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Complexity in fault zone structure and implications for fault seal prediction
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Complexity in fault zone structure and implications for fault seal prediction C. Childs, J.J. Walsh and J. Watterson
In their simplest form, brittle faults consist of a single zone of intense deformation which macroscopically is seen as a slip surface and/or a zone of fault rock. More generally, fault zones have complex geometries with multiple slip surfaces and/or deformation zones. The most common pattern in complex fault zones observed at outcrop is a fault zone bounded by a pair of sub-parallel slip surfaces. In three dimensions, fault zones bounded by paired slip surfaces alternate both laterally and up/down dip with areas of only one slip surface. Within this overall framework, a range of fault rocks is irregularly distributed as spatially impersistent sheets and lenses. Due to seismically irresolvable complexities of fault zone structure, the juxtapositions of footwall and hangingwall rocks predicted from seismic data will in most cases be different from those actually present. The importance of such differences to the prediction of across-fault connectivity, of both hydraulically passive and hydraulically active fault zones, is strongly dependent on the reservoir sequence. Connectivities are calculated for hydraulically passive and active faults offsetting an Upper Brent Reservoir sequence. Shaley fault rocks within brittle fault zones often represent a spatially persistent, although variable thickness, component of the zones and provide a basis for the application of empirical methods of fault seal prediction to brittle faults. The distribution of fault rocks cannot be characterised from well data, raising the question of whether purely deterministic methods for fault seal prediction can ever be successful. The way forward is refinement of current empirical methods by achieving a more detailed characterisation of sub-surface faults, allowing more quantitative comparisons of target faults with those of known sealing behaviour.
Introduction
Data for characterisation of faults in the subsurface are limited to two sources, seismics and wells. Seismic reflection data allow the displacement distribution over a fault surface to be mapped while well and core data may allow determination of fault rock types and deformation mechanisms at specific points, in addition to characterising the lithologies of the host sequence. It is evident from outcrop studies that the internal geometries of fault zones are usually complex, in terms of the numbers of individual slip surfaces, the partitioning of slip between them and in the distribution of different fault rocks, all of which vary over a fault surface. This 3-D complexity of fault zone structure may not be apparent from either seismic or core data but is nevertheless crucial to the bulk hydraulic properties of a fault. A model for the development of the complex internal structures of fault zones has recently been proposed (Childs et al., 1996). Although this model does not increase the predictability of sub-surface fault zone structure, it demonstrates how complexity can arise from the operation of simple processes and provides a framework for consideration of the uncertainties inherent in prediction. The purpose of this paper is to describe and develop this model in terms relevant to the problems of fault seal prediction. While
the model represents a further step towards development of a deterministic method of fault seal prediction, the successful application of a reductionist approach to seal prediction remains a remote possibility. The fault sealing mechanisms considered are those which occur as a direct result of the faulting process, i.e., those due to either across-fault juxtapositions of reservoir and non-reservoir units or to the presence of sealing fault rocks, i.e., membrane seals. The diagenetic contribution to seals (Knipe, 1992) is not considered. Fault zone structure
Brittle fault zones comprise discrete slip surface(s) and fault rocks. There is a general positive correlation between fault displacement and the thickness and complexity of the fault zones (Robertson, 1983; Hull, 1988). Complex fault zones generally comprise multiple slip surfaces or zones of intense shear (Childs et al., 1996). The simplest and most common multi-slip fault zones observed in outcrop are structures with two discrete bounding slip surfaces, enclosing fault rock which may vary from intensely deformed to virtually undeformed (Koestler and Ehrmann, 1991; Childs et al., 1996). Where sufficient data are available, areas of a fault zone with the paired slip surface geometry can be seen to alternate with areas with a
Hydrocarbon Seals: Importancefor Exploration and Production edited by P. Mr
and A.G. Koestler. NPF Special Publication 7, pp. 61-72, Elsevier, Singapore. 9 Norwegian Petroleum Society (NPF) 1997
C. Childs, J.J. Walsh and J. Watterson
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Fig. I. Cartoon illustrating the asperity bifurcation model of fault zone widening. An irregularity on a fault surface (grey fill) in (a) is sheared off by the formation of a new slip surface in (b). Subsequent fault movement may result in deformation of the newly formed slip surface bounded lens.
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single slip surface or zone of intense deformation. This type of structure, with rock lenses bounded by slip surfaces, is developed by either or both of the processes, asperity bifurcation and tip-line bifurcation (Childs et al., 1996), illustrated in Figs. 1 and 2, respectively. Asperity bifurcation is due to the shearing off of fault surface irregularities by the formation of new slip surfaces. These irregularities may occur anywhere on a fault surface and on any scale. Irregular Fig. 2. Successive stages of the tip-line bifurcation process of fault zone widening and generation of paired bounding slip surfaces (see text). The tip-line of a fault surface (e), part of which is shown shaded in (a)-(d), propagates upwards through a rock volume. The area shown in (a)-(d) is indicated by the rectangle in (e). With fault growth the elliptical tip-line bounding the fault surface propagates radially to the successive positions, a-d, shown in (e). The lines labelled I-III in (a) indicate successive positions of the fault surface tipline.
e
Complexity in fault zone structure and implications f or fault seal prediction
fault surfaces can either be inherited from non-planar surfaces formed when a fault propagated or be developed during continued fault growth, for example by bedding-parallel slip. Strain of a rock volume adjacent to a fault (Barnett et al., 1987) is often partly accommodated by bedding-plane slip which disrupts existing planar fault surfaces. Asperity bifurcation at outcrop or larger scales is equivalent to the grain scale wear process described by Engelder (1978). A paired bounding slip surface geometry will persist until the newly formed active slip surface has significantly higher displacement than its predecessor. Tip-line bifurcation is a process related to the radial propagation of the fault surface tip-line which accompanies increase in fault displacement, as shown in Fig. 2e. Tip-lines are locally retarded where they encounter mechanical heterogeneities (Huggins et al., 1995) and tip-line embayments are formed as shown in Fig. 2a by local arrest of a propagating tip-line. An embayment locally divides the fault surface into two lobes which are free to propagate independently and slightly out of plane with respect both to the main fault surface and to one another (Fig. 2b). At this stage the overall fault tip-line has by-passed the point of embayment, as shown in Fig. 2e. With continued fault growth the two fault lobes propagate laterally and overlap one another (Fig. 2c) to form a relay zone (Peacock and Sanderson, 1994; Huggins et al., 1995). With further fault growth, failure occurs by linkage of the overlapping fault surfaces (Fig. 2d). This evolution is accompanied by increasing strain of the sliver of rock between the slip surfaces. The end result of this process is the formation of a fault bounded lens of relatively undeformed rock, a cross-section through which displays a paired bounding slip surface geometry. Both bifurcation processes are independent of scale and can result in the formation of slip surface separations and lens dimensions on different scales, often simultaneously. Whereas asperity bifurcation can occur at any point on a fault surface, the tip-line bifurcation process is restricted to tip-lines but can occur on a range of scales at the same location. The varied scales can be visualised by thinking of the simple elliptical tip-line shown in Fig. 2e as having embayments on all scales. Offset and overlapping fault geometries (Fig. 2b,c) occur on faults of all sizes (Griffiths, 1980; Larsen, 1988; Peacock and Sanderson, 1991; Stewart and Hancock, 1991; Peacock and Sanderson, 1994). The action of either, or both, bifurcation processes at a point on a fault surface generally results in a stepwise increase in fault zone thickness and in a highly complex internal fault zone geometry. The asperity bifurcation process, and the formation of new slip surfaces within a fault zone, may also cause
63
fault zone thinning by "structural erosion" of previously formed fault rock as fault displacement increases. Both bifurcation processes result, at least initially, in lenses or pods of relatively undeformed rock becoming incorporated in a fault zone. Fig. 3a-c shows an outcropping fault zone consisting of lenses of fault rock each of which is bounded by discrete slip surfaces, These lenses range from intensely deformed to virtually undeformed. Each lens of fault rock may differ from neighbouring lenses in respect of both deformation intensity and rock composition. Each lens is a distinct element many of which are interpreted as having been incorporated into the fault zone by a fault surface bifurcation event. In the fault zone illustrated (Fig. 3), the numerous slip surface bounded lenses of similar size are consistent with this fault zone having widened dominantly by asperity bifurcation. Many of the slip surfaces which form boundaries to lenses within this fault zone may have formed within the existing fault zone and therefore would not have contributed to widening of the fault zone. Fault zone thickness
Measurement of fault zone and fault rock thicknesses in complex fault zones (Fig. 4) can be very subjective (Evans, 1990). In particular, the distinction in either outcrop or core between a single multi-slip surface fault zone and two or more individual faults is dependent on the distances between slip surfaces relative to their displacements, their relative orientations, the deformation state of the intervening rock and the larger scale context. Slip surfaces which at one scale of observation appear as separate faults may, with a more extended view, be clearly seen to be part of a single fault zone. As this problem can occur on any scale of observation and is effectively intractable, it should be borne in mind when assessing fault zone thickness data. The fault zone thickness versus fault displacement data in Fig. 4 are assigned to one of three fault zone categories. These are (i) single zones of fault rock, (ii) complex fault zones containing both fault rock and weakly deformed lenses of rock enclosed by two or more slip surfaces, and (iii) unclassified zones. The data distribution is broadly similar to that shown by previous workers (Otsuki, 1978; Robertson, 1983; Segall and Pollard, 1983; Hull, 1988; Blenkinsop, 1989) except that the data in Fig. 4 define a wider band and larger range of fault zone thicknesses for a given throw value than previously published datasets. This difference is largely, but not entirely, due to inclusion of category (ii) fault zones. Inclusion of these
C. Childs, J.J. Walsh and J. Watterson
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a
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Fig. 3. (a) A normal fault with a throw of 18 m in a Carboniferous mixed sandstone/shale sequence from a quarry in Lancashire, UK. The fault zone dips towards the observer. The hangingwall rocks have been removed by quarrying operations to expose the fault zone rocks which occur as lenses. Outlines of the most prominent lenses are shown in (b). Lenses dominantly of sandstone are stippled and those dominantly of shale are shaded. Mixed sandstone/shale lenses also occur (shaded and heavy stipple). Rocks in the lenses range from almost undeformed to highly deformed. (c) Detail of the sandstone breccia lens marked SB in (b), which, during quarrying operations, has slipped along its lower bounding slip surface and broken open, revealing a ca. 1 m thickness of sandstone breccia which tapers in all directions. The hammer at the centre of the photograph is 0.5 m long.
