures in a core through a typical fault zone. Fig. 7. Location of deformation bands in relation to footwall and hanging wall deformation associated with normal faults. Note that the footwall area is involved in the deformation of the fault zone. The cataclasis is unlikely to be due to hanging wall deformation of a separate fault which lies in the footwall of the illustrated fault because the seams dip in the opposite direction to those in the hanging wall. response to strain in the hanging wall and footwall that is a consequence of faulting. In modelling fault geometries using the Chevron or modified Chevron methods (Williams & Vann 1987) antithetic simple shear of the hanging wall commonly works well, particularly for growth faults (Xiao & Suppe 1992). The orientation of the parallel deformation bands suggests they developed as a manifestation of antithetic simple shear in the hanging wall (Fig. 7). The variations in cross-sectional fault zone geometry in the study area can be simplified to 11 idealized characteristics (Fig. 8). A fault zone may consist of a single type or more commonly a combination. Kinks or ramps in the fault plane may reflect lithology changes that influenced the initial fault trajectories, the linkage of two initially separate faults, or may be due to to folding of the fault by activity on an antithetic fault in the footwall. Downwards-broadening zones and splays are associated with the development of synthetic faults (Fig. 9). They are most frequently found at lithological boundaries where passing from a shale into a sandstone the fault splays downwards. Pull-apart transfer zones (Fig. 9b) are common in
218 M. Burhannudinnur and C. K. Morley the fault where sandstone and shale beds have been dragged into the fault zone (Weber et al. 1978; Bouvier et al. 1989; Berg & Avery 1995). The sandstones are broken up by numerous minor faults and bedding may be rotated parallel to the fault plane. They are similar to shale smears except the behaviour of the rock was more brittle and bedding can still be observed. Well developed listric detachments are associated with footwall sheared zones. The width of observed sheared zones varies from 10 to 150 cm. The listric faults commonly detach within a fine grain unit or at a bedding plane. Listric detachment faults can also form in the hanging wall (Fig. 10). These enable one fault strand to die out and be replaced by a different strand at depth. Fig. 8. Schematic characteristics of fault zone geometries in cross-section based upon outcrop examples. The evidence for such geometries on seismic reflection profiles is discussed in Morley & Burhannudinnur (1997). cross-sectional view, but have only been recognized in sandstones. The zone between the fault strands consists of cataclasis bands and fault breccias. Faults will die out over a certain length, they do not persist along-strike indefinitely. The termination of a fault is indicated by zero displacement. Displacement usually dies out gradually from a centrally located maximum (Barnett et al. 1987). Two types of fault termination are recognized in the study area. First is a gradual decrease in displacement upwards and laterally, and in some cases downwards, into deformation bands or a number of small faults (Figs 9 & 10). This geometry has only been clearly observed in sandstones. In such cases faults with narrow zones of intense deformation change gradually to broader zones of less intense deformation. They branch into a number of smaller faults with decreasing displacement until the density of small faults decreases and the fault zone dies out. In many cases the small faults disappear abruptly at a lithological boundary, particularly between sandstones and shales. In Fig. 10 a low-angle fault exists below the minor faults and they are confined to the hanging wall of the fault. In another type of termination a fault in the footwall of a fault of opposite dip may end abruptly at the fault plane, with no offset continuation of the fault visible in the hanging wall. Four of the 11 types (Fig. 8) occur relatively infrequently, these are the three listric detachment styles and horses (Fig. 10). All are best developed in interbedded sandstones and shales. The term sheared zone is used to describe localized areas along Height displacement relationships The outcrops are not continuous enough to expose the complete strike-length of major faults. However smaller faults in cross-section can be seen to die out both upwards and downwards. Hence it is possible to examine the height (H) displacement (D) relationships. Values of displacement plotted against height for minor faults are shown in Fig. 11. The relationship is based on measurements of 182 faults and cataclastic seams around five main faults. The minimum measurement is 3 cm in height and 0.05 cm of displacement. The maximum measurement is 1800 cm in height with 30 cm displacement. Generally, the fault displacement increases as the fault height increases. Distribution of the data is excellent for determining the best fit line. The best line of the data can be done by linear or power examination line. The linear examination resulted in D=0.0173 H with a regression ratio (R 2 )of 0.813. The power examination resulted in D=0.0133 H 1.007 with a best fit ratio (R 2 ) of 0.83. By comparing the ratios it can be shown that the power examination is statistically more reasonable than the linear one. Nevertheless, both values are generally reasonable. The smallest sheared zones measured were