anatomy of a normal fault with shale smear implications for fault seal

AAPG Bulletin, v. 86, no. 8 (August 2002), pp. 1367–1381 1367

Anatomy of a normal faultwith shale smear: Implicationsfor fault sealAtilla Aydin and Yehuda Eyal

ABSTRACT

We describe the geometry and structural attributes of an excep-tionally well-exposed normal fault with shale smear in southernIsrael. We discuss the mechanism by which the shale was emplacedinto the fault zone and compare and contrast it with other shaleemplacement mechanisms. The shaly unit, the Ora formation, isabout 110 m thick in normal stratigraphic position and is composedof a lower shale member of approximately 60 m and an upper shalemember of approximately 30 m, separated by a middle carbonate-bearing unit approximately 20 m thick. One or both of these shaleunits occur along the entire 2 km length of the fault, although witha drastically reduced thickness. The upper shale member vanishesalong a large part of the fault, but the lower shale appears to survivein the fault rock with less than 0.5 m thickness. Both shale unitshave been stretched for more than 250m (the fault throw) betweennearly planar discontinuities defined by the footwall or hanging-wallcutoff planes (duplex). Thus, the fault geometry, position, and dis-tribution of the remaining shale rocks reveal a smearing process bywhich the shale units reduce their thickness or vanish by thinningperpendicular to the fault and stretching parallel to the fault. Thecontinuity of the lower shale unit as a fault rock appears to be barelymaintained for a throw/thickness ratio of approximately 4 but notfor the upper shale unit, which has a throw/thickness ratio of ap-proximately 8. The brittle carbonate-bearing rocks within the shalyrocks are fractured and faulted and show boudinage at variousscales, which result in significant variation in the lithological andmechanical character of the fault zone along the throw interval. Thefaults and joints, however, do not appear to degrade the integrityof the smeared shale as a lateral barrier along the fault zone.

INTRODUCTION

Many societal issues in the earth sciences are closely related tothe flow of water, natural gas, oil, and chemical and radioactive

Copyright �2002. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received September 18, 2000; revised manuscript received January 11, 2002; final acceptanceFebruary 1, 2002.

AUTHORS

Atilla Aydin � Shale Smear Project,Department of Geological and EnvironmentalSciences, Stanford University, California;[email protected]

Atilla Aydin received his B.S. degree ingeological engineering from Istanbul TechnicalUniversity (Turkey) and his M.S. degree andPh.D. in geology from Stanford University.After three years of teaching at IstanbulTechnical University and ten years of teachingat Purdue University, he moved to StanfordUniversity in 1991 as a research professor ofstructural geology and geomechanics. He iscodirector of the Rock Fracture Project anddirector of the Shale Smear Project atStanford. His research interest includes howrocks break and fluids flow through fracturesand faults with a primary application tohydrocarbon entrapment, migration, andrecovery.

Yehuda Eyal � Department of Geology andMineralogy, Ben Gurion University, BeerSheva, Israel

Yehuda Eyal is currently a professor ofstructural geology in the Department ofGeological and Environmental Sciences at theBen-Gurion University of the Negev, Israel. Hereceived his B.S. degree (1965), M.S. degree(1967), and Ph.D. (1976) from the HebrewUniversity of Jerusalem, Israel. His currentresearch interests are focused ondevelopment of the Syrian Arc and therelationships between the Syrian Arc and theDead Sea stress fields in the Middle East;fractures and other mesostructures as strainand stress indicators; and kinematic analysisof shear zones, such as the eastern Sinai andcentral Sinai-Negev shear zones.

ACKNOWLEDGEMENTS

We thank Rich Gibson, Grant Skerlec,Al Lacazette, and Neil Hurley for theirthoughtful reviews of the manuscript. Thework by Aydin was funded by the StanfordShale Smear Project, including Conoco, Elf,Japan National Oil Corporation (JNOC), andNorsk Hydro as supporting members at thetime of the study. Aydin would like to thankthese companies for their generous support.Special thanks to Pete D’Onfro of Conoco forhis continuous encouragement.