complex fault zones increases by an order of magnitude the range of fault zone thicknesses for a given displacement. A complex multi-slip surface fault zone may even have a thickness greater than the fault displacement. Although prediction of fault hydraulic properties is relevant to both exploration and production, a differ-
ent prediction is required in each of the two situations. For exploration, prediction of the sealing capacity of a particular fault is required, while for production purposes prediction of the combined effects of many faults within a reservoir volume may be the main concern. Uncertainties in the two cases are therefore viewed differently. In exploration, a fault is
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Fig. 4. Plot of fault displacement versus fault zone thickness. Faults zones are distinguished as (i) simple fault zones comprising a single slip surface or zone of deformation (small crosses), (ii) complex fault zones comprising multiple (normally two) slip surfaces separated by undeformed or slightly deformed rock (squares), and (iii) fault zones n o t assigned to either category (filled circles). Many of the data assigned to category (i) are from published sources (Robertson, 1983; Hull, 1988; Otsuki, 1978; Segall and Pollard, 1983; Blenkinsop, 1989). Published data in category (ii) are from Wolf (1985). Data from a coastal section exposing small normal and oblique-slip faults cutting a chalk sequence at Flamborough Head, UK, represent maximum (large crosses) and minimum (small circumscribed crosses) fault rock thicknesses (category (i)) measured on individual fault traces.
predicted to be either sealing or non-sealing, and a level of uncertainty is attached to the prediction. In the case of production, uncertainty in the estimation of hydraulic properties of faults is expressed as uncertainty (or probability) in the results of reservoir flow simulation. The crucial difference between the two types of requirement can be summarised by saying that production requires estimation of the average hydraulic properties of a typical reservoir fault on a relatively short timescale, while exploration requires estimation of the hydraulic properties of the "weakest link" on a fault surface on a geological timescale. For production purposes, data relating fault displacement to fault thickness (Fig. 4) can readily be applied by taking a median line through the data distribution. For individual fault seal prediction it is the minimum rather than the average fault rock thickness in a fault zone which is crucial in determining whether or not an oil column can be supported. Fig. 4 shows that for a given throw, fault rock thickness varies by more than two orders of magnitude. B lenkinsop (1989) showed that, on a fault with 23 m strike-slip move-
65
ment, the fault rock thickness varied by an order of magnitude over an exposed fault trace length of only 12 m. If thickness had been measured over the entire fault surface rather than on a single short section, this range is likely to have been greatly extended. Fault rock thickness data from small (throw <11 m) normal and oblique slip faults from Flamborough Head, UK, show that on individual fault traces, fault rock thicknesses vary by more than two orders of magnitude (Fig. 4). The bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strikenormal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 5 0 - 1 0 0 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. Across-fault juxtapositions" sequence
Upper Brent
Hydraulically passive fault rocks The consequences of sub-resolution fault zone complexity can be assessed for both hydraulically passive and hydraulically active fault rocks. Hydraulically passive here refers to fault rocks which have hydraulic properties identical with their host rocks and, therefore, only primary juxtaposition effects need to be considered. Fig. 5a shows two fault zones which offset an Upper Brent sequence, each with an aggregate displacement of ca. 40 m. On the scale of observation, fault zone A comprises a single slip surface, while fault zone B comprises two parallel slip surfaces each of which accommodates about half of the total displacement. In this case the paired slip surfaces are separated by ca. 15 m of rock with low shear strain, as indicated by bedding re-orientation. Two slip surfaces would not be distinguished even with good quality seismic data. Across-fault juxtapositions calculated on the basis of a single slip surface would be valid in the case of fault A, but invalid for fault B. The consequences of incorrect juxtapositions on fault B are illustrated in Fig. 5b-d), for the Upper Brent sequence shown in Fig. 5a. Fig. 5b shows the range of across-fault juxtaposi-
66
C. Childs, J.J. Walsh and J. Watterson
a
b
Complexity in fault zone structure and implications for fault seal prediction
tions for the 84 m thick Upper Brent sequence for fault throws from 0 to 84 m; all the juxtapositions represented in this figure would occur at some point on a fault with throws from 0 to 84 m. The traces of fault zone A and both of the slip surfaces of fault zone B (Fig. 5a) are sufficiently short for the throws along them to be constant, and so to be represented in Fig. 5b by a vertical line. If the traces were longer and throws along them varied, then the locus of each fault trace in Fig. 5b would be curved rather than straight. Across-fault reservoir connectivity for a single slip surface, over the 0-84 m range of throw values, is represented in Fig. 5c. For hydraulically passive faults the across-fault connectivity is 0.65, or 65%, at zero throw and is identical with the sequence net/ gross. The connectivity is sharply decreased by even a small throw. The subsequent, approximately linear, reduction in connectivity with increasing throw, reflects the relatively regular and frequent alternation of reservoir and non-reservoir lithologies within the gross reservoir sequence (Knott, 1993). Minor irregularities in the curve are due to connectivity peaks when individual sandstone units are juxtaposed. Connectivities for fault zones comprising two slip surfaces with equal displacements are shown in Fig. 5d for aggregate throw values over the range 0-84 m; an aggregate throw of 40 m is represented by two slip surfaces each with a throw of 20 m. The connectivity for paired slip surfaces is derived from Fig. 5b as follows. For each reservoir unit within the fault zone and bounded by slip surfaces, the connectivity across each slip surface is measured and the minimum of these taken as the net connectivity for that unit. The process is repeated for all reservoir units within the fault zone and the aggregate of the net connectivities represents the connectivity of the fault zone. For the Brent sequence shown here, across-fault connectivity is higher for a single slip surface than paired slip surfaces for all throws less than 80 m. For this sequence the across-fault sandstone connectivity of fault zone A is 23% of the gross reservoir thickness, and that of fault zone B is 14%. The difference in connectivity between the two fault zones is relatively small in this example. In general, however, the
67
difference in connectivity between single and paired slip surface geometries is highly sequence dependent. For some reservoir sequences, fault zone B would have a higher connectivity than fault zone A.
Hydraulically active fault rocks Fault zones with hydraulically active fault rocks may act as either conduits or barriers to flow, but only the latter case is discussed here. Hydraulically active fault rocks in clastic reservoir sequences form primarily either by cataclasis of sandstones or by incorporation of shale into fault zones, or both. The effects on fault seal potential of fault rocks formed by cataclasis of reservoir quality sandstones are not considered. The hydraulic properties of these cataclastic rocks as measured in outcrop samples suggest that they can form hydrocarbon seals (Antonellini and Aydin, 1994), but there is no consensus on their importance in the trapping of large oil columns (Smith, 1980; Gibson, 1994). By contrast, shaley fault rocks are believed to support significant hydrocarbon columns and there is a variety of methods for relating the amount and distribution of shale in a faulted sequence to the sealing potential of faults within it (Bouvier et al., 1989; Gibson, 1994; Fristad et al., 1997, this volume). In one of the simplest and most widely applied methods, the percentage shale of that part of the sequence which has been displaced past a point is calculated for each point on the fault surface. The percentage shale calculated in this way is expressed as the shale gouge ratio (SGR), as shown colour coded in Fig. 5b. Given data on faults of known sealing behaviour in a particular reservoir or province, SGR can be calibrated to provide cut-off values for fault seal for target faults within the same reservoir or province (Fristad et al., 1997, this volume; see Implications for fault seal: fault rock distribution). The software used to produce Fig. 5b calculates SGRs, and a range of other fault seal parameters, to provide a range of uncalibrated values for a given reservoir sequence. The reservoir sequence is represented either as lithological units classified as either reservoir or non-reservoir or shale, or as a percentage shale curve as derived, for example, from a gamma log.