individual, isolated cataclastic seams. Hence the deformation bands appear to exhibit similar displacement height characteristics to the minor faults. DEFORMATION CHARACTERISTICS OF BROAD FAULT ZONES To examine the strain associated with cataclasis and its effects on reservoir properties a single sandstone unit was sampled at intervals in both the hanging- and footwalls of a fault (Fig. 12). The changes in average grain size, porosity and percentage of fine grained material were examined within the deformation bands and in the surrounding sandstone. The sandstone is a poorly lithified, medium to fine grained, clean sand. The sub angular to sub rounded grains range in size between 0.02 and 0.5 mm, the average grain size is around 0.25 mm. It is composed of 65 75% quartz grains, up to 0.5% opaque minerals, 0 0.5% sedimentary rock clasts, up to 2% unidentified minerals, and the average porosity in undeformed samples is about 27%. Clay minerals are rare and iron oxides fill 1 3% of the pores spaces. Cataclasis zones In thin section deformation bands formed by cataclasis in sandstones are generally characterized by bands of reduced grain size and porosity, which visually can be determined by a lighter colour than the surrounding rock (Fig. 13). Reductions
Growth fault zones, outcrop study 219 Fig. 9. Section illustrating faults which splay or broaden downwards. (a) Fault Hn-I, note the significant offset of the paired sandstone layers by splaying faults that are almost subvertical. At a larger scale such geometries might be difficult to identify on seismic data. (b) Fault Hn-B. For seismic example see Morley & Burhannudinnur (1997), Figs 2 and 6. of grain size in anastomosing seams are shown in Fig. 13. Secondary minerals, i.e. iron oxides or clay, commonly fill the porosity outside the cataclasis zone (Fig. 13) but tend not to fill the cataclasis zone, demonstrating its reduced porosity and permeability. Undulose extinction of stressed quartz grains was found in regions of cataclasis bands, but rarely in undeformed sandstones. It is probably a precursor to the creation of sub-grains. Under high magnification, grain to grain contacts in anastomosing cataclasis seams show planar and stressed grain contacts. Point counting of porosity, average grain size and the percentage of fine grained material in each cataclasis zone and in the surrounding rock was used to document the manifestation of strain associated with each type of cataclasis zone. Grain sizes between different cataclastic zones cannot be compared directly because of grain size variations in the parent sandstone, hence percentage changes in average grain size by comparison with grains immediated adjacent to the cataclasis seams was used. In comparison with the surrounding rocks the deformation associated with cataclasis seams has follow effects: porosity is reduced from 16 27% to 0 5%, reduction of average grain size ranges from about 11 23% and the percentage of fine grained material is increased from less than 3% up to 37%, i.e. approximately 10 times. Anastomosing cataclasis displays the highest degree of grain size reduction, porosity reduction and percentage of fine grained material (Table 1). Hence, in keeping with the mapped structural zonation anastomosing cataclasis represents relatively high strain and parallel deformation bands are relatively low strain. Fault gouge Petrographically fault gouge zones are yellowish with quartz grains highly reduced in size. They are zones of cataclasis where in outcrop there are no visible microlithons that separate the individual seams (unlike type 1 anastomosing seams). The percentage of fine grain material is up to 74%, with micro faults well developed parallel or at a high angle to the main fault orientation (Fig. 13b). Grain shape is sub angular to angular with sharp, linear edges. Single quartz grains under high magnification are fractured and surrounded by subgrains. Shale smears In thin section shale smears are dark gray to greenish in colour and dominated by fine grained material which comprises up to 91% of the zone. Fragmented quartz grains represent 5 10%, their diameter ranges between 0.02 and 0.05 mm and they are angular to sub angular. Their presence is probably due to mixing of shale smear and cataclastic products. Under high magnification the grains and fine grained material show micro fractures. The porosity of shale smears is 0%. Lateral changes in porosity approaching a fault zone Porosity values for the sandstone show a tendency to diminish approaching the fault zone, commencing about 10 m from the fault zone (Fig. 13). This reflects the presence of relatively low
220 M. Burhannudinnur and C. K. Morley Fig. 10. Outcrop examples of listric faults associated with normal faults. (a) Vertical termination of a fault into small faults and deformation bands. (b) Hanging wall listric splay of fault Hn-Km. See Morley & Burhannudinnur (1997), Fig. 5 for a seismic example. (c) Footwall listric fault in fault Hn-I. See Morley & Burhannudinnur (1997), Fig. 2 for a seismic example. strains associated with parallel and sub-parallel seams. The porosity reduction is most significant between the zones of anastomosing cataclasis and the main displacement planes, reflecting the closer spacing and higher displacement of the cataclasis seams (Fig. 12). In Fig. 12 local decreases in porosity are shown at 4 m (in the footwall), 2.5 and 7 m (in the hanging wall), they are caused by higher strains associated with minor faults near the sample