1368 Normal Fault with Shale Smear

contaminants through rock (Nelson, 1985; Long et al.,1996). We commonly desire to know about the loca-tion and flow velocity of these substances and whatwould be required to enhance or retard their flow in agiven situation. Finding answers to these questions iscommonly difficult because fluid flow in rocks isstrongly heterogeneous, primarily because of faults andother types of fractures. The effect of faults on fluidflow is complex (Smith, 1980; Caine et al., 1996; Ay-din, 2000): Some faults are conduits for fluids, whereasothers are barriers. A good example of this conduit/barrier paradox occurs in the Monterey basins in Cali-fornia (Aydin, 2000), where faults in low-permeabilitysiliceous rocks transport fluids (Dholakia et al., 1998)and faults in high-permeability sandstones retard fluidflow (Antonellini and Aydin, 1994). The so-called seal-ing faults have been well known in the oil and gas in-dustry because of their dramatic impact on reservoirperformance and compartmentalization (for an over-view, see Weber [1997] and Knipe et al. [1998]). Thereduction of pore-throat size through granulation andthe presence of clay or shale in fault zones are the twomajor mechanisms proposed to explain the relativelylow permeability and high sealing capacity of thesefaults (Weber et al., 1978; Smith, 1980; Pittman,1981, 1992; Downey, 1984; Bouvier et al., 1989;Knipe et al., 1991; Hippler, 1993; Antonellini and Ay-din, 1994; Gibson, 1994; Weber 1997).

The occurrence of shale in subsurface fault zonesin major oil- and gas-producing provinces the worldover, including the Gulf Coast (Perkins, 1961; Smith,1966, 1980), the Niger Delta (Weber et al., 1978; Bou-vier et al., 1989; Jev et al., 1993; Koledoye et al.,2000), the North Sea (Hardmann and Booth, 1991;Sassi et al., 1992; Knott, 1993; Fristad et al., 1997),and the Columbus Basin, Trinidad (Gibson, 1994), iscommon knowledge. This information was largely ob-tained from gamma-ray and dipmeter logs. Core re-covery from these fault zones is very poor because ofdisintegration of fault rock during the coring process,and the resolution of seismic data is not high enoughto reveal fault-zone architecture and its content. Nev-ertheless, well tests, together with seismic data andwell logs, show that these faults have relatively lowpermeability, thereby providing lateral seals for hydro-carbons (Bouvier et al., 1989; Jev et al., 1993; Gibson,1994), very much like shale layers that commonly pro-vide top seals for reservoirs (Downey, 1984; Grunau,1987; Watts, 1987; Skerlec, 1999). However, there isevidence that flow parallel to shale-bearing fault zonesdoes occur (Weber et al., 1978) and that some of the

faults may leak laterally (Perkins, 1961; Smith, 1966),perhaps because of breakdown in the continuity of theshale body (Weber et al., 1978; Gibson, 1994; Weber,1997). These problems have necessitated field studiesof the geometry and distribution of shale bodies alongfault zones in structural analogs and the mechanismsof their emplacements (Weber et al., 1978; Lindsay etal., 1993; Gibson, 1994; Lehner and Pilaar, 1997; Hey-nekamp et al., 1999). Detailed field-based studies ofreservoir-scale faults with shale are rather few; never-theless, these few studies have described some fieldcases and identified mechanisms for incorporation ofshale into fault zones. The term “shale smearing” ap-parently is being used as a broad term to encompass allshearing-based mechanisms responsible for incorporat-ing shale into fault zones. We elaborate on this in afollowing section.

In this article, we describe the geometry and struc-tural features of a normal fault with shale smear in asequence of sedimentary rocks overlying a crystallinebasement in the Shelomo graben in eastern Sinai,southern Israel. The arid climate and a stratigraphicsequence dominated by rocks that are highly resistiveto erosion provide a unique opportunity to study thearchitecture of such a fault. We discuss the mechanismof the shale emplacement and compare this mecha-nism with other similar structures. This contributionshould help visualize the body of a smeared shale in arelatively large fault that could be detectable seismi-cally in a typical reservoir setting. Such an understand-ing is vital in evaluating the sealing potential of faultzones with shale smear and assessing where and howthis potential is likely to be reduced or breached.

GEOLOGIC SETT ING

The Shelomo graben is located about 10 km west-northwest of the town of Elat in the Gulf of Elat/Aqaba region (Figure 1) and was studied previously byseveral researchers, including Eyal (1967, 1973) andGarfunkel (1970). The graben structure defines anelongated body of sedimentary rocks (Figure 2) about22 km long and 750 m to 6 km wide and trends nearlynorth-south in the south and north-northeast–south-southwest in the north. The graben is surrounded onboth sides by crystalline rocks of Precambrian age (Fig-ure 3a). The two major boundary faults of the grabenin the south are the Shelomo fault in the east and theGishron fault in the west (Figures 1, 3a). Both faultsare steep normal faults (�80�), with offsets ranging be-

Aydin and Eyal 1369

Figure 1. Location map of the study area, northwest of the Gulf of Elat/Aqaba. The Shelomo graben with the bounding faults, theGishron in the east and Shelomo in the west, is in the center and trends northeast-southwest in the north and north-south in thesouth. The bold “Y” in the lower center of the graben marks the Yehoshafat fault. The coordinates are in Mercator grid of Israel, andeach grid is 5 � 5 km.

tween 600 and 1500 m and between 200 and 600 m,respectively. The fault system also has a sinistral strike-slip component estimated, based on a displaced syn-cline axis, to be about 450 m for the Shelomo fault(Eyal, 1973) and, based on a displaced Precambrianmetamorphic contact (Garfunkel, 1970), 8 km for abroader fault belt that includes the Shelomo andnearby subparallel faults.

The stratigraphy of the rock units exposed in thegraben comprises about 1500 m of sedimentary rocksof Cambrian–Miocene age (Eyal, 1967, 1973; Bartovet al., 1972) over the crystalline basement. Figure 2 isa geological column showing the formation names,thicknesses, and ages. A Precambrian regional pene-plain was overlain by the thick (470 m) Nubia sand-stone of continental affinity. The most relevant sectionto the present article is a carbonate shelf deposit ofnearly 900 m thick. This sequence includes the Ora

shales, which we refer to as the Ora formation in thisarticle to avoid confusion due to lithological implica-tions. This formation factors prominently in faulting aswell as folding. As shown in the right-hand column ofFigure 2, it is composed of an approximately 60 m–thick lower shale member and an approximately 30m–thick upper shale member separated by a middlecarbonate-bearing unit approximately 20 m thick,which also has minor thin shale beds. The lower mem-ber consists of three units, which from the bottom areabout 20 m of shale, about 15 m of alternating shaleand marly limestone layers of 5–30 cm thickness, andabout 25 m of shale in an upper shale unit.

Some shale units exist above and below the Oraformation; for instance, a 50 m–thick green shale isfound at the base of Paleogene, but this unit is outsideof the interval that we studied. Some thinner argilla-ceous units exist within the Hazera Formation, the


Figure 2. Geologic column. Note the Ora formation (about110 m thick) within a sedimentary sequence (1500 m thick). Asdescribed in the right-hand column, the Ora formation includesan upper shale unit (�30 m thick), a middle carbonaceous unit(�20 m thick), and a lower shaly unit (�60 m thick). The lowershaly member, from the bottom to the top, is approximately 20m of shale, approximately 15 m of alternating shale and marlylimestone layers, and approximately 25 m of shale in part ofthe lower upper shale unit. Other shale units in the regionalcolumn include a lower Paleogene unit (50 m thick) and a 20m–thick shale within the Hazera, the unit immediately belowthe Ora formation. Data are from Eyal (1973).

most important of which is an approximately 20 m–thick unit in the upper section of this formation.

The rocks within the graben are strongly faultedand folded. Tight folds, less than 1 km long, charac-terize the southern, narrow part of the graben, whereasbroad, gentle synclines characterize the northern,wider part. The common trend of the fold axes isnorth-northeast to northeast, oblique to the boundaryfaults. Shortening of the strata perpendicular to fold

axes in the southern, narrower part of the graben is upto 25%, whereas along the northern part it is only 3–4%. An area characterized by strike-slip faulting existsbetween the tight and gentle folding, where the She-lomo fault changes its trend from north-south to north-northeast.

Faulting responsible for the Shelomo graben oc-curred in at least two phases (Eyal, 1973). The firstphase occurred between the post–middle Eocene andpre–middle Miocene (Garfunkel et al., 1974) and isassociated with the early phase of the opening of theGulf of Elat/Aqaba, estimated to be at 20 Ma (Eyal etal., 1981). The second phase, of post–middle Mioceneage, is inferred from the central and southern parts ofthe graben along the Shelomo fault.

Numerous faults, predominantly normal with asmall sinistral component, occur within the graben.One of these faults, the Yehoshafat fault near the cen-ter of the graben (Figures 1, 3), is the focus of ourmoredetailed study.

ANATOMY OF A NORMAL FAULT WITHSHALE SMEAR

The Yehoshafat fault is a normal fault with a tracelength of about 2 km (Figure 3) and a maximum throwof about 250 m. The fault dips to the east at a highangle (�80�). The map trace of the fault zone is con-tinuously exposed between the two lateral ends.Deeply incised drainage (Figure 4), including a 35 m–high waterfall (Figure 5), provides an opportunity toclosely examine a vertical interval along more than 300m of topographic and structural relief.

Along its entire length and vertical trace, the faultis marked by what remains of the Ora formation (Fig-ures 3, 4). This spatial association is direct evidence forthe involvement of the shale units in the faulting pro-cess beyond an ordinary juxtaposition, because themaximum offset (250 m) across the fault far exceedsthe total thickness of the Ora formation (�110 m).The geometry of the shale units within the fault zone,and its comparison with that of the shales in normalstratigraphic positions in the footwall and hangingwall,make it possible to evaluate the geometric and me-chanical changes that occurred as the shale was en-trained into the fault zone (Figure 4).

The first important geometric feature of the faultzone is that the shaly units within the Ora formationform a monoclinal structure with a steeply dippingmiddle section and shallowly dipping limbs (Figures 4,

Aydin and Eyal 1371

Figure 3. (a) Geologic map ofthe southern part of theShelomo graben with thebounding faults, the Shelomo inthe east and the Gishron in thewest. The Yehoshafat fault, a 2km–long normal fault zone withabout 250 m maximum throw,is in the middle of the map.The rectangles show the loca-tions of Figure 3b and thewaterfall in Figures 4 and 5.(b) Enlargement of the faultzone showing schematically thedetails of the fault architecturedue to boudinage of the car-bonate units. (c) West-eastcross section (see Figure 3b forlocation) showing the shales ofthe lower member within thefault zone and the associatedcarbonates. The upper shalemember vanished along thefault.


Figure 4. Panoramic view due north showing the Ora formation in its normal position on the top left and entering into the Yehoshafatfault on the top right. Note that remains of this formation occur continuously along the fault zone (see also the geologic map inFigure 3a). Also shown is the location of the waterfall and its northern (N) and southern (S) exposures (see Figure 5a, b, respectively).

5, 6). However, unlike a simple monocline, here thesteeply dipping middle section corresponds to the faultzone in the form analogous to a duplex (Figures 6; 7a,b). The remains of the Ora formation are juxtaposedagainst the older footwall units and younger hanging-wall units in such a way that the shaly members dipsteeply in an orientation approximately parallel withthe fault zone defined by the footwall and hanging-wallcutoff planes of the older and younger units (Figure 6),respectively. This is why we use the term “duplex” todescribe the overall geometry, although the juxtapo-sition angles between the shaly layers within the faultzone and the brittle layers in the footwall and hangingwall are here much higher than those in typical duplexstructures in compressional tectonic environments.

Both topographically and structurally the lowestpart of the Ora formation is exposed at the bottom ofthe waterfall (Figure 5a). Here the geometry of thelayers suggests that the lower carbonate units of theformation diverge from the fault zone to assume theirnormal stratigraphic position in the hanging wall.

Thus, the emerging overall fault geometry and the po-sition of the shale units define the shale smear config-uration depicted schematically in Figure 6.

Detailed examination of the fault zone in the wa-terfall exposure shown in Figure 5 sheds light onto thecontent of the fault rock and the attenuation of thesmeared shale. Thinning perpendicular to the fault andstretching parallel to the fault of all units of the Oraformation in the fault zone are remarkable: The shaleswithin the lower member, with a composite thicknessof more than 45 m in the normal stratigraphic position(Figure 2), have been reduced to a thickness less than0.5 m in a few places on the waterfall exposure. Thisunit appears to have vanished in other locationsnearby, but the quality of the outcrop does not allowverification of this. The uppermost shale, which isabout 30 m thick, disappeared along significant partsof the fault and may be preserved only in isolatedpockets.

The carbonate-bearing members of the Ora for-mation appear to be extensively deformed, even brec-

Aydin and Eyal 1373

ciated. The alternating shale and marly limestone lay-ers of the lower Ora member are broken up by a seriesof small faults and joints within the fault zone (lowerright-hand side in Figure 5a). This process representsthe initial stage of the formation of boudins, whichoccurs along the fault zone in various scales, depend-ing on the relative thicknesses of shale and carbonateunits. An excellent example of a small-scale boudi-nage in the lowermost section of the carbonate-bear-ing package occurs along a limestone layer about 2 mthick, the offset pieces of which form a train within ashaly matrix (Figure 5a). A larger scale example forthis kind of fragmentation is suggested by Figure 5b,which shows a marly limestone block within theshales of the lower Ora formation. Similar boudins aresuggested by a train of limestone bodies of the lowerOra carbonates along the fault zone, as shown in Fig-ures 3b and 6.

The waterfall exposure (Figure 5) also providesvaluable information about the interaction betweenthe smeared shale and adjacent structures within thebrittle rock units in the footwall. Vertical fractures(thicker red lines) in the northern site (Figure 5a),probably with an initial extension origin, sliced apartpieces of the Nubia sandstone in the footwall (left-hand side). Some sandstone lenses (defined by a com-bination of thick red and yellow lines) were conse-quently removed from their original positions byshearing along the joints and bedding planes and wereincorporated into the fault zone. However, the Nubiasandstone in the southern waterfall area, about 20 mto the south (see Figure 4 for location), displays well-developed deformation-band faults (Figure 5b), in con-trast to the shearing along the joints and bedding planesin the northern outcrop.

The type, geometry, and distribution of the jointsand faults, shown in Figure 5, can help to evaluate thepossible effects these structures may have on the per-meability along and across the fault zone. Unambigu-ous evidence exists for faulting along the lateral bound-aries of the smeared shale. For example, immediatelysouth of the waterfall, a zone of the lower shale unit iswell defined by two border faults separating it from theNubia sandstone on the west side and from thecarbonate-bearing interval of the Ora formation on theeast side. The upper shale is also bounded by a pair offaults.

Many of the smaller faults in the alternating shaleand marly limestone at high angle to the fault zone,shown on the right-hand side of Figure 5a, do not ap-pear to cut across the smeared lower shale unit: They

terminate against the smeared shale. If this observationcan be generalized, it has important implications forthe integrity of the smeared shale and its sealing ca-pacity. Note that one set of the small faults at the pres-ent position displays apparent reverse offset (Figure5a); however, whether these faults formed as reversefaults or they represent rotated normal faults is notclear. Regardless of the sense of offset across the indi-vidual fault sets, the combined current kinematical ef-fect is such that the rocks are stretched parallel to themain fault zone and thinned perpendicular to it.

The smeared shale has been deformed by internalfaulting, jointing, and folding. The internal faultswithin the shale units are mostly parallel with thebounding faults and do not pose much hazard for thelateral sealing efficiency. However, in an environmentwith abnormally high fluid pressure, these faults maycontribute to fault-parallel fluid flow in vertical andlateral directions. The most conspicuous structurewithin the smeared lower shale is a network of gypsumveins (see photo inset in Figure 5a), with individual setsapproximately parallel with and perpendicular to thefault zone. As the photo shows, the set parallel withthe fault zone (G in photo inset in Figure 5a) is betterdeveloped. However, gypsum veins in this orientationare known to exist in the undeformed lower Ora shale(Eyal et al., 1981). Therefore, how gypsum veins arefactors in the stretching and thinning of the shale andtheir impact on the permeability and sealing potentialof the fault zone during and after faulting areuncertain.

DISCUSSION

We have shown that the Ora shales smeared betweena pair of fault surfaces, referred to as a fault duplex inthis article. Such a structure is defined by the footwalland hanging-wall cutoff planes along the Yehoshafatfault (Figure 6). This architecture and the associatedjuxtaposition angles between the smeared shale layersand the brittle layers in the footwall and hanging wall,as idealized in Figure 7a, uniquely separate it from amonocline over a high-angle basement fault (Figure7b) and from a ductile shear zone (see, for example,Twiss and Moores, 1992) with no angular juxtaposi-tion across the fault (Figure 7c). Furthermore, this ar-chitecture also differs from those of shale intrusions(dikes as illustrated in Figure 7d or mud volcanoes) andcommon juxtapositions due to offsets by brittle faults(Figure 7e).

Aydin and Eyal 1375

Below the point at which the lower Ora shale di-verges away from the fault zone to resume its normalposition in the hanging wall (Figure 5a), the fault con-tinues downward, probably between the Nubia sand-stone and the Hazera carbonates. Thus, judging fromthe existence of extensional fractures and faults, thecharacter of the fault zone, including its sealing prop-erty, is somewhat different below this point. A similarrelationship is inferred for the part of the fault abovethe point equivalent to the hanging-wall cutoff of theupper Ora shale (Figures 4, 6). These results highlightthe variation of the faulting process in the vertical di-

rection and its impact on the fault rock distribution ina multilithology sequence.

The spatial characteristics of the whole fault zoneelucidated in the preceding paragraphs may also be in-terpreted in a temporal sense (Figure 8): The initialfracturing may have occurred in one of the competentunits in a brittle mode, given that the tensile strengthof rocks is lower than their shear strength (Figure 8a).This process leads to the formation of normal faults byshearing across opening mode fractures. The shale unitsimply reacts to this offset in a ductile manner. Theshearing and stretching of the incompetent units

Figure 5. Waterfall exposures (see Figures 3a and 4 for locations). (a) Northern exposure (view looking toward north; outcropdelineated by blue line on the top and purple line on the bottom) showing details of the fault-zone architecture, as well as theassociated structures. The lower shale of the lower member of the Ora formation has been reduced to less than 0.5 m in a fewlocalities. The marly limestone layers within the carbonate-bearing unit have been faulted and jointed. The faults are of both reverseand normal types. Some isolated layers at the bottom of the sequence show boudinage. The inset shows the gypsum veins (G) withinthe smeared shale in the lower part of Figure 5a. The upper shale member vanished at the northern waterfall. (b) Southern exposure(view looking toward south) showing a steeply dipping fault surface with high-angle slickensides. Also shown is wedging of thecarbonate-bearing unit between two shale units in the lower member. The top shale wedges out toward the northern exposure inFigure 5a.


within a fault zone increase with progressive offsetacross the two competent units (Figure 8b). Eventu-ally, the bounding faults overlap and juxtapose thecompetent units against the incompetent shale,thereby forming a shale smear structure that is analo-gous to an extensional stepover or a duplex. This struc-ture elongates and thins as the throw increases (Figure8c). Note that in our scenario depicted in Figure 8a–c,the growth has been assumed to progress from the bot-tom to the top (a basement-driven deformation). Start-ing with the failure of the upper brittle unit, however,should result in similar fault architecture but with aprogression from the top to the bottom.

In the case of a lithologic sequence in which mul-tiple shale units occur in close proximity to each other,

the neighboring shale units merge into a compositefault rock with polygenetic shale units (Figure 8d).This has been described by Weber et al. (1978) andLehner and Pilaar (1997) and conceptualized by Kole-doye et al. (2000). This phenomenon is quite obviousalong the normal fault that we studied (see Figures 3b,5b). In some cases, however, it is not easy to identifythe origin of the smeared shale and determine whetherit contains a single shale unit or multiple shale units.

Remarkably, the integral thickness of the shaly lay-ers within the Ora formation (see Figure 2) has beenreduced from a total of 75–90 m to less than 0.5 m andmay even be at the critical vanishing point in a fewplaces along the fault. The continuity of the uppershale member (�30 m), the upper shale unit of the

Figure 6. Cross section show-ing a conceptual model basedon competency rather than li-thology. The incompetent uppershale member vanishes, but theother incompetent units withinthe lower shale member barelymaintain their presence withinthe fault zone. The overall faultstructure is analogous to a du-plex with internal complexitiesdue to the competent unitswithin the Ora formation.

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lower member (�25 m), and the thickest shaly unitwithin the Hazera carbonates (�20 m) underlying theOra formation has been broken down along a signifi-cant stretch of the fault. Considered individually, thesenumbers suggest a range of throw/thickness ratios fromapproximately 8 to approximately 13. The continuityof the lower shale unit (20 m or throw/thickness of�13) is difficult to explain; however, if the lowermember is considered (including the alternating shaleand marly limestone sequence), the throw/thicknessratio is calculated to be approximately 4. Thus, giventhat the continuity of the lower shale member appearsto be near a breakdown, the critical throw/thicknessratio corresponding to the loss of continuity of individ-ual smeared shale is more likely to be closer to ap-proximately 4. This number compares well with thevalues of 7 measured by Lindsay et al. (1993; “shalesmear factor” in their terminology) in the United King-

dom, 4 calculated by Gibson (1994) in the ColumbusBasin, and 5 measured by A. Younes and A. Aydin(1998, unpublished data) in the Gulf of Suez, Egypt.

An alternative method uses the shale gouge ratio,defined as the ratio of the cumulative shale thicknesswithin the throw interval to the fault throw (Fristad etal., 1997; Yielding et al., 1997). This calculation pro-vides a range of 0.3–0.4 (30–40%) depending on howthe shales within the carbonate-bearing units are ac-counted for and if the shale in the Hazera Formationis included in the total shale budget. In any case, thelower bound of this range is significantly higher than0.18–0.20, the critical value separating sealing andnonsealing faults in the subsurface at the North Sea bythis method. All we can conclude is that the range ofthe shale gouge ratio values from the fault zone in ourstudy, in which the continuity of the smeared shale ismerely maintained or may even be broken down, is

Figure 7. Comparison of thearchitecture of (a) a fault withshale smear, (b) a fault-coredmonocline, (c) a ductile shearzone, (d) a shale dike intrusion,and (e) a brittle fault zone withslices of semibrittle shale dis-placed by individual faultstrands.


nearly twice the critical ratio obtained from the faultsin the North Sea. Another method commonly used inthe industry is the smear gouge ratio (Skerlec, 1996,1999) (sand/shale ratio), which in principle shouldgive a result similar to the shale gouge ratio.

The distribution of fault-plane surfaces and otherassociated small faults and joints within and adjacentto the fault zone is important for fluid flow across andalong the fault zone. We have observed multiple slipsurfaces within the fault. With regard to other small-scale structures near or within a fault zone that hasshale smear, the only relevant studies are those byWe-ber et al. (1978) and Lehner and Pilaar (1997), whopublished schematic maps showing a series of smalldip-slip faults associated with a fault with shale smear.These secondary faults and their patterns are similar tothose observed in our study (Figure 5a) and idealizedin our conceptual model (Figure 6).

Heynekamp et al. (1999) and Gibson (1994) re-ported the presence of deformation-band faults withina sandstone adjacent to smeared shale. We have ob-served similar structures within the Nubia sandstoneadjacent to the normal fault that we studied (see, forexample, the right-hand side of Figure 5b). However,we also have observed faults formed by shearing acrosshigh-angle joints within the same formation about 20m north of this location (Figure 5a). The reason forsuch a dramatic change in the sandstone deformationmechanism in a short distance is puzzling and remainsto be investigated.

With regard to fluid flow across the smearedshale, we note that absolute permeability and capillarypressure determination of the smeared Ora shales ona micro scale (in the sense defined by Downey [1984,p. 1752]) is not of interest within the context of thisarticle. Methods of performing such a study are avail-able in the literature (Purcell, 1949; Hubbert, 1953;Berg, 1975; Brace, 1980; Watts, 1987). Suffice it tosay that a shale layer a few centimeters thick can sup-port a high column of hydrocarbons (Downey, 1984).Of interest, however, is evaluating the influence ofboth continuous and discontinuous deformation as-sociated with the smearing of the Ora shales on thefault permeability in a macro sense (Downey, 1984),which would provide a framework for hydraulic char-acterization of fault zones with shale smear in otherenvironments.

The small faults within the carbonate-bearingrocks in the Ora formation terminate against the du-plex boundary faults as described in a previous section(Figure 5a) and may not degrade the sealing potentialof the smeared lower shale. These faults display a con-jugate geometry (Figure 6), and regardless of the senseof slip across the individual fault sets, the faults resultin stretching parallel to, and thinning perpendicular to,the fault zone as defined by the duplex structure.

Figure 8. Conceptual model showing temporal evolution of afault with shale smear: (a) initial fracturing in lower brittle unit,(b) faulting of the older and younger brittle units and shearingof the shale, (c) stretching and thinning of the shale within anextensional fault step analogous to a duplex structure definedby the initial faults, and (d) merging of shale units if there ismore than just one shale unit in close proximity where the throwexceeds the thickness of the middle brittle unit. In this case, thefault rock is composite with polygenetic shales. Note the juxta-position of smeared shale and the fault rock formed by brittleprocesses in nonshaly units, primarily in Figure 8c, d.

Aydin and Eyal 1379

These observations are consistent with the picture de-picted by Weber et al. (1978) and Lehner and Pilaar(1997).

A large body of literature on experimental studiesof the deformation of both consolidated and uncon-solidated shale (Foster and De, 1971; Arch and Malt-man, 1990) and on core-based studies from ocean sed-iments (Knipe et al., 1991; Moore and Vrolijk, 1992)exists. These studies provide evidence for the rotationand realignment of clay minerals in localized zones,compaction, and brittle fracturing, as well as crystalplasticity. Of special interest is the effect of shearingon permeability: Permeability increases parallel to, anddecreases perpendicular to, the shear zone (Arch andMaltman, 1990). The gypsum veins were fluid-flowpathways at the time of their formation, and judgingfrom their aperture, they must have contributed to thefault-parallel flow of briny water. This relationship issimilar to that established for the sediments deformedin accretionary environments (Moore and Vrolijk,1992). The impact of the veins on the permeability ofthe smeared shale matrix following the precipitation ofgypsum has probably been negligible.

In the geological literature, there are various no-tions about mechanical mixing of various lithologicalrock units within fault zones and themechanism of thismixing. For example, based on their field observations,Lehner and Pilaar (1997) suggested that either no orvery little lithological admixing occurs in the smearedshale. However, a group of authors from Shell (Weberet al., 1978), including Lehner and Pilaar, also de-scribed lensoidal sand bodies within the smeared shalein the Niger Delta, suggesting that the internal struc-ture of the faults is more complicated than that madeup of a tabular body of deformed shale. Similarly, atrain of sandstone bodies surrounded by smeared shaleshas been schematically illustrated by Gibson (1994) inhis conceptual diagram. Our observations show thattrains of nonshale units are common within thesmeared zone and this mixing is due to boudinage ofbrittle units. The distribution of shale and nonshale li-thologies in both vertical and horizontal directionsvaries dramatically because of the boudinage configu-ration, and it changes from a carbonate-dominated sec-tion to an almost 100% shale section, as illustrated inFigure 5b. Although the boudinage geometry de-scribed in this article appears to be different from thenecking geometry proposed by Lehner and Pilaar(1997), the extreme variability of the smeared shalethickness is the common denominator for both atten-uation mechanisms. Other gouge concepts (FAPS

News, 1995; Skerlec, 1996, 1999; Fristad et al., 1997;Yielding et al., 1997) are hypothetical and stochasticand do not say anything about the fault architecture; aphysical interpretation of these other results is difficultbecause of the lack of a conceptual framework for themechanism of faulting in these methods.

A poorly understood aspect of shale smear is theeffects of the mechanical properties of the lithologiesinvolved. Direct surface observations and the subsur-face data show that the phenomenon occurs in bothunlithified sediments and lithified rocks (Weber et al.,1978; Lindsay et al., 1993). A certain degree of duc-tility is clearly a prerequisite for shale to be smeared;however, the details of this relationship are not known.The example given in this article represents a series ofUpper Cretaceous shale units within a carbonate shelfenvironment. The faulting started from the middleMiocene and probably continued to the present, giventhat the area is situated in a tectonically active region.

Another question regarding the scale effect onshale smear has been raised by Lindsay et al. (1993).Unfortunately, almost all of the outcrop examples re-ported by previous investigators come from eithersmall-scale faults with throws on the order of a fewmeters or less or from a rather restricted window ofobservation at a mine or a quarry (Weber et al., 1978;Lindsay et al., 1993). A broadscale range of faults withshale smear must be studied and analyzed for under-standing possible scale effects. This article provides in-formation from a large fault that should fill in the upperrange of scales and shows that the smearing occurs inreservoir-scale faults in a fashion similar to that insmall-size examples described from other locations.

CONCLUDING REMARKS

We described the architecture of a reservoir-scale nor-mal fault with shale. The Ora formation marks thetrace of the fault zone within the 250 m throw inter-val. The internal architecture of the fault zone isstrongly influenced by the distribution of the shaleunits, as well as the carbonate-bearing units, withinthe formation. The spatial distribution of these lithol-ogies shows significant variation along the faulted in-terval. The thickness of the smeared shale is reducedto less than 0.5 m and may be close to the vanishingpoint in a few places. The carbonate-bearing rock bod-ies show significant boudinage that results in a highdegree of lithological and structural variation alongthe fault zone. The relative proportion of the shale


within the carbonate-bearing units may have impor-tant implications for the behavior of these units.

We discussed several parameters related to(1) where and how faults may incorporate shale intothe fault zones, therein acting as a lateral seal for fluidmovement, and (2) where and how the sealing poten-tial of these faults may be breached because of thebreakdown in the continuity of the smeared shale andother secondary faults and joints. We compared andcontrasted our observations and interpretation withthose from other studies. In general, our results areconsistent with those from previous field-basedstudies.

Although we studied one fault, the results pre-sented in this article should help to visualize faultswithshale smear and the distribution of attendant structuresin analogous subsurface situations. We elucidated apossible temporal evolution of the fault zone in brittle/ductile multilayers. Quite probably, this temporal pro-gression and the dimensional characteristics of the brit-tle and ductile units are critical for the final fault-zonearchitecture. For example, the shale layers of the lowermember may have merged to form a composite shaleunit at a throw value of 15 m, the thickness of thealternating shale and marly limestone in the middle ofthe member. After this stage, the sum of the shaleswithin the lower member (�45 m) represents the ef-fective thickness for the shales for calculating thethrow/thickness ratio as used in our calculation pre-sented in a previous section.

Several problems are promising candidates for fu-ture studies in the area. Among these are (1) a detailedcharacterization of the distribution of the fault-zonematerial along the entire 2 km exposure of the fault,(2) a better lithological and petrophysical characteriza-tion of alternating shale and limestone layers and theirtemporal behavior within the fault zone, and (3) vari-ation of the faulting-related structures within the foot-wall or hanging-wall rocks adjacent to the fault zone.A good understanding of these topics is a prerequisitefor subsurface applications in which problems are al-most always underdetermined.

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anatomy of a normal fault with shale smear implications for fault seal

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