Fig. 5. (a) A typical Upper Brent gross reservoir sequence (reservoir units, grey; non-reservoir (shale), black) offset by two fault zones, A and B. The fault geometries are from a fault map of strike-slip faults offsetting vertical beds (Childs et al., 1996). (b) Sequence/throw juxtaposition diagram (Bentley and Barry, 1991) for the reservoir sequence shown on the left (same sequence as in (a)). The diagram illustrates the across-fault juxtapositions for the range of throw values 0-84 m, non-reservoir units are shown black. Footwall shale layers are represented by horizontal black lines and hangingwall shales by oblique lines. Reservoir/reservoir across-fault juxtapositions are colour coded (see key) according to the SGR (see text) calculated for each element of the surface area. (c) Across-fault connectivity curves for the reservoir sequence in (a) and (b). The reservoir/reservoir across-fault connectivity for a given SGR cut-off value is expressed as a proportion of the gross reservoir sequence thickness (84 m). Connectivity curves for SGR cut-off values are based on the premise that reservoir/reservoir juxtapositions are not connected if the SGR is above the specified cut-off value. The connectivity curve for SGR = 100 is the curve for simple juxtaposition, i.e., hydraulically passive faults. (d) Across fault connectivity curves for the same sequence as (c) but with the total throw distributed equally on two slip surfaces.
68 Connectivity curves for a single slip surface are shown in Fig. 5c for a range of SGR cut-off values, for the Upper Brent sequence shown in Fig. 5b. The forms of the connectivity curves are very different. For an SGR cut-off = 20 there is no connectivity for throws >33 m. For an SGR cut-off = 30, there is no connectivity for throws from 40 to 60 m, but 8% connectivity at a throw of 65 m. For an SGR cutoff = 40, there is a sharp increase in connectivity at 45 m when the Tarbert and Lower Ness reservoirs become juxtaposed. Connectivity curves for paired hydraulically active slip surfaces can be calculated by the same method used for hydraulically passive faults. Connectivity curves for a range of SGR cut-off values are shown in Fig. 5d. For the Upper Brent sequence the connectivity of a single slip surface, for a given SGR cutoff, is generally higher than that for a fault zone with paired slip surfaces. The difference in connectivity between paired and single slip surface geometries is greatest at an SGR cut-off = 40. Although connectivities for the single and paired slip surfaces, at SGR cut-off = 40, are equal at a throw of 25 m, they are 24% (the same as that for a hydraulically passive fault) and 3%, respectively, at a throw of 50 m. For an SGR cut-off = 20 the connectivities for the single and paired slip surfaces are similar at all throws, so the sequence is insensitive to fault zone structure at this cut-off value. These variations in relative connectivities between single and paired slip surface geometries illustrate the complexity and non-linear consequences of multiple slip surfaces.
Summary The significance of sub-seismic fault zone complexity to calculation of across-fault connectivities and of fault seal potential is very strongly dependent not only on the amount but also on the distribution of non-reservoir and shale units within the gross reservoir sequence. The effect of an increased number of slip surfaces within a fault zone is twofold, namely increased average connectivity across individual slip surfaces and increased probability of an individual reservoir unit being juxtaposed against a nonreservoir layer or shaley fault gouge. Complex interplay between these two opposing effects determines the overall connectivity of a fault zone. Fault surface bifurcation processes result in areas of a fault zone with paired bounding slip surfaces alternating with areas with only a single slip surface. Either laterally or up-/down-dip, the two slip surfaces of fault zone B may give way to a single slip surface. Similarly, it is unlikely that fault zone A is characterised everywhere by only a single slip surface. As
c. Childs, J.J. Walsh and J. Watterson
there is no requirement for displacement to be partitioned equally between two slip surfaces, the range of possible connectivities across a 40 m fault zone is much greater than is indicated by the simple example illustrated. If more than two slip surfaces are present, the range of possible connectivities is increased further. The complexity of sequence juxtapositions due to sub-seismic fault zone structure is therefore compounded by the 3-D variation of fault zone structure. We consider that the sensitivity of individual sequences to partitioning of slip onto multiple slip surfaces in 3-D can be assessed only in very general terms. Although sensitivity studies can be performed, existing empirical methods, which implicitly incorporate sub-seismic fault zone complexity, may represent the best approach to fault seal prediction (see below).
Implications for fault seal: fault rock distribution The combined effects of the asperity and tip-line bifurcation processes can result in the formation of fault zones with highly complex internal geometries and highly variable fault rock compositions and distributions. Even detailed information on the fault rocks at a point on the fault zone shown in Fig. 3a-c, as would be provided by a well for example, would not enable valid extrapolation even to nearby parts of the fault zone. The implications of fault zone complexity for the distribution of potential sealing lithologies over a fault surface are illustrated in Fig. 6a which shows a cross-section through a shale-rich normal fault zone. Two distinct layers of shale-derived fault rock are present, a thicker layer derived from an 8 m thick shale source layer and a thinner gouge layer derived from thinner shale units within the footwall sequence. This fault zone has paired external slip surfaces with the intervening rocks intensely deformed. One slip surface occurs along the very thin lower layer of shale gouge and the other along the contact between the thick shale layer and the hangingwall sandstones. The fault zone thins upwards from 2 m at the base of the illustrated section to 1 m at the top. Both the relative and absolute thicknesses of sandstone and shale within the fault zone are determine by lenses of footwall derived sandstone, so the thickness of shalederived fault rock varies rapidly within the section. Rapid changes in both fault rock deformation state and fault rock composition also occur along the fault strike. Fig. 7 shows a map section through a fault of similar throw and offsetting the same sequence as the fault shown in Fig. 6. The mapped surface is 1-2 m below the footwall cut-off of the 8 m thick shale unit and shows paired bounding slip surfaces. The fault
Complexity in fault zone structure and implications f or fault seal prediction
a
69
distribution precludes the possibility of these breccias forming a seal. The shales within these fault zones show highly irregular thickness variations but, nevertheless, display a high degree of conl~inuity. The sealing integrity of a fault would be maintained if this continuity occurred over the whole fault surface. However, given the very small proportion of an entire fault surface which is represented by any single crosssection or map trace, nothing can be concluded about the likelihood of a continuous shale layer over the whole fault surface. A cross-section through a series of minor faults offsetting a mixed sandstone shale sequence (Fig. 8)
b
Fig. 6. (a) Cross-section through a normal fault in a Carboniferous mixed sandstone/shale sequence from a quarry in Lancashire, UK. The fault dips to the left, with a throw of ca. 15 m. (b) Sketch of the outcrop in (a). The well developed shale gouge layer within the fault zone (black) is derived from an 8 m thick shale unit. The base of this shale unit is ca. 1 m above the top of the exposed face in the footwall and 1.5 m below ground level in the hangingwall. The fault separates sandstones in the hangingwall (stippled) from a mixed sandstone (no ornament) and shale (black) footwall sequence. Dense coarse stipple indicates sandstone breccia. Mixed sandstone and shale breccias are shown shaded with a coarse stipple. The boundary between the fault zone and the footwall country rock is a slip surface with a thin (<2 cm), but continuous, layer of clay gouge; this gouge layer is too thin to be represented in the figure. Only structural features which are clearly not the result of quarrying activity are shown in (b).
rocks include shale gouge, sandstone breccias and relatively undeformed blocks of sandstone and shale within which bedding has been rotated towards the hangingwall, as shown in the vertical cross-sections. The fault zone is of more or less constant thickness (2 m) over its mapped length but the fault rock content is highly variable. On cross-section C the fault zone contains 2 m of shale, of varying degree of deformation, while 8 m along-strike (cross-section B) the shale component is reduced to 3 cm of shale gouge. These rapid variations in fault rock content and, particularly, in the content of potentially sealing lithologies raise the question of whether or not deterministic predictions of fault seal potential are feasible. Sandstone derived breccias within the fault zones shown in Figs. 3, 6 and 7 occur either as thin sheets of limited extent or as localised lenses. This patchy
Fig. 7. Map (left) and three cross-sections, A-C, through a normal fault with ca. 15 m throw. The fault offsets the same sequence as shown in Fig. 7. The mapped surface is at a level ca. 1 m below the top of the outcrop in Fig. 6. Deformed shale, either strongly foliated or shale gouge, is shown in black. Shale with preserved bedding is dark shaded with broken lines indicating approximate bedding orientations. Hangingwall sandstones are ornamented with regular stipple and footwall sandstones with irregular stipple. Sandstone bedding directions on cross-sections are shown by thin lines. Sandstone breccias are indicated by a coarse stipple. The boundary between the fault zone and the footwall sandstones on cross-sections B and C is a discrete slip surface.