location. Generally, porosity values around 27% are constant beyond 10 m distance from the fault zone in the undeformed sandstone. The main fault has only 4 m displacement, yet it is associated with porosity reduction up to 10 m away from the fault into both the footand hangingwalls; hence the total width of the damaged zone is about 20 m. In the surrounding outcrops at least 30 faults are present (16 faults are under 2 m displacement, 11 faults have around 4 m displacement, three faults are over 4 m or have unknown displacement). They occur in nine fault zones, which can be assumed to have the same displacement amount as fault Hn-A. Based on Fig. 12, it can be estimated that the zone of porosity damage caused by the nine fault zones, in an area of 350 m could affect 58% (cross-sectional area) of the reservoir rocks. However the length of the entire outcrop is around 600 m, so the amount of reservoir rock likely to be affected by some degree of porosity reduction due to faulting could be about 37%. Hence, even if faults do not act as seals they can affect reservoir properties in other ways. In particular, porosity reduction will affect reserve estimates, and the cataclasis seams
Growth fault zones, outcrop study 221 may affect fluid flow, and cause a strong permeability anisotropy. Fig. 11. Plots of displacement vs height for minor faults in ouctrop in the Miri Formation. DISCUSSION One critical aspect of fault studies is assessing the ability of a fault to act as a barrier to hydrocarbon migration (over geological and oil field production time scales). Examination of sealing faults has to involve fault properties, these properties include texture, composition, structure, permeability and porosity, which ultimately relates to displacement pressure associated with capillary pressure (Berg & Avery 1995). Sealing faults have been widely recognized and studied. The common sealing fault is juxtaposition of shale and sand across a fault (Allan 1989). In this type of seal the nature of the fault plane itself is regarded as immaterial. It has also been recognized that faults planes themselves can form seals (sometimes called membrane seals). Clay smear has been recognized as a possible seal in faults zone (Weber et al. 1978, Berg & Avery 1995). Deformation bands or cataclastic shear zones are also a possible sealing mechanism since they are zones of reduced porosity and permeability (Antonellini & Aydin 1994). Fault zones have high displacement pressures when a fine-grained sheared zone is present (Berg & Avery 1995). Fig. 12. Changing porosity values in a single sandstone unit approaching a fault zone. The changes in porosity approaching the fault zone are attributed to different intensities of cataclastic deformation, and are an indicator of increasing strain approaching the fault zone. The data suggest that even small fault zones, if closely spaced, can affect the storage capacity of reservoir rocks by reducing porosity in both the hanging walls and footwalls of the fault zone.
222 M. Burhannudinnur and C. K. Morley Fig. 13. Different cataclastic deformation styles in thin section from a single sandstone unit (Fig. 12). The darkest areas represent iron oxides filling pores, they are located next to cataclasis bands. (a) Fault zone has a high intensity of micro faults, micro fractured grains (e.g. gr. n) are surrounded by sub-grains. Iron oxide fills the pore spaces. (b) detail of anastomosing cataclasis band where average grain size is reduced by 23% compared with the surrounding rock. (c) Parallel deformation bands, (d) anastomosing cataclasis. Table 1. Summary of point counting grain size and percentage of fine grained material within and outside deformation bands associated with normal faults in the Miri Formation Average grain size (mm) Range Average Percentage of grain size reduction Percentage of fine grained material Fault breccia (5 samples) 2.04 73 Clay smears (5 samples) 1.64 91 Parallel deformation bands (5 samples) Inside zone Outside zone 2.23 2.79 11.1 5 8 Sub-parallel deformation bands (5 samples) Inside zone Outside zone 2.08 2.62 11.3 15 1 Anastomozing deformation bands (15 samples) Inside zone Outside zone (1.82 2.30) (2.86 3.41) 2.05 3.06 23.1 2 26 The deformation styles associated with faults in the study area can modify the reservoir characteristics of a rock (i.e. porosity and permeability) to varying degrees. In the study area, fault zones have reduced porosity (Figs 12 and 13). Porosity in the undeformed rocks is higher than in the deformed rocks, and decreases toward the fault zone. Curves of reducing porosity in the hanging wall and the footwall show the same trend. Shale smears were identified in the study area. They are known to be important to the sealing capability of faults (Gibson 1994; Berg & Avery 1995); and are important in Baram Delta related oil fields (e.g. James 1984; Bait & Banda 1994). In the study area they displayed 0% porosity. Berg & Avery (1995), in tackling the problem as to why fault zones appear to act as seals under some circumstances and fluid conduits under others, suggested that in places (e.g. the centre of a fault) the fault surface tends to be dilational, while towards the fault tip compressional stresses tend to promote the development of sheared zones. The field observations presented here show no evidence for such distributions. Sheared zones occur in most places along a fault where a shale is present, their occurrence seems to be purely due to the presence of the right lithology.
Growth fault zones, outcrop study 223 Even where there is insufficient displacement on the fault to connect up all the shale smears the anastomosing seams and fault gouge zones are continuous enough to provide a good seal. Evidence for this is seen by the presence of iron staining fronts in the regions of type 2 anastomosing cleavage. There appear to be certain zones along a fault that are weak spots regarding sealing ability; in particular the regions of conjugate fault sets. The presence of secondary minerals filling the porosity in surrounding sandstones and in the conjugate faults themselves demonstrate that such regions of the fault zone had good fracture porosity and permeability. In the field the conjugate fault sets are associated with fault tips or transfer zones. One can infer that before cementation some transfer zones and conjugate tips faults were areas of high permeability. This suggests that certain parts of a fault zone can act as a focus for fluid migration. Hence the timing of secondary mineral formation is critical since mineralization (prior to hydrocarbon migration) may plug the leaks in the fault system. As a minor conclusion in their excellent paper Antonellini & Aydin (1994) concluded that localized porosity loss associated with deformation band and fault zone porosity loss does not generally affect the storage property of a porous sandstone at a reservoir scale. That may well be the case for their area of study, however the situation appears to be different for the growth faults examined in this study. The fault zone they studied was about 20 30 m wide and had an offset of about 40 m. It is difficult to acertain the state of lithification of the sandstone at the time of deformation, but probably it was better lithified than the Miri Formation. In NW Borneo growth faults with only a few metres displacement can significantly reduce porosity (by a few percent) in the hanging wall and footwall up to 10 m away from even a small fault zone. This means that small, sub-seismically visible faults have the capability to affect the porosity, and direction of maximum permeability of a large percentage of reservoir rock, depending upon their spacing. If the field example studied is more widely applicable then it is possible for 40% of the reservoir volume to be affected to some degree by porosity reduction related to growth faulting. In better lithified rocks the damage zone is likely to be narrower than those in poorly lithified rocks, and the fault population characteristics (including fault spacing) may be different, and hence explain the differences in conclusions between this study and that of Antonellini & Aydin (1994). CONCLUSIONS Mapping has shown that fault zones in the Miri Formation contain up to four zones of deformation bands and cataclasis in order of increasing strain, they are: parallel seams, sub-parallel seams, anastomosing seams and fault gouge/shale smear zones (Figs 3&4). Single cataclasis seams can reduce porosity to 0%. This is achieved by grain size reduction and incorporation of fine grained material. So, a single cataclasis seam, especially anastomosing cataclasis, can function as an excellent porosity and permeability barrier. Secondary minerals, particularly iron oxides tend to fill up pore spaces surrounding the cataclasis seams. In most cases secondary minerals do not pass through the cataclasis zone, indicating that the cataclasis zones are permeability barriers. The cataclasis and clay smears combine to permit the fault zone to seal over a broad area in different lithologies, which is in agreement with the conclusions of Gibson (1994). The assumption that faults act as seals across one homogeneous plane seems to be borne out by examples from oil fields in Brunei and Sarawak (Miri) where there are numerous examples of sealing fault planes (James 1984; Bait & Bander 1994). However, field data shows that there are particular areas where fault zones have been the sites of fluid flow, rather than seals. These sites, which tend to occur at fault splays, are potential weak points in the fault seal. They were probably areas of non plane strain, where material was moved out sideways towards the unconstrained margin of the fault. The structural damage to reservoirs by faulting may affect permeability and flow characteristics beyond the narrow principal displacement planes of a fault zone. It appears that faults with just a few metres throw can affect a relatively large rock volume. The effect of deformation extends beyond the fault zone and can significantly reduce porosity in the hanging wall and footwall several metres away from even a small fault zone. Although the parallel cataclasis zone has discontinuous seams and cannot act as a long-term permeability barrier it will affect the production characteristics of a reservoir. Since these zones can extend several metres from small fault zones it means that even small faults can affect the porosity, and direction of maximum permeability of a large percentage of reservoir rock. Hence at least some fault systems should be regarded as strongly three dimensional features, not just two dimensional planes that act as a sheet-like seal. REFERENCES ALLAN, U. A. 1989. Model for hydrocarbon migration and entrapment within faulted structures. American Association of Petroleum Geologists Bulletin, 73, 803 811. ANTONELLINI, M. A. & AYDIN, A. 1994. 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