Anatomy of Growth Fault Zones in Poorly Lithified Sandstones and Shales

8/9/2019 Anatomy of Growth Fault Zones in Poorly Lithified Sandstones and Shales

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Anatomy of growth fault zones in poorly lithified sandstones and shales:implications for reservoir studies and seismic interpretation: part 1,

outcrop study

M. Burhannudinnur1,2and C. K. Morley11D epartment of Petroleum Geosciences, U niversity of Brunei D arussalam Bandar Seri Begawan, 2028,

N egara Brunei D arussalam2Present address: Jurusan Teknki Geologi, Fakultas Teknologi Mineral, U niversitas Trisakti, Jl. Kyai Tapa N o 1, Grogol,

Jakarta Barat, I ndonesia

ABSTRACT: Some normal faults developed in poorly lithified sediments inMiocenePliocenedeposits of NW Borneo in the vicinity of Brunei display regularzonationsof deformation bands. For fault displacementsof afewmetres thezonesof deformation bands extend up to about 10m into both the hanging wall and

footwall. They range from closely spaced anastomosingseams within or adjacent tothe main slip planes, to more widely spaced sub-parallel and parallel seams passingawayfromthefault zone. Theyreduceporosityandpermeability, andif thefaults areclosely spaced, are likely to impact reservoir production characteristics and reserveestimates. In cross-section and map view fault zones are commonly composed ofseveral important gouge and cataclasis zones, which branch and join, display listricdetachments and various types of hard and soft linkage. Some of these geometrieshave been described as common characteristics of faults, others are comparitivelyrare. They havesignificant implications for theinterpretation of seismic data.

KEYWORDS: sealing faults, deformation bands, cataclasis, porosity, permeability

INTRODUCTIONRecently it has been recognized that shale smears or shearedzones and cataclasis zones play an important role in makingfault zones act as seals to hydrocarbon migration and entrap-ment (Weber et al. 1978;Bouvier et al. 1989; Gibson 1994; Berg& Avery1995). Despitethegenerally poor exposureof growthfaults and poor recovery of fault zones from cores (Berg &Avery, 1995) details about the complexity of fault zones havebegun to emerge. In particular the study of faults from theArches National Park by Antonellini & Aydin (1994) demon-strated thereduction of both porosity and permeability due todeformation bands in broad areas around fault zones.

Thisstudydescribes thedetails of growth fault geometries inoutcrop from the Baram delta province of NW Borneo,

centered around Brunei Darussalem (Fig. 1). The significanceof the growth fault geometry described from outcrop isdiscussed with respect to reservoir modelling and seismicinterpretation in Part 2 (Morley & Burhannudinnur 1997).

Seismic studies provide much better information on theentirefault systemthan outcrop studies but cannot resolvethedetailedgeometryof a fault zone. Both types of studies can beused together to build amorecompletepictureof growth faultgeometry and evolution.

GEOLOGICAL SETTING

Faults were examined in a number of outcrops of the MiriFormation,whichis foundnear thecoast of Brunei Darussalem

from the Jerudong area in Brunei Darussalam to the LambirHill area south of Miri in neighbouring Sarawak (Fig. 1). In

particular the Jerudong area contains numerous exposures ofnormal faults. The formation is a sequence of sandstones andshales which, based on benthonic foraminifera assemblages(Liechti 1960; Wilford 1960), were deposited in a marineenvironment during the late MiocenePliocene.

TheMiri Formation was deposited intheBaramprovinceorBaramBelait depocenter. This depocenter, of middleMioceneto Present day age, formed after southsoutheastward obliquesubduction of theChinaSeaPlateunder Borneo ceased (James1984). Magnetic and gravity surveys indicate up to 15kmsubsidencehas occurred in thedepocenter (James 1984).It wasgravitational instability of the young, thick deltaic sediments,overlying a thick, mobile, overpressured marine shale sub-stratum that caused the development of the growth faultsexamined in this study. The following descriptions are ofgrowth fault zones developed in a deltaic setting.

Fault geometries in outcrop

A number of studies have described deformation band zo-nation in faulted sandstones(e.g. Aydin 1978; Aydin & Johnson1978, 1983; Jamison & Stearns 1982). The term deformationband (Aydin 1978) is used to describe small planar features,about 1mm thick that accommodate small offsets with dis-placements ranging up to a few centimetres. Recently, detailedstudies have shown that deformation band development con-sistsof initial dilatancyfollowed bycompactionandcrushingofgrains (Antonellini et al. 1994). Variations in porosity andconfiningpressureaffect whether theseamsaredilatant withno

cataclasis or undergo compaction with cataclasis (Antonelliniet al. 1994; Antonellini & Pollard 1995).

Petroleum Geoscience, Vol. 3 1997, pp. 211224 1354-0793/ 97/ $10.001997 EAGE/ Geological Society, London


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Theexact conditions of lithification of theMiri Formation atthetimeof deformationareunknown,but coresfromwells andmanyoutcropsarepoorly lithified at present, hencethesameislikely to be true for the Miri Formation in the past. The high

pore fluid pressures and low degree of lithification wereprobablyconducivefor granular flowas well as cataclastic flowduring extension. Most of the outcrops that contain growthfaults display some degree of diffuse zones of deformationbands extending up to several metres beyond the main faultzone. In some areas remarkably regular arrays of seams ofdifferent intensity could bedistinguished, that laysub-parallel tothe fault zone. Such zones can bemanymetres widefor faultswith onlyafewmetres displacement. Commonlythezones arecharacterized by complex linkage of multiple intense cataclasisbands, fault gouge zones and shale smears set in a zone ofless intense deformation bands. Where the faults displayedregular zonation of deformation bands three main types couldbe 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 thefault zone.They are characterized by light coloured, parallel defor-mation bands 240cmlong, 0.10.5cmwideand spaced560cm apart. They represent very low strain defor-mation and throw on individual seams is generally lessthan 1mm. Thebelt of parallel deformationbandsisup to10m wide in map view, for faults with throws of severalmetres (Fig. 2).Seams in some areas exhibit dilatant behaviour since, incomparison to the surrounding rock, they are preferen-tially stained andfilled bylateformingiron oxidecements.

Such examples are relatively rare. In most cases theopposite is found, the seams stand out as light colouredbands, andthesurroundingrock is stained byiron oxides.This indicates that the seams are compactional and acted

as barriers to fluid migration. These visual observationsare confirmed by examination of porosity changes inthinsection. Thetwo types of behaviour occur in adjacentfault zones, in similar sandstones. Hence subtle variations

in strain rather than variations in porosity or confin-ing pressure (e.g. Antonellini et al. 1994) are probablyresponsible for thedifferences.

(2) Sub-parallel seams occur between, and are transitional to,the anastomosing zones and parallel zones. They arecharacterized by light coloured sub-parallel deformationbands 260cm long, 0.10.5cm wide and spaced220cm apart. The sub-parallel seams are generallyparallel with some oblique low-angle and wavy connec-tions. Displacement on individual seams is about 1mm.In map view, for fault displacements of several metres,the belt of sub-parallel deformation bands is up to 4mwide.

(3) Anastomosingseamsoccur in theareanearestto or within

thenarrow zones of largest displacement. They are char-acterizedbyeither black or white anastomosing seams. Inboth cases theseams are closely spaced, 0.10.8cmwideand individually range in length fromabout 2cm to 1m,however they link to form systems almost as long as thefault length. For fault displacementsof several metres thebelt of anastomosing deformation bands are up to 75cmwide. In detail anastomosing seams can be divided intotwo types. Type 1 represents the highest intensity defor-mation. They are usually dark grey to black in colour andformin theprincipal displacementzonesof thefaultzone.Consequently they are commonly associated with shalesmears. The black colour is primarily due to fine grainedcataclastic material and smeared shale, not iron oxide

staining. They form zones 1cm to 15cm wide in mapview. Type 2 are characterized by light coloured seamsthat in places are coloured dark red or black due toprecipitation of iron oxides. They are spaced 0.5mm to

Fig. 1. Location map of Brunei withinBorneo showing theextent of theBaramdelta province, and themain areas wherethe Miri Formation is present in outcrop(based on James 1984).

212 M. Burhannudinnur and C. K. Morley


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Fig.2a.

Growth fault zones, outcrop study 213


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Fig.2b.

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5cm apart and displacement on individual seams isgenerally around 15mm. They represent relatively highstrain zones, but lower strain than the type1 zones.

Different types of lateral variation of fault geometry were

described based on thepattern of principal shear planes in mapview (Fig. 2). The lateral variations of fault geometry illustratehowfaultsdevelop; link anddieout. Six types wererecognizedin the study area (Fig. 4).

(1) Relay structures, this typeof transfer zone can belocatedanywhere within a fault zone, as one set of type 1anastomosing seams is offset from another set, anddisplacement is transferred between thetwo sets.

(2) Horses areusuallydefined bynarrow type1anastomosingcataclastic bands or fault gouge.

(3) Splays are most frequently found at the termination of afault or at atransfer zonewithin thefault zone. Themainfault branches into a number of smaller faults.

(4) Linking cross-faults form another type of transfer zone

involvingoblique slip on a system of minor faults that liebetween twomajor displacement strandsof thefault zone,but do not extend beyond the two strands. The majorstrands transfer displacement to one another via thesesmall faults.

(5) Conjugate fault zones are regions where numerous minorconjugate minor faults, fractures and deformation bandsare present. The two sets of faults not only displaydifferent dip directions, but also intersect in strike view atan acuteangle. Thespacingof minor cross-cuttingfaultsisaround 230cm. Secondary minerals, in particular ironoxides, commonly preferentially stain the cross-cuttingfault zones (but not adjacent zones), suggesting they areregions of high porosity and permeability. The conjugate

fault zones tend to form between splays in type 1anastomosing seams or fault gouge zones near the faulttip, in particular they are usually associated with loss ofdisplacement on a fault in sandstone. Despite normal

dip-slip indicators in nearby areas, the conjugate faultzones display horizontal or low-angled oblique slicken-slides. This suggest material is being moved laterally awayfrom the fault zone at the unconstrained margin of thefault.

(6) Fault gougeandtype1anastomosingzonesarecommonlyoffset byminor obliquefaults. Such faults extend beyondtheboundaries of thefaultsthey offset andmaybeduetoindependent antithetic faults or second order tear faultswithin the fault zone.

CROSS-SECTION CHARACTERISTIC OF GROWTHFAULTS IN OUTCROP

Faults in cross-sectional view are commonly developed aszones, not discrete planes. One control on the width of thefault zoneappears to belithology. A fault zoneis composed ofbundlesof high-strain cataclasis zones, noneof which extendsthroughout the width of the fault. The high strain zonespass into others laterally and vertically via a variety of transferzonetypeswhich areusuallycharacterized byahigh intensityofcataclastic deformation; cross-cutting minor faults andsmall-scale pull-apart structures.

The fault Hn-A is a small growth fault that has up to 4 mdisplacement in syn-tectonic sediments. Thesandstonewas notdrag folded, but shales as interbeds or lenses in thesandstonewere strongly drag folded and incorporated within the maincataclasis zones as shale smears (Fig. 5). The parallel and

sub-parallel cataclasis zones that are clearly visiblein map vieware harder to observe in cross-section, particularly in sectionsthrough interbedded sandstones and shales. Anastomosingseams are the easiest to distinguish, they merge with, and lay

Fig. 3. Schematic sketches of themain types of deformation bandgeometry in map view based upon map view exposures of faultzones (see Fig. 2 above). Fig. 4. Summary cartoons of the main map view variations in

fault geometry observed from faults in the Miri Formation. Theselinking or cross-cutting features can be seen in Fig. 2. Mostly theyenable transfer of displacement from one fault gouge zone oranastamosing cataclasis zone to another, or they occur at the tipsof these zones where the fault zonedies out.



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sub-parallel to themain fault gougezone. Whereclearly visiblethe parallel and sub-parallel deformation band zones describedfrom map view vary dependingupon whether they are locatedin the foot- or hanging-walls (Fig. 6). I n the hanging wall the

parallel seams tend to havehigh-angle (7045) dips, antitheticto themain fault zone, though someseams may havesyntheticdips. Thesub-parallel seamstendto beamixtureof high angledsynthetic and antithetic dips, with synthetic dips becoming

more dominant approaching the fault zone. I n the footwall asimilar pattern is produced except that the dominant parallelcataclasis dip direction is synthetic to themajor fault zone. Thispattern of cataclasis orientation should produce a predictable

pattern in cores, and indicate the location of important faultzones, even if they are zones of poor recovery (Fig. 6).The parallel seams in particular seem to be forming not

directlyin response to the shear within the fault zone, but as a

Fig. 5. Cross-section of fault Hn-A from outcrop. This fault zone displays many typical features of growth faults in outcrop from theBaram Delta province. There is expansion of the hanging wall section into the fault, suggesting it was a growth fault. The fault plane iskinked, 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, andtowards the bottom are small displacement high-angle listric faults. Along the fault zone there are numerous examples of shale drag andsandstone lenses intensely deformed by cataclasis seams.



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response to strain in the hanging wall and footwall that is aconsequence of faulting. In modelling fault geometries usingtheChevron or modified Chevron methods (Williams & Vann1987) antithetic simple shear of the hanging wall commonlyworks well, particularlyfor growth faults (Xiao & Suppe1992).Theorientation of theparallel deformation bands suggests they

developed as a manifestation of antithetic simple shear in thehanging wall (Fig. 7).The variations in cross-sectional fault zone geometry in the

studyareacan besimplified to 11 idealized characteristics (Fig.

8). A fault zonemayconsist of asingletypeor morecommonlya combination. Kinks or ramps in the fault plane may reflectlithologychangesthat influenced theinitial fault trajectories,thelinkage of two initially separate faults, or may be due to tofolding of the fault by activity on an antithetic fault in thefootwall. Downwards-broadening zones and splays are associ-

ated with thedevelopment of synthetic faults(Fig. 9). Theyaremost frequently found at lithological boundarieswhere passingfrom a shale into a sandstone the fault splays down-wards. Pull-apart transfer zones (Fig. 9b) are common in

Fig. 6. (a) Schematic block diagram illustrating the type of deformation band and fault gouge geometries found around fault zones in theMiri Formation. PC, parallel cataclasis; SPC, subparallel cataclasis. (b) Schematic section illustrating the likely arrangement of structures in acore through a typical fault zone.

(a) (b)

Fig. 7. Location of deformation bandsin relation to footwall and hanging walldeformation associated with normalfaults. Note that the footwall area is

involved in the deformation of the faultzone. The cataclasis is unlikely to bedueto hanging wall deformation of a separatefault which lies in the footwall of theillustrated fault because the seams dip inthe opposite direction to those in thehanging wall.



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cross-sectional view, but have only been recognized in sand-stones.Thezonebetween thefault strandsconsistsof cataclasisbands and fault breccias.

Faults will dieout over acertain length, they do not persistalong-strike indefinitely. Thetermination of a fault is indicatedby zero displacement. Displacement usually dies out graduallyfrom a centrally located maximum (Barnett et al. 1987). Twotypes of fault termination arerecognized in thestudyarea. Firstis agradual decreasein displacement upwardsandlaterally, and

in somecases downwards, into deformation bandsor anumberof small faults (Figs 9 & 10). This geometry has only beenclearly observed in sandstones.In suchcases faults withnarrowzones of intense deformation change gradually to broaderzones of less intense deformation. They branch into a numberof smaller faults with decreasingdisplacement until thedensityof small faults decreases and the fault zone dies out. In manycases the small faults disappear abruptly at a lithologicalboundary, particularly between sandstones and shales. In Fig.10 a low-angle fault exists below the minor faults and they areconfined to the hanging wall of the fault. In another type ofterminationafault in thefootwall of afault of oppositedipmayend abruptly at the fault plane, with no offset continuation ofthe fault visible in the hangingwall.

Four of the 11 types (Fig. 8) occur relatively infrequently,thesearethethreelistricdetachment stylesandhorses(Fig. 10).All are best developed in interbedded sandstones and shales.Thetermsheared zoneis used to describelocalized areas along

the fault where sandstone and shale beds have been draggedinto thefault zone (Weber et al. 1978; Bouvier et al. 1989; Berg& Avery 1995). The sandstones are broken up by numerousminor faults and bedding may be rotated parallel to the faultplane. Theyaresimilar to shale smears except thebehaviour ofthe rock was more brittle and bedding can still be observed.Well developed listric detachmentsareassociated with footwallsheared zones. The width of observed sheared zones variesfrom10to 150cm. Thelistricfaultscommonlydetachwithin afine grain unit or at a bedding plane. Listric detachment faultscan also formin the hanging wall (Fig. 10). These enable onefault strand to die out and be replaced by a different strand atdepth.

Heightdisplacement relationships

The outcrops are not continuous enough to expose thecomplete strike-length of major faults. However smaller faultsin cross-section can be seen to die out both upwards and

downwards. Hence it is possible to examine theheight (H) displacement (D) relationships. Values of displacement plottedagainst height for minor faults are shown in Fig. 11. Therelationship is based on measurements of 182 faults andcataclastic seams around five main faults. The minimummeasurement is 3cm in height and 0.05cm of displacement.The maximum measurement is 1800cm in height with 30cmdisplacement. Generally, thefault displacement increases as thefault height increases. Distribution of the data is excellent fordetermining the best fit line. The best line of the data can bedone by linear or power examination line. The linear examin-ation resulted in D=0.0173 H with a regression ratio (R2) of0.813. The power examination resulted in D=0.0133 H1.007

with abest fit ratio (R2) of 0.83. Bycomparingtheratiosit can

be shown that the power examination is statistically morereasonable than the linear one. Nevertheless, both values aregenerally reasonable. The smallest sheared zones measuredwere individual, isolated cataclastic seams. Hence the defor-mation bands appear to exhibit similar displacementheightcharacteristics to the minor faults.

DEFORMATION CHARACTERISTICSOF BROADFAULT ZONES

To examine the strain associated with cataclasis and its effectson reservoir properties a single sandstone unit was sampled atintervals in both thehanging- andfootwalls of afault (Fig. 12).The changes in average grain size, porosity and percentage of

fine grained material were examined within the deformationbands and in the surrounding sandstone.

The sandstone is a poorly lithified, medium to fine grained,clean sand. Thesubangular to subrounded grainsrangein sizebetween 0.02 and 0.5mm, the average grain size is around0.25mm. It is composed of 6575% quartz grains, upto 0.5%opaque minerals, 00.5% sedimentary rock clasts, up to 2%unidentified minerals, and theaverage porosity in undeformedsamplesis about 27%.Clayminerals arerareandiron oxidesfill13% of thepores spaces.

Cataclasis zones

In thin section deformation bands formed by cataclasis insandstones are generally characterized by bands of reducedgrain size and porosity, which visually can bedetermined byalighter colour than thesurrounding rock (Fig. 13). Reductions

Fig. 8. Schematic characteristics of fault zone geometries in

cross-section based upon outcrop examples. The evidencefor suchgeometries on seismic reflection profiles is discussed in Morley &Burhannudinnur (1997).



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of grain size in anastomosing seams are shown in Fig. 13.Secondary minerals, i.e. iron oxides or clay, commonly fill theporosityoutsidethecataclasis zone(Fig. 13) but tendnot to fillthe cataclasis zone, demonstrating its reduced porosity andpermeability. Unduloseextinction of stressed quartz grains wasfound in regions of cataclasis bands, but rarely in undeformedsandstones. It is probably a precursor to the creation ofsub-grains. Under high magnification, grain to grain contactsinanastomosing cataclasis seams show planar and stressed graincontacts.

Point counting of porosity, average grain size and the

percentage of finegrained material in each cataclasis zoneandin the surrounding rock was used to document the manifes-tation of strain associated with each type of cataclasis zone.Grain sizes between different cataclastic zones cannot becompared directlybecauseof grain sizevariations in theparentsandstone, hence percentage changes in average grain size bycomparison with grains immediated adjacent to the cataclasisseams was used. In comparison with thesurroundingrocksthedeformation associated with cataclasis seams has follow effects:porosity is reduced from 1627% to 05%, reduction ofaverage grain sizeranges from about 1123% and thepercent-age of fine grained material is increased from less than 3% upto 37%, i.e. approximately 10 times. Anastomosing cataclasisdisplays 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 anas-tomosingcataclasis represents relatively high strain and paralleldeformation bands are relatively low strain.

Fault gouge

Petrographically fault gouge zones are yellowish with quartzgrains highly reduced in size. They are zones of cataclasiswherein outcrop thereareno visiblemicrolithonsthat separatethe individual seams (unlike type 1 anastomosing seams). Thepercentage of fine grain material is up to 74%, with microfaults well developed parallel or at a high angle to the mainfault orientation (Fig. 13b). Grain shape is sub angular toangular with sharp, linear edges. Single quartz grains underhigh magnification are fractured and surrounded by sub-grains.

Shale smears

In thin section shalesmears aredark gray to greenish in colourand dominated byfinegrained material which comprises up to91% of thezone. Fragmented quartz grains represent 510%,their diameter ranges between 0.02 and 0.05mm and they areangular to sub angular. Their presence is probably due tomixing of shale smear and cataclastic products. Under highmagnification the grains and fine grained material show microfractures. Theporosity of shale smears is 0%.

Lateral changes in porosity approaching a fault zone

Porosity values for thesandstone showatendency to diminishapproachingthefault zone, commencingabout 10m from thefault zone (Fig. 13). This reflects the presence of relatively low

Fig. 9. Section illustrating faults which splay or broaden downwards. (a) Fault Hn-I, note the significant offset of the paired sandstonelayers by splaying faults that are almost subvertical. At a larger scale such geometries might be difficult to identify on seismic data. (b) FaultHn-B. For seismic example see Morley & Burhannudinnur (1997), Figs 2 and 6.



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strains associated with parallel and sub-parallel seams. Theporosity reduction is most significant between the zones ofanastomosing cataclasis and the main displacement planes,reflecting the closer spacing and higher displacement of thecataclasis seams (Fig. 12). In Fig. 12 local decreases in porosityare shown at 4m (in the footwall), 2.5 and 7m (in thehanging wall), they are caused byhigher strains associated withminor faults near the sample location. Generally, porosityvaluesaround 27%areconstant beyond10mdistancefromthefault zone in the undeformed sandstone. The main fault has

only 4m displacement, yet it is associated with porosityreduction up to 10m away from the fault into both the foot-andhangingwalls;hencethetotal width of thedamaged zoneisabout 20m.

In thesurroundingoutcropsat least 30 faultsarepresent (16faults are under 2m displacement, 11 faults have around 4mdisplacement, three faults are over 4m or have unknowndisplacement). They occur in nine fault zones, which can beassumed to havethesamedisplacement amount as fault Hn-A.Based on Fig. 12, it can beestimated that the zone of porositydamage caused by the nine fault zones, in an area of 350mcould affect 58% (cross-sectional area) of the reservoir rocks.However the length of the entire outcrop is around 600m, sothe amount of reservoir rock likely to be affected by some

degree of porosity reduction due to faulting could be about37%. Hence, even if faults do not act as seals they can affectreservoir properties in other ways. In particular, porosityreduction will affect reserve estimates, and the cataclasis seams

Fig. 10. Outcrop examples of listricfaults associated with normal faults.(a) Vertical termination of a fault intosmall faults and deformation bands.(b) Hanging wall listric splay of faultHn-K m. See Morley & Burhannudinnur(1997), Fig. 5 for a seismic example.(c) Footwall listric fault in fault Hn-I. SeeMorley & Burhannudinnur (1997), Fig. 2

for a seismic example.



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may affect fluid flow, and cause a strong permeabilityanisotropy.

DISCUSSION

One critical aspect of fault studies is assessing theability of afault to act as a barrier to hydrocarbon migration (overgeological andoil field production timescales). Examination ofsealing faults has to involve fault properties, these propertiesincludetexture, composition, structure, permeability andporos-ity, whichultimately relates to displacement pressureassociatedwith capillary pressure(Berg & Avery 1995).

Sealing faults havebeen widely recognized and studied. Thecommon sealing fault is juxtaposition of shale and sand acrossa fault (Allan 1989). In this type of seal the natureof the faultplane itself is regarded as immaterial. It has also been recog-nized that faults planes themselves can form seals (sometimescalled membrane seals). Clay smear has been recognized as apossible seal in faults zone (Weberet al. 1978, Berg & Avery1995). Deformation bands or cataclastic shear zones are also a

possible sealing mechanism since they are zones of reducedporosity and permeability (Antonellini & Aydin 1994). Faultzones have high displacement pressures when a fine-grainedsheared zone is present (Berg & Avery 1995).

Fig. 11. Plots of displacement vs height for minor faults inouctrop in the Miri Formation.

Fig. 12. Changing porosity values in a single sandstone unit approaching a fault zone. The changes in porosity approaching the fault zoneare attributed to different intensities of cataclastic deformation, and are an indicator of increasing strain approaching thefault zone. The datasuggest that even small fault zones, if closely spaced, can affect the storage capacity of reservoir rocks by reducing porosity in both thehanging walls and footwalls of thefault zone.



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The deformation styles associated with faults in the studyarea can modify the reservoir characteristics of a rock (i.e.porosity and permeability) to varying degrees.

In thestudyarea, fault zones havereduced porosity (Figs12and13). Porosityin theundeformed rocksis higher than in thedeformed rocks, anddecreasestoward thefault zone. Curvesofreducingporosityin thehangingwall andthefootwall showthe

same trend.Shale smears were identified in the study area. They areknown to be important to the sealing capability of faults(Gibson1994; Berg& Avery1995);andareimportantin Baram

Deltarelated oil fields(e.g. James 1984; Bait & Banda1994). Inthestudyareatheydisplayed0%porosity.Berg& Avery(1995),in tackling the problem as to whyfault zones appear to act asseals under some circumstances and fluid conduits underothers, suggested that in places (e.g. thecentreof a fault) thefault surfacetends to bedilational, while towards the fault tipcompressional stresses tend to promote the development of

sheared zones. Thefield observations presented here show noevidence for such distributions. Sheared zones occur in mostplaces along a fault where a shale is present, their occurrenceseems to be purely dueto the presence of the right lithology.

Fig. 13. Different cataclastic deformation styles in thin section from a single sandstone unit (Fig. 12). The darkest areas represent ironoxides 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 isreduced by 23% compared with thesurroundingrock. (c) Parallel deformation bands, (d) anastomosing cataclasis.

Table1. Summary of point countinggrain sizeand percentageof finegrained materi al within andoutsidedeformation bandsassociated with normal faults

in the Miri Formation

Average grain size(mm) Percentage

of grain size

reduction

Percentage of

fine grained

materialRange Average

Fault breccia (5 samples) 2.04 73Clay smears (5 samples) 1.64 91Parallel deformation bands (5samples) Inside zone

Outside zone

2.23

2.79

11.1 5

8Sub-parallel deformation bands (5 samples) Insidezone

Outside zone

2.08

2.62

11.3 15

1Anastomozingdeformation bands (15 samples) Insidezone

Outside zone

(1.822.30)

(2.863.41)

2.05

3.06

23.1 2

26



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Even where there is insufficient displacement on the fault toconnect up all the shale smears the anastomosing seams andfault gouge zones are continuous enough to provide a goodseal. Evidence for this is seen bythe presence of iron stainingfronts in theregions of type2 anastomosing cleavage.

There appear to be certain zones along afault that areweakspots regarding sealing ability; in particular the regions ofconjugate fault sets. Thepresence of secondary minerals fillingthe porosity in surrounding sandstones and in the conjugatefaults themselves demonstrate that such regions of the faultzone had good fracture porosity and permeability. In the fieldtheconjugatefault sets areassociated with fault tipsor transferzones. One can infer that before cementation some transferzones and conjugatetips faults wereareas of high permeability.Thissuggeststhat certainpartsof afaultzonecan act asafocusfor fluid migration. Hence the timing of secondary mineralformation is critical since mineralization (prior to hydrocarbonmigration) may plug theleaks in thefault system.

As aminor conclusion in their excellent paper Antonellini &Aydin (1994) concluded that localized porosity loss associated

with deformation band and fault zone porosity loss does notgenerally affect thestoragepropertyof aporoussandstoneat areservoir scale. That maywell bethecasefor their areaof study,however the situation appears to be different for the growthfaults examined in this study. Thefault zone they studied wasabout 2030m wide and had an offset of about 40m. It isdifficult to acertain thestateof lithification of thesandstone atthe time of deformation, but probably it was better lithifiedthan the Miri Formation. In NW Borneo growth faults withonly a few metres displacement can significantly reduceporos-ity (by a few percent) in the hanging wall and footwall up to10mawayfromeven asmall fault zone. Thismeansthat small,sub-seismically visible faults have the capability to affect theporosity, and direction of maximum permeability of a large

percentage of reservoir rock, depending upon their spacing. Ifthe field example studied is more widely applicable then it ispossiblefor 40% of thereservoir volumeto beaffected to somedegree by porosity reduction related to growth faulting. Inbetter lithified rocks the damage zone is likely to benarrowerthan those in poorly lithified rocks, and the fault populationcharacteristics (including fault spacing) may be different, andhenceexplain thedifferences in conclusions between this studyand that of Antonellini & Aydin (1994).

CONCLUSIONS

Mapping has shown that fault zones in the Miri Formationcontain upto four zonesof deformationbandsandcataclasis in

order of increasingstrain, they are: parallel seams, sub-parallelseams, anastomosingseams and fault gouge/ shale smear zones(Figs 3 & 4).

Single cataclasis seams can reduce porosity to 0%. This isachieved by grain size reduction and incorporation of finegrained material. So, a single cataclasis seam, especially anasto-mosing cataclasis, can function as an excellent porosity andpermeability barrier. Secondary minerals, particularly iron oxidestend to fill up pore spaces surrounding the cataclasis seams. Inmostcasessecondaryminerals do not passthrough thecataclasiszone, indicating that the cataclasis zones are permeability bar-riers. Thecataclasisand clay smears combineto permit thefaultzoneto seal over abroad areain different lithologies, which is inagreement with theconclusionsof Gibson (1994).

The assumption that faults act as seals across one homo-geneous plane seems to be borne out by examples from oilfields in Brunei and Sarawak (Miri) where there are numerousexamples of sealing fault planes (James 1984; Bait & Bander

1994). However, field datashows that thereareparticular areaswherefault zones havebeen thesites of fluid flow, rather thanseals. These sites, which tend to occur at fault splays, arepotential weak pointsin thefault seal. Theywereprobablyareasof non plane strain, where material was moved out sidewaystowards theunconstrained margin of thefault.

The structural damage to reservoirs by faulting may affectpermeability and flow characteristics beyond thenarrow prin-cipal displacement planes of a fault zone. It appears that faultswith just a few metres throw can affect a relatively large rockvolume. The effect of deformation extends beyond the faultzone and can significantly reduceporosity in thehanging walland footwall several metres away from even asmall fault zone.Although the parallel cataclasis zone has discontinuous seamsand cannot act as a long-termpermeabilitybarrier it will affecttheproduction characteristics of a reservoir. Since thesezonescan extend several metres from small fault zones it means thateven small faults can affect the porosity, and direction ofmaximumpermeability of a large percentage of reservoir rock.Hence at least some fault systems should be regarded as

strongly three dimensional features, not just two dimensionalplanes that act as a sheet-like seal.

REFERENCES

ALLAN, U. A. 1989. Model for hydrocarbon migration and entrapmentwithin faulted structures. A meri can A ssociation of Petroleum Geologists Bulleti n,73, 803811.

ANTONELLINI, M. A. & AYDIN, A. 1994. Effectof faultingonfluid flowin porous sandstones: petrophysical properties. A meri can A ssociati on of Petroleum Geologists Bulletin, 78, 355377.

& POLLARD, D. D. 1995. Distinct element modelingof deformationbandsin sandstone. Journal of Structural Geology, 17, 11651182.

, AYDIN, A. & POLLARD, D. D. 1994. Microstructureof deformationbands in porous sandstones at Arches National Park, Utah. Journal of Structural Geology, 16, 941959.

AYDI N, A. 1978. Small faults formed as deformation bands in standstones.Pure and A pplied Geophysics, 116, 913930.

& JOHNSON, A. M. 1978. Development of faults as zones ofdeformation bands and as slip surfaces in sandstones.Pure and A ppliedGeophysics, 116, 913930.

& 1983. Analysis of faulting in porous sandstones. Journal of Structural Geology, 5, 1931.

BAIT, B. & BANDA, R. M. 1994. Terti ary Basin of N orth Sarawak Malaysia: Afield excursion in the TatauBintulu and Mi ri areas. AAPG InternationalConference and Exhibition, 2124 August 1994.

BARNETT, J. A. M., MORTIMER, J., RIPPON, J. H., WALSH, J. J. &WATTE RSON, J. 1987. Displacement geometryin thevolumecontaininga single normal fault. A meri can A ssociation of Petroleum Geologists Bulletin, 71,925938.

BERG, R. R & AVERY, A. H. 1995. Sealing properties of Tertiary growthfaults, Texas Gulft Coast. A meri can Association of Petroleum Geologists Bulletin,79, 375393.

BOUVIER, J. D., KARRS-SIJPESTEINIJN, C. H., KLUESNER, D. F.,ONYEJEK WE, C. C. & VAN DER PAL, R. C. 1989. Three-dimensionalseismic interpretation and fault sealing investigations, Nun RiverField, Nigeria. A meri can A ssociation of Petroleum Geologists Bulletin, 73, 13971414.

GI BSON, R. G. 1994. Fault-zoneseals in siliciclastic strataof theColumbusBasin, offshore Trinidad. A meri can A ssociation of Petroleum Geologists Bulletin,78, 13721385.

JAMES, D. M. D. 1984. The Geology and H ydrocarbon Resources of N egaraBrunei D arussalam. Special Publication, Muzium Brunei and Brunei ShellPetroleum Company Berhad.

JAMISON, W. R. & STEARNS, D. W. 1982. Tectonic deformation ofWingate Sandstone, Colorado National Monument. AA PG Bulletin, 66,25842608.

LIECHTI, P. 1960. The geology of Sarawak, Brunei and the western part of N orthBorneo. Geological Survey Department British Territories in Borneo:Bulletin 3, KuchingSarawak.

MORLEY, C. E. & BURHANNUDI NNUR, M. 1997. Anatomy of growthfault zones in poorly lithified sandstones and shales: implications forreservoir studies and seismic interpretation: part 2, seismic reflectiongeometries. Petroleum Geoscience, 3, 225231.



14/14

WEBER, K. J.,MANDL, G., PILAAR,W. F., LEHNER,F. & PRECIOUS,R. G. 1978. Therole of faults in hydrocarbon migration and trapping inNigerian growth fault structures. Tenth A nnual Offshore Technology ConferenceProceedings, 4, 26432653.

WILFORD, G. E. 1960. The geology and mineral resources of Brunei and adjacentparts of Sarawak with description of Seri a and Miri oil fields. Memoir 10 the

Geological Survey Department, British Territories in Borneo, 2nd edition,Brunei State.

WILLIAMS, G. & VANN, I. 1987. Thegeometry of listric normal faults anddeformation in their hangingwalls. Journal of Structural Geology, 9, 789795.

XI AO, H. & SUPPE, J. 1992. Origin of rollover. A merican A ssociati on ofPetroleum Geologists Bulletin, 76, 509529.

Received 12November 1996; revised typescript accepted 26March 1997.


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Anatomy of Growth Fault Zones in Poorly Lithified Sandstones and Shales