70
C. Childs, J.J. Walsh and J. Watterson
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Fig. 8. Cross-section showing a number of small faults offsetting a carboniferous sandstone/shale sequence. Thin shales are shown as subhorizontal lines. Fault traces are shown as steep lines and those with shale smears are shown as wide grey lines. There is no horizontal connectivity across this 2-D section but 3-D connectivity is likely.
shows the frequency with which shale is incorporated into the fault zones and the apparent absence of sandstone connectivity. However, this 2-D section allows no conclusion to be drawn concerning the sandstone connectivity in 3-D. The observations reported here are consistent with those of Lindsay et al. (1993) who concluded that for faults in the same working quarry as Figs. 3, 6, 7 and 8, shale smears in outcrop can be discontinuous when the displaced sequence has an SGR value of <15. This SGR value does not, however, imply a sharp cut-off and continuous shale smears do occur at SGR values <15. These results are broadly comparable with those of Fristad et al. (1997), who find that significant static seal on subsurface faults does not occur for SGR values <15 but does occur at SGR values >18. Gibson (1994) estimates the SGR cut-off value to be 2 2 - 3 0 . Variation in SGR cut-off values is expected for different datasets from different geological settings (Fristad et al., 1997). Observations of shale smear continuity from outcrop are not directly comparable with SGR cutoffs derived from sub-surface datasets, because outcrop observations are small scale and generally only 2-D (Gibson, 1994) and do not allow assessment of the sealing effects of shale-derived fault rock in the 3-D system. A mechanism by which regular and continuous clay smears are generated on relatively low strain rate faults in soft sediments has been described by Weber (1978) and Lehner and Pilaar (1997, this volume). Soft-sediment faults are effectively ductile shear zones and may lack the discontinuities and complexity typical of fault zones, in rocks of either low or high shear strength, characterised by high instantaneous slip rates, i.e., "brittle" faults. Although the faults we have described formed in already lithified sedi-
ments (Lindsay et al., 1993) and the clay smears do not show the thickness regularity of those described by Weber (1978), the continuity of the clay smears may be comparable. The frequency of thick shale smears in the quarry (as in Figs. 6 and 7) is such that the local quarrymen refer to them as "vertical shales".
Discussion Faults are not directly imaged on seismic sections but interpreted, where reflections are offset, as discrete fault surfaces. It is acknowledged by interpreters that these discrete surfaces may comprise two or more seismically unresolved slip surfaces. The bifurcation model of fault growth implies that multi-slip surface fault zones are the rule rather than the exception and that there will be a minimum of two discrete slip surfaces on at least some places on a fault zone. More generally, the high degree of complexity within brittle fault zones, which is evident on all scales of observation, means that no fault surface can be fully defined in three dimensions and on the relevant scale. The fault zone structure observed either at a point or on a cross-section of the fault surface, cannot be extrapolated any significant distance from the point or line of observation. Given these limitations, we question whether it is possible, even theoretically, to predict deterministically the sealing potential of fault zones in the sub-surface. Clearly, detailed observations of fault zones have a role in fault seal prediction. Data from wells which penetrate fault zones can be used to determine both the deformation mechanisms which operated and the fault rocks which are present or which are likely to occur. However, well data provide no constraints on the spatial distribution of the fault rocks and on the sealing potential of faults
Complexity in fault zone structure and implications f or fault seal prediction
on geological timescales. The poroperm properties of fault rock sampled from wells may provide useful constraints on the permeabilities of faults for production timescale flow simulations A contrasting approach to fault seal prediction is to apply the empirical method by which a prediction is made simply on the basis of the known behaviour of comparable faults. We suggest that progress will best be made by moderating the extreme empirical approach by applying deterministic methods to quantifying the degree of comparability between a target fault and faults of known behaviour. An important aspect of an empirical approach is quantifying the degree to which one fault can be considered a valid hydraulic analogue of another. Methods of fault seal prediction employing empirical databases in which faults of known sealing capacities are characterised by reference to shale smear parameters (Gibson, 1994; Fristad et al., 1997, this volume; Lehner and Pilaar, 1997, this volume) represent a combination of the deterministic and empirical approaches. The methods of shale smear parameterisation are, implicitly or otherwise, based on the assumption of a regular and predictable distribution of shale-derived fault rock within a fault zone. While the complexity in fault zones suggests that the juxtaposition diagrams on which these methods are based may not represent the actual across-fault juxtapositions, the methods have nevertheless achieved a measure of credibility within the industry. This apparent contradiction can, however, be resolved. Although the constructions on which fault seal potential calculations are based take no account of subseismic complexity of fault zones, the complexity is incorporated implicitly in the databases against which fault seal potentials are calibrated. For some reservoir sequences, calculated fault seal potentials are to some degree independent of a certain level of fault zone complexity. Figs. 6 and 7 illustrate the extreme spatial variation in thickness of shale-derived fault rock within a fault zone but, together with Fig. 8, they also demonstrate the high degree of continuity of shalederived fault rocks which can occur. It is this continuity of shale-derived fault rock which, despite the complexity of its distribution, provides a physical basis for methods of prediction of fault seal potential based on shale smear parameterisation.
Conclusions Observations show that the internal structure of fault zones is highly complex, largely as a consequence of bifurcation processes. Simple examples of fault zone complexity, in which displacement on a fault of constant displacement is partitioned onto two
71
slip surfaces, illustrate the sensitivity of across-fault connectivity to fault zone structure. In general fault zones have multiple slip surfaces separating volumes of mildly to intensely deformed rock, with chaotic spatial variation in structure of the fault zone. We question whether it is even theoretically possible to characterise deterministically, complex fault zone architectures of subsurface faults, given the limited data available. Empirical risking methods implicitly take account of the unpredictable complexities in fault zone structure which inevitably are present. The way forward is by further refinement of current empirical methods, i.e., by more detailed characterisation of sub-surface faults to allow more objective comparison of target faults and faults of known sealing behaviour.
Acknowledgements This research was part funded by the EC Joule II Reservoir Engineering Project (Contract No. JOU2CT92-0182) and the EC Joule III PUNQ Project (Contract No. JOF3-CT95-0006). The lithological sequence in Fig. 5 was interpreted by Kees Geel from a well log provided by Norsk Hydro. We are grateful to Barry Murphy and Nick Lindsay for fieldwork leading to the production of Fig. 8. We thank Marie Eeles for help in preparing the manuscript. Peter Keller and Andreas Koestler are thanked for their helpful reviews of the manuscript.
References Antonellini, M. and Aydin, A. 1994. Effect of faulting on fluid flow in porous sandstones: petrophysical properties. Am. Assoc. Pet. Geol. Bull., 78: 355-377. Barnett, J.A.M., Mortimer, J., Rippon, J., Walsh, J.J. and Watterson, J. 1987. Displacement geometry in the volume containing a single normal fault. Am. Assoc. Pet. Geol. Bull., 71: 925-937. Bentley, M.R. and Barry, J.J. 1991. Representation of fault sealing in a reservoir simulation: Cormorant block IV UK North Sea. In: 66th Ann. Tech. Conf. and Exhibition of the Soc. Pet. Engineers, Dallas TX, pp. 119-126. Blenkinsop, T.G. 1989. Thickness-displacement relationships for deformation zones: discussion. J. Struct. Geol., 11: 1051-1054. Bouvier, J.D., Kaars-Sijpesteijn, C.H., Kluesner, D.F., Onyejekwe, C.C. and Van Der Pal, R.C. 1989. Three-dimensional seismic interpretation and fault sealing investigations, Nun River Field, Nigeria. Am. Assoc. Pet. Geol. Bull., 73: 1397-1414. Childs, C., Watterson, J. and Walsh, J.J. 1996. A model for the structure and development of fault zones. J. Geol. Soc. London, 153: 337-340. Engelder, T. 1978. Aspects of asperity-surface interaction and surface damage of rocks during experimental frictional sliding. Pure Appl. Geophys., 116: 705-716. Evans, J.P. 1990. Thickness-displacement relationships for fault zones. J. Struct. Geol., 12: 1061-1065. Fristad, T., Groth, A., Yielding, G. and Freeman, B. 1997. Quantitative fault seal prediction: a case study from Oseberg Syd. In: P. Mr and A.G. Koestler (Editors), Hydrocarbon Seals:
C. Childs, J.J. Walsh and J. Watterson
72 Importance for Exploration and Production. Norwegian Petroleum Society (NPF), Special Publication No. 7. Elsevier, Singapore, pp. 107-124. Gibson, R.G. 1994. Fault zones seals in siliciclastic strata of the Columbus Basin, offshore Trinidad. Am. Assoc. Pet. Geol. Bull., 78: 1372-1385. Griffiths, P.S. 1980. Box-fault systems and ramps: atypical associations of structures from the eastern shoulder of the Kenya Rift. Geol. Mag., 117: 579-586. Huggins, P., Watterson, J., Walsh, J.J. and Childs, C. 1995. Relay zone geometry and displacement transfer between normal faults recorded in coal-mine plans. J. Struct. Geol., 17: 1741-1755. Hull, J. 1988. Thickness-displacement relationships for deformation zones. J. Struct. Geol., 10: 431-435. Knipe, R.J. 1992. Faulting processes and fault seal. In: R.M. Larsen, H. Brekke, B.T. Larsen and E. Talleras (Editors), Structural and Tectonic Modelling and its Application to Petroleum Geology. Norwegian Petroleum Society Special Publication (NPF), Special Publication 1. Elsevier, Amsterdam, pp. 325-342. Knott, S.D. 1993. Fault seal analysis in the North Sea. Am. Assoc. Pet. Geol. Bull., 77: 778-792. Koestler, A.G. and Ehrmann, W.U. 1991. Description of brittle extensional features in chalk on the crest of a salt ridge (NW Germany). In: A.M. Roberts, G. Yielding and B. Freeman (Editors), The Geometry of Normal Faults, Special Publication 56. Geol. Soc. London, pp. 113-123. Larsen, P.-H. 1988. Relay structures in a Lower Permian basementinvolved extension system, East Greenland. J. Struct. Geol., 10: 3-8. Lehner, F.K. and Pilaar, W.F. 1997. On a mechanism of clay smear emplacement in synsedimentary normal faults. In: P. Mr Pedersen and A.G. Koestler (Editors), Hydrocarbon seals: Importance for Exploration and Production. Norwegian Petroleum Society (NPF), Special Publication 7. Elsevier, Singapore, pp. 39-50.
C. CHILDS J.J. WALSH J. WATTERSON
Lindsay, N.G., Murphy, F.C., Walsh, J.J. and Watterson, J. 1993. Outcrop studies of shale smears on fault surfaces. In: S. Hint and A.D. Bryant (Editors), The Geological Modelling of Hydrocarbon Reservoirs and Outcrop Analogues. Int. Assoc. Sediment., 15: 113-123. Otsuki, K. 1978. On the relationship between the width of shear zone and the displacement along fault. J. Geol. Soc. Jpn., 84:661-669. Peacock, D.C.P. and Sanderson, D.J. 1991. Displacements, segment linkage and relay ramps in normal fault zones. J. Struct. Geol., 13: 721-733. Peacock, D.C.P. and Sanderson, D.J. 1994. Geometry and development of relay ramps in normal fault systems. Am. Assoc. Pet. Geol. Bull., 78: 147-165. Robertson, E.C. 1983. Relationship of fault displacement to gouge and breccia thickness. Soc. Mining Eng., Am. Inst. Mining Eng. Trans., 35: 1426-1432. Segall, P. and Pollard, D.D. 1983. Joint formation in granitic rock of the Sierra Nevada. Am. Assoc. Pet. Geol. Bull., 94: 563-575. Smith, D.A. 1980. Sealing and non-sealing faults in Louisiana Gulf Coast salt basin. Am. Assoc. Pet. Geol. Bull., 64: 145-172. Stewart, I.S. and Hancock, P.L. 1991. Scales of structural heterogeneity with neotectonic normal fault zones in the Aegean region. J. Struct. Geol., 13:191-204. Weber, K.J., Mandl, G., Pilaar, W.F., Lehner, F. and Precious, R.G. 1978. The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures. In: 10th Ann. Offshore Technology Conf., Soc. Pet. Eng., Paper No. 3356, pp. 2643-2653. Wolf, R. 1985. Tiefentektonik des linksneiderrheinischen Steinkohlengebietes. In: G. Drozdzewski, H. Engel, R. Wolf and V. Wrede (Editors), Beitr~ige zur Tiefentektonik westdeutscher Steinkolhlenlagerst~itten, Geologisches Landesamt NordrheinWestfalen, Krefeld.
Fault Analysis Group, Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK Fault Analysis Group, Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK Fault Analysis Group, Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK
Complexity in fault zone structure and implications for fault seal prediction C. Childs, J.J. Walsh and J. Watterson
In their simplest form, brittle faults consist of a single zone of intense deformation which macroscopically is seen as a slip surface and/or a zone of fault rock. More generally, fault zones have complex geometries with multiple slip surfaces and/or deformation zones. The most common pattern in complex fault zones observed at outcrop is a fault zone bounded by a pair of sub-parallel slip surfaces. In three dimensions, fault zones bounded by paired slip surfaces alternate both laterally and up/down dip with areas of only one slip surface. Within this overall framework, a range of fault rocks is irregularly distributed as spatially impersistent sheets and lenses. Due to seismically irresolvable complexities of fault zone structure, the juxtapositions of footwall and hangingwall rocks predicted from seismic data will in most cases be different from those actually present. The importance of such differences to the prediction of across-fault connectivity, of both hydraulically passive and hydraulically active fault zones, is strongly dependent on the reservoir sequence. Connectivities are calculated for hydraulically passive and active faults offsetting an Upper Brent Reservoir sequence. Shaley fault rocks within brittle fault zones often represent a spatially persistent, although variable thickness, component of the zones and provide a basis for the application of empirical methods of fault seal prediction to brittle faults. The distribution of fault rocks cannot be characterised from well data, raising the question of whether purely deterministic methods for fault seal prediction can ever be successful. The way forward is refinement of current empirical methods by achieving a more detailed characterisation of sub-surface faults, allowing more quantitative comparisons of target faults with those of known sealing behaviour.
Introduction
Data for characterisation of faults in the subsurface are limited to two sources, seismics and wells. Seismic reflection data allow the displacement distribution over a fault surface to be mapped while well and core data may allow determination of fault rock types and deformation mechanisms at specific points, in addition to characterising the lithologies of the host sequence. It is evident from outcrop studies that the internal geometries of fault zones are usually complex, in terms of the numbers of individual slip surfaces, the partitioning of slip between them and in the distribution of different fault rocks, all of which vary over a fault surface. This 3-D complexity of fault zone structure may not be apparent from either seismic or core data but is nevertheless crucial to the bulk hydraulic properties of a fault. A model for the development of the complex internal structures of fault zones has recently been proposed (Childs et al., 1996). Although this model does not increase the predictability of sub-surface fault zone structure, it demonstrates how complexity can arise from the operation of simple processes and provides a framework for consideration of the uncertainties inherent in prediction. The purpose of this paper is to describe and develop this model in terms relevant to the problems of fault seal prediction. While
the model represents a further step towards development of a deterministic method of fault seal prediction, the successful application of a reductionist approach to seal prediction remains a remote possibility. The fault sealing mechanisms considered are those which occur as a direct result of the faulting process, i.e., those due to either across-fault juxtapositions of reservoir and non-reservoir units or to the presence of sealing fault rocks, i.e., membrane seals. The diagenetic contribution to seals (Knipe, 1992) is not considered. Fault zone structure
Brittle fault zones comprise discrete slip surface(s) and fault rocks. There is a general positive correlation between fault displacement and the thickness and complexity of the fault zones (Robertson, 1983; Hull, 1988). Complex fault zones generally comprise multiple slip surfaces or zones of intense shear (Childs et al., 1996). The simplest and most common multi-slip fault zones observed in outcrop are structures with two discrete bounding slip surfaces, enclosing fault rock which may vary from intensely deformed to virtually undeformed (Koestler and Ehrmann, 1991; Childs et al., 1996). Where sufficient data are available, areas of a fault zone with the paired slip surface geometry can be seen to alternate with areas with a
Hydrocarbon Seals: Importancefor Exploration and Production edited by P. Mr
and A.G. Koestler. NPF Special Publication 7, pp. 61-72, Elsevier, Singapore. 9 Norwegian Petroleum Society (NPF) 1997
C. Childs, J.J. Walsh and J. Watterson
62
a
a
b
b C
Fig. I. Cartoon illustrating the asperity bifurcation model of fault zone widening. An irregularity on a fault surface (grey fill) in (a) is sheared off by the formation of a new slip surface in (b). Subsequent fault movement may result in deformation of the newly formed slip surface bounded lens.
" ,~iiiiii!iiii!iiiiiii!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiili!i!i~ ............ ~,iiiiiiiiiiiiiiiiiiiiiiiiiiiiiii!!ii!iiiii~ ......
single slip surface or zone of intense deformation. This type of structure, with rock lenses bounded by slip surfaces, is developed by either or both of the processes, asperity bifurcation and tip-line bifurcation (Childs et al., 1996), illustrated in Figs. 1 and 2, respectively. Asperity bifurcation is due to the shearing off of fault surface irregularities by the formation of new slip surfaces. These irregularities may occur anywhere on a fault surface and on any scale. Irregular Fig. 2. Successive stages of the tip-line bifurcation process of fault zone widening and generation of paired bounding slip surfaces (see text). The tip-line of a fault surface (e), part of which is shown shaded in (a)-(d), propagates upwards through a rock volume. The area shown in (a)-(d) is indicated by the rectangle in (e). With fault growth the elliptical tip-line bounding the fault surface propagates radially to the successive positions, a-d, shown in (e). The lines labelled I-III in (a) indicate successive positions of the fault surface tipline.
e
Complexity in fault zone structure and implications f or fault seal prediction
fault surfaces can either be inherited from non-planar surfaces formed when a fault propagated or be developed during continued fault growth, for example by bedding-parallel slip. Strain of a rock volume adjacent to a fault (Barnett et al., 1987) is often partly accommodated by bedding-plane slip which disrupts existing planar fault surfaces. Asperity bifurcation at outcrop or larger scales is equivalent to the grain scale wear process described by Engelder (1978). A paired bounding slip surface geometry will persist until the newly formed active slip surface has significantly higher displacement than its predecessor. Tip-line bifurcation is a process related to the radial propagation of the fault surface tip-line which accompanies increase in fault displacement, as shown in Fig. 2e. Tip-lines are locally retarded where they encounter mechanical heterogeneities (Huggins et al., 1995) and tip-line embayments are formed as shown in Fig. 2a by local arrest of a propagating tip-line. An embayment locally divides the fault surface into two lobes which are free to propagate independently and slightly out of plane with respect both to the main fault surface and to one another (Fig. 2b). At this stage the overall fault tip-line has by-passed the point of embayment, as shown in Fig. 2e. With continued fault growth the two fault lobes propagate laterally and overlap one another (Fig. 2c) to form a relay zone (Peacock and Sanderson, 1994; Huggins et al., 1995). With further fault growth, failure occurs by linkage of the overlapping fault surfaces (Fig. 2d). This evolution is accompanied by increasing strain of the sliver of rock between the slip surfaces. The end result of this process is the formation of a fault bounded lens of relatively undeformed rock, a cross-section through which displays a paired bounding slip surface geometry. Both bifurcation processes are independent of scale and can result in the formation of slip surface separations and lens dimensions on different scales, often simultaneously. Whereas asperity bifurcation can occur at any point on a fault surface, the tip-line bifurcation process is restricted to tip-lines but can occur on a range of scales at the same location. The varied scales can be visualised by thinking of the simple elliptical tip-line shown in Fig. 2e as having embayments on all scales. Offset and overlapping fault geometries (Fig. 2b,c) occur on faults of all sizes (Griffiths, 1980; Larsen, 1988; Peacock and Sanderson, 1991; Stewart and Hancock, 1991; Peacock and Sanderson, 1994). The action of either, or both, bifurcation processes at a point on a fault surface generally results in a stepwise increase in fault zone thickness and in a highly complex internal fault zone geometry. The asperity bifurcation process, and the formation of new slip surfaces within a fault zone, may also cause
63
fault zone thinning by "structural erosion" of previously formed fault rock as fault displacement increases. Both bifurcation processes result, at least initially, in lenses or pods of relatively undeformed rock becoming incorporated in a fault zone. Fig. 3a-c shows an outcropping fault zone consisting of lenses of fault rock each of which is bounded by discrete slip surfaces, These lenses range from intensely deformed to virtually undeformed. Each lens of fault rock may differ from neighbouring lenses in respect of both deformation intensity and rock composition. Each lens is a distinct element many of which are interpreted as having been incorporated into the fault zone by a fault surface bifurcation event. In the fault zone illustrated (Fig. 3), the numerous slip surface bounded lenses of similar size are consistent with this fault zone having widened dominantly by asperity bifurcation. Many of the slip surfaces which form boundaries to lenses within this fault zone may have formed within the existing fault zone and therefore would not have contributed to widening of the fault zone. Fault zone thickness
Measurement of fault zone and fault rock thicknesses in complex fault zones (Fig. 4) can be very subjective (Evans, 1990). In particular, the distinction in either outcrop or core between a single multi-slip surface fault zone and two or more individual faults is dependent on the distances between slip surfaces relative to their displacements, their relative orientations, the deformation state of the intervening rock and the larger scale context. Slip surfaces which at one scale of observation appear as separate faults may, with a more extended view, be clearly seen to be part of a single fault zone. As this problem can occur on any scale of observation and is effectively intractable, it should be borne in mind when assessing fault zone thickness data. The fault zone thickness versus fault displacement data in Fig. 4 are assigned to one of three fault zone categories. These are (i) single zones of fault rock, (ii) complex fault zones containing both fault rock and weakly deformed lenses of rock enclosed by two or more slip surfaces, and (iii) unclassified zones. The data distribution is broadly similar to that shown by previous workers (Otsuki, 1978; Robertson, 1983; Segall and Pollard, 1983; Hull, 1988; Blenkinsop, 1989) except that the data in Fig. 4 define a wider band and larger range of fault zone thicknesses for a given throw value than previously published datasets. This difference is largely, but not entirely, due to inclusion of category (ii) fault zones. Inclusion of these
C. Childs, J.J. Walsh and J. Watterson
64
a
b
C
Fig. 3. (a) A normal fault with a throw of 18 m in a Carboniferous mixed sandstone/shale sequence from a quarry in Lancashire, UK. The fault zone dips towards the observer. The hangingwall rocks have been removed by quarrying operations to expose the fault zone rocks which occur as lenses. Outlines of the most prominent lenses are shown in (b). Lenses dominantly of sandstone are stippled and those dominantly of shale are shaded. Mixed sandstone/shale lenses also occur (shaded and heavy stipple). Rocks in the lenses range from almost undeformed to highly deformed. (c) Detail of the sandstone breccia lens marked SB in (b), which, during quarrying operations, has slipped along its lower bounding slip surface and broken open, revealing a ca. 1 m thickness of sandstone breccia which tapers in all directions. The hammer at the centre of the photograph is 0.5 m long.
complex fault zones increases by an order of magnitude the range of fault zone thicknesses for a given displacement. A complex multi-slip surface fault zone may even have a thickness greater than the fault displacement. Although prediction of fault hydraulic properties is relevant to both exploration and production, a differ-
ent prediction is required in each of the two situations. For exploration, prediction of the sealing capacity of a particular fault is required, while for production purposes prediction of the combined effects of many faults within a reservoir volume may be the main concern. Uncertainties in the two cases are therefore viewed differently. In exploration, a fault is
Complexity in fault zone structure and implications for fault seal prediction
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Fig. 4. Plot of fault displacement versus fault zone thickness. Faults zones are distinguished as (i) simple fault zones comprising a single slip surface or zone of deformation (small crosses), (ii) complex fault zones comprising multiple (normally two) slip surfaces separated by undeformed or slightly deformed rock (squares), and (iii) fault zones n o t assigned to either category (filled circles). Many of the data assigned to category (i) are from published sources (Robertson, 1983; Hull, 1988; Otsuki, 1978; Segall and Pollard, 1983; Blenkinsop, 1989). Published data in category (ii) are from Wolf (1985). Data from a coastal section exposing small normal and oblique-slip faults cutting a chalk sequence at Flamborough Head, UK, represent maximum (large crosses) and minimum (small circumscribed crosses) fault rock thicknesses (category (i)) measured on individual fault traces.
predicted to be either sealing or non-sealing, and a level of uncertainty is attached to the prediction. In the case of production, uncertainty in the estimation of hydraulic properties of faults is expressed as uncertainty (or probability) in the results of reservoir flow simulation. The crucial difference between the two types of requirement can be summarised by saying that production requires estimation of the average hydraulic properties of a typical reservoir fault on a relatively short timescale, while exploration requires estimation of the hydraulic properties of the "weakest link" on a fault surface on a geological timescale. For production purposes, data relating fault displacement to fault thickness (Fig. 4) can readily be applied by taking a median line through the data distribution. For individual fault seal prediction it is the minimum rather than the average fault rock thickness in a fault zone which is crucial in determining whether or not an oil column can be supported. Fig. 4 shows that for a given throw, fault rock thickness varies by more than two orders of magnitude. B lenkinsop (1989) showed that, on a fault with 23 m strike-slip move-
65
ment, the fault rock thickness varied by an order of magnitude over an exposed fault trace length of only 12 m. If thickness had been measured over the entire fault surface rather than on a single short section, this range is likely to have been greatly extended. Fault rock thickness data from small (throw <11 m) normal and oblique slip faults from Flamborough Head, UK, show that on individual fault traces, fault rock thicknesses vary by more than two orders of magnitude (Fig. 4). The bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strikenormal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 5 0 - 1 0 0 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. Across-fault juxtapositions" sequence
Upper Brent
Hydraulically passive fault rocks The consequences of sub-resolution fault zone complexity can be assessed for both hydraulically passive and hydraulically active fault rocks. Hydraulically passive here refers to fault rocks which have hydraulic properties identical with their host rocks and, therefore, only primary juxtaposition effects need to be considered. Fig. 5a shows two fault zones which offset an Upper Brent sequence, each with an aggregate displacement of ca. 40 m. On the scale of observation, fault zone A comprises a single slip surface, while fault zone B comprises two parallel slip surfaces each of which accommodates about half of the total displacement. In this case the paired slip surfaces are separated by ca. 15 m of rock with low shear strain, as indicated by bedding re-orientation. Two slip surfaces would not be distinguished even with good quality seismic data. Across-fault juxtapositions calculated on the basis of a single slip surface would be valid in the case of fault A, but invalid for fault B. The consequences of incorrect juxtapositions on fault B are illustrated in Fig. 5b-d), for the Upper Brent sequence shown in Fig. 5a. Fig. 5b shows the range of across-fault juxtaposi-
66
C. Childs, J.J. Walsh and J. Watterson
a
b
Complexity in fault zone structure and implications for fault seal prediction
tions for the 84 m thick Upper Brent sequence for fault throws from 0 to 84 m; all the juxtapositions represented in this figure would occur at some point on a fault with throws from 0 to 84 m. The traces of fault zone A and both of the slip surfaces of fault zone B (Fig. 5a) are sufficiently short for the throws along them to be constant, and so to be represented in Fig. 5b by a vertical line. If the traces were longer and throws along them varied, then the locus of each fault trace in Fig. 5b would be curved rather than straight. Across-fault reservoir connectivity for a single slip surface, over the 0-84 m range of throw values, is represented in Fig. 5c. For hydraulically passive faults the across-fault connectivity is 0.65, or 65%, at zero throw and is identical with the sequence net/ gross. The connectivity is sharply decreased by even a small throw. The subsequent, approximately linear, reduction in connectivity with increasing throw, reflects the relatively regular and frequent alternation of reservoir and non-reservoir lithologies within the gross reservoir sequence (Knott, 1993). Minor irregularities in the curve are due to connectivity peaks when individual sandstone units are juxtaposed. Connectivities for fault zones comprising two slip surfaces with equal displacements are shown in Fig. 5d for aggregate throw values over the range 0-84 m; an aggregate throw of 40 m is represented by two slip surfaces each with a throw of 20 m. The connectivity for paired slip surfaces is derived from Fig. 5b as follows. For each reservoir unit within the fault zone and bounded by slip surfaces, the connectivity across each slip surface is measured and the minimum of these taken as the net connectivity for that unit. The process is repeated for all reservoir units within the fault zone and the aggregate of the net connectivities represents the connectivity of the fault zone. For the Brent sequence shown here, across-fault connectivity is higher for a single slip surface than paired slip surfaces for all throws less than 80 m. For this sequence the across-fault sandstone connectivity of fault zone A is 23% of the gross reservoir thickness, and that of fault zone B is 14%. The difference in connectivity between the two fault zones is relatively small in this example. In general, however, the
67
difference in connectivity between single and paired slip surface geometries is highly sequence dependent. For some reservoir sequences, fault zone B would have a higher connectivity than fault zone A.
Hydraulically active fault rocks Fault zones with hydraulically active fault rocks may act as either conduits or barriers to flow, but only the latter case is discussed here. Hydraulically active fault rocks in clastic reservoir sequences form primarily either by cataclasis of sandstones or by incorporation of shale into fault zones, or both. The effects on fault seal potential of fault rocks formed by cataclasis of reservoir quality sandstones are not considered. The hydraulic properties of these cataclastic rocks as measured in outcrop samples suggest that they can form hydrocarbon seals (Antonellini and Aydin, 1994), but there is no consensus on their importance in the trapping of large oil columns (Smith, 1980; Gibson, 1994). By contrast, shaley fault rocks are believed to support significant hydrocarbon columns and there is a variety of methods for relating the amount and distribution of shale in a faulted sequence to the sealing potential of faults within it (Bouvier et al., 1989; Gibson, 1994; Fristad et al., 1997, this volume). In one of the simplest and most widely applied methods, the percentage shale of that part of the sequence which has been displaced past a point is calculated for each point on the fault surface. The percentage shale calculated in this way is expressed as the shale gouge ratio (SGR), as shown colour coded in Fig. 5b. Given data on faults of known sealing behaviour in a particular reservoir or province, SGR can be calibrated to provide cut-off values for fault seal for target faults within the same reservoir or province (Fristad et al., 1997, this volume; see Implications for fault seal: fault rock distribution). The software used to produce Fig. 5b calculates SGRs, and a range of other fault seal parameters, to provide a range of uncalibrated values for a given reservoir sequence. The reservoir sequence is represented either as lithological units classified as either reservoir or non-reservoir or shale, or as a percentage shale curve as derived, for example, from a gamma log.
Fig. 5. (a) A typical Upper Brent gross reservoir sequence (reservoir units, grey; non-reservoir (shale), black) offset by two fault zones, A and B. The fault geometries are from a fault map of strike-slip faults offsetting vertical beds (Childs et al., 1996). (b) Sequence/throw juxtaposition diagram (Bentley and Barry, 1991) for the reservoir sequence shown on the left (same sequence as in (a)). The diagram illustrates the across-fault juxtapositions for the range of throw values 0-84 m, non-reservoir units are shown black. Footwall shale layers are represented by horizontal black lines and hangingwall shales by oblique lines. Reservoir/reservoir across-fault juxtapositions are colour coded (see key) according to the SGR (see text) calculated for each element of the surface area. (c) Across-fault connectivity curves for the reservoir sequence in (a) and (b). The reservoir/reservoir across-fault connectivity for a given SGR cut-off value is expressed as a proportion of the gross reservoir sequence thickness (84 m). Connectivity curves for SGR cut-off values are based on the premise that reservoir/reservoir juxtapositions are not connected if the SGR is above the specified cut-off value. The connectivity curve for SGR = 100 is the curve for simple juxtaposition, i.e., hydraulically passive faults. (d) Across fault connectivity curves for the same sequence as (c) but with the total throw distributed equally on two slip surfaces.
68 Connectivity curves for a single slip surface are shown in Fig. 5c for a range of SGR cut-off values, for the Upper Brent sequence shown in Fig. 5b. The forms of the connectivity curves are very different. For an SGR cut-off = 20 there is no connectivity for throws >33 m. For an SGR cut-off = 30, there is no connectivity for throws from 40 to 60 m, but 8% connectivity at a throw of 65 m. For an SGR cutoff = 40, there is a sharp increase in connectivity at 45 m when the Tarbert and Lower Ness reservoirs become juxtaposed. Connectivity curves for paired hydraulically active slip surfaces can be calculated by the same method used for hydraulically passive faults. Connectivity curves for a range of SGR cut-off values are shown in Fig. 5d. For the Upper Brent sequence the connectivity of a single slip surface, for a given SGR cutoff, is generally higher than that for a fault zone with paired slip surfaces. The difference in connectivity between paired and single slip surface geometries is greatest at an SGR cut-off = 40. Although connectivities for the single and paired slip surfaces, at SGR cut-off = 40, are equal at a throw of 25 m, they are 24% (the same as that for a hydraulically passive fault) and 3%, respectively, at a throw of 50 m. For an SGR cut-off = 20 the connectivities for the single and paired slip surfaces are similar at all throws, so the sequence is insensitive to fault zone structure at this cut-off value. These variations in relative connectivities between single and paired slip surface geometries illustrate the complexity and non-linear consequences of multiple slip surfaces.
Summary The significance of sub-seismic fault zone complexity to calculation of across-fault connectivities and of fault seal potential is very strongly dependent not only on the amount but also on the distribution of non-reservoir and shale units within the gross reservoir sequence. The effect of an increased number of slip surfaces within a fault zone is twofold, namely increased average connectivity across individual slip surfaces and increased probability of an individual reservoir unit being juxtaposed against a nonreservoir layer or shaley fault gouge. Complex interplay between these two opposing effects determines the overall connectivity of a fault zone. Fault surface bifurcation processes result in areas of a fault zone with paired bounding slip surfaces alternating with areas with only a single slip surface. Either laterally or up-/down-dip, the two slip surfaces of fault zone B may give way to a single slip surface. Similarly, it is unlikely that fault zone A is characterised everywhere by only a single slip surface. As
c. Childs, J.J. Walsh and J. Watterson
there is no requirement for displacement to be partitioned equally between two slip surfaces, the range of possible connectivities across a 40 m fault zone is much greater than is indicated by the simple example illustrated. If more than two slip surfaces are present, the range of possible connectivities is increased further. The complexity of sequence juxtapositions due to sub-seismic fault zone structure is therefore compounded by the 3-D variation of fault zone structure. We consider that the sensitivity of individual sequences to partitioning of slip onto multiple slip surfaces in 3-D can be assessed only in very general terms. Although sensitivity studies can be performed, existing empirical methods, which implicitly incorporate sub-seismic fault zone complexity, may represent the best approach to fault seal prediction (see below).
Implications for fault seal: fault rock distribution The combined effects of the asperity and tip-line bifurcation processes can result in the formation of fault zones with highly complex internal geometries and highly variable fault rock compositions and distributions. Even detailed information on the fault rocks at a point on the fault zone shown in Fig. 3a-c, as would be provided by a well for example, would not enable valid extrapolation even to nearby parts of the fault zone. The implications of fault zone complexity for the distribution of potential sealing lithologies over a fault surface are illustrated in Fig. 6a which shows a cross-section through a shale-rich normal fault zone. Two distinct layers of shale-derived fault rock are present, a thicker layer derived from an 8 m thick shale source layer and a thinner gouge layer derived from thinner shale units within the footwall sequence. This fault zone has paired external slip surfaces with the intervening rocks intensely deformed. One slip surface occurs along the very thin lower layer of shale gouge and the other along the contact between the thick shale layer and the hangingwall sandstones. The fault zone thins upwards from 2 m at the base of the illustrated section to 1 m at the top. Both the relative and absolute thicknesses of sandstone and shale within the fault zone are determine by lenses of footwall derived sandstone, so the thickness of shalederived fault rock varies rapidly within the section. Rapid changes in both fault rock deformation state and fault rock composition also occur along the fault strike. Fig. 7 shows a map section through a fault of similar throw and offsetting the same sequence as the fault shown in Fig. 6. The mapped surface is 1-2 m below the footwall cut-off of the 8 m thick shale unit and shows paired bounding slip surfaces. The fault
Complexity in fault zone structure and implications f or fault seal prediction
a
69
distribution precludes the possibility of these breccias forming a seal. The shales within these fault zones show highly irregular thickness variations but, nevertheless, display a high degree of conl~inuity. The sealing integrity of a fault would be maintained if this continuity occurred over the whole fault surface. However, given the very small proportion of an entire fault surface which is represented by any single crosssection or map trace, nothing can be concluded about the likelihood of a continuous shale layer over the whole fault surface. A cross-section through a series of minor faults offsetting a mixed sandstone shale sequence (Fig. 8)
b
Fig. 6. (a) Cross-section through a normal fault in a Carboniferous mixed sandstone/shale sequence from a quarry in Lancashire, UK. The fault dips to the left, with a throw of ca. 15 m. (b) Sketch of the outcrop in (a). The well developed shale gouge layer within the fault zone (black) is derived from an 8 m thick shale unit. The base of this shale unit is ca. 1 m above the top of the exposed face in the footwall and 1.5 m below ground level in the hangingwall. The fault separates sandstones in the hangingwall (stippled) from a mixed sandstone (no ornament) and shale (black) footwall sequence. Dense coarse stipple indicates sandstone breccia. Mixed sandstone and shale breccias are shown shaded with a coarse stipple. The boundary between the fault zone and the footwall country rock is a slip surface with a thin (<2 cm), but continuous, layer of clay gouge; this gouge layer is too thin to be represented in the figure. Only structural features which are clearly not the result of quarrying activity are shown in (b).
rocks include shale gouge, sandstone breccias and relatively undeformed blocks of sandstone and shale within which bedding has been rotated towards the hangingwall, as shown in the vertical cross-sections. The fault zone is of more or less constant thickness (2 m) over its mapped length but the fault rock content is highly variable. On cross-section C the fault zone contains 2 m of shale, of varying degree of deformation, while 8 m along-strike (cross-section B) the shale component is reduced to 3 cm of shale gouge. These rapid variations in fault rock content and, particularly, in the content of potentially sealing lithologies raise the question of whether or not deterministic predictions of fault seal potential are feasible. Sandstone derived breccias within the fault zones shown in Figs. 3, 6 and 7 occur either as thin sheets of limited extent or as localised lenses. This patchy
Fig. 7. Map (left) and three cross-sections, A-C, through a normal fault with ca. 15 m throw. The fault offsets the same sequence as shown in Fig. 7. The mapped surface is at a level ca. 1 m below the top of the outcrop in Fig. 6. Deformed shale, either strongly foliated or shale gouge, is shown in black. Shale with preserved bedding is dark shaded with broken lines indicating approximate bedding orientations. Hangingwall sandstones are ornamented with regular stipple and footwall sandstones with irregular stipple. Sandstone bedding directions on cross-sections are shown by thin lines. Sandstone breccias are indicated by a coarse stipple. The boundary between the fault zone and the footwall sandstones on cross-sections B and C is a discrete slip surface.
70
C. Childs, J.J. Walsh and J. Watterson
t
2m
Fig. 8. Cross-section showing a number of small faults offsetting a carboniferous sandstone/shale sequence. Thin shales are shown as subhorizontal lines. Fault traces are shown as steep lines and those with shale smears are shown as wide grey lines. There is no horizontal connectivity across this 2-D section but 3-D connectivity is likely.
shows the frequency with which shale is incorporated into the fault zones and the apparent absence of sandstone connectivity. However, this 2-D section allows no conclusion to be drawn concerning the sandstone connectivity in 3-D. The observations reported here are consistent with those of Lindsay et al. (1993) who concluded that for faults in the same working quarry as Figs. 3, 6, 7 and 8, shale smears in outcrop can be discontinuous when the displaced sequence has an SGR value of <15. This SGR value does not, however, imply a sharp cut-off and continuous shale smears do occur at SGR values <15. These results are broadly comparable with those of Fristad et al. (1997), who find that significant static seal on subsurface faults does not occur for SGR values <15 but does occur at SGR values >18. Gibson (1994) estimates the SGR cut-off value to be 2 2 - 3 0 . Variation in SGR cut-off values is expected for different datasets from different geological settings (Fristad et al., 1997). Observations of shale smear continuity from outcrop are not directly comparable with SGR cutoffs derived from sub-surface datasets, because outcrop observations are small scale and generally only 2-D (Gibson, 1994) and do not allow assessment of the sealing effects of shale-derived fault rock in the 3-D system. A mechanism by which regular and continuous clay smears are generated on relatively low strain rate faults in soft sediments has been described by Weber (1978) and Lehner and Pilaar (1997, this volume). Soft-sediment faults are effectively ductile shear zones and may lack the discontinuities and complexity typical of fault zones, in rocks of either low or high shear strength, characterised by high instantaneous slip rates, i.e., "brittle" faults. Although the faults we have described formed in already lithified sedi-
ments (Lindsay et al., 1993) and the clay smears do not show the thickness regularity of those described by Weber (1978), the continuity of the clay smears may be comparable. The frequency of thick shale smears in the quarry (as in Figs. 6 and 7) is such that the local quarrymen refer to them as "vertical shales".
Discussion Faults are not directly imaged on seismic sections but interpreted, where reflections are offset, as discrete fault surfaces. It is acknowledged by interpreters that these discrete surfaces may comprise two or more seismically unresolved slip surfaces. The bifurcation model of fault growth implies that multi-slip surface fault zones are the rule rather than the exception and that there will be a minimum of two discrete slip surfaces on at least some places on a fault zone. More generally, the high degree of complexity within brittle fault zones, which is evident on all scales of observation, means that no fault surface can be fully defined in three dimensions and on the relevant scale. The fault zone structure observed either at a point or on a cross-section of the fault surface, cannot be extrapolated any significant distance from the point or line of observation. Given these limitations, we question whether it is possible, even theoretically, to predict deterministically the sealing potential of fault zones in the sub-surface. Clearly, detailed observations of fault zones have a role in fault seal prediction. Data from wells which penetrate fault zones can be used to determine both the deformation mechanisms which operated and the fault rocks which are present or which are likely to occur. However, well data provide no constraints on the spatial distribution of the fault rocks and on the sealing potential of faults
Complexity in fault zone structure and implications f or fault seal prediction
on geological timescales. The poroperm properties of fault rock sampled from wells may provide useful constraints on the permeabilities of faults for production timescale flow simulations A contrasting approach to fault seal prediction is to apply the empirical method by which a prediction is made simply on the basis of the known behaviour of comparable faults. We suggest that progress will best be made by moderating the extreme empirical approach by applying deterministic methods to quantifying the degree of comparability between a target fault and faults of known behaviour. An important aspect of an empirical approach is quantifying the degree to which one fault can be considered a valid hydraulic analogue of another. Methods of fault seal prediction employing empirical databases in which faults of known sealing capacities are characterised by reference to shale smear parameters (Gibson, 1994; Fristad et al., 1997, this volume; Lehner and Pilaar, 1997, this volume) represent a combination of the deterministic and empirical approaches. The methods of shale smear parameterisation are, implicitly or otherwise, based on the assumption of a regular and predictable distribution of shale-derived fault rock within a fault zone. While the complexity in fault zones suggests that the juxtaposition diagrams on which these methods are based may not represent the actual across-fault juxtapositions, the methods have nevertheless achieved a measure of credibility within the industry. This apparent contradiction can, however, be resolved. Although the constructions on which fault seal potential calculations are based take no account of subseismic complexity of fault zones, the complexity is incorporated implicitly in the databases against which fault seal potentials are calibrated. For some reservoir sequences, calculated fault seal potentials are to some degree independent of a certain level of fault zone complexity. Figs. 6 and 7 illustrate the extreme spatial variation in thickness of shale-derived fault rock within a fault zone but, together with Fig. 8, they also demonstrate the high degree of continuity of shalederived fault rocks which can occur. It is this continuity of shale-derived fault rock which, despite the complexity of its distribution, provides a physical basis for methods of prediction of fault seal potential based on shale smear parameterisation.
Conclusions Observations show that the internal structure of fault zones is highly complex, largely as a consequence of bifurcation processes. Simple examples of fault zone complexity, in which displacement on a fault of constant displacement is partitioned onto two
71
slip surfaces, illustrate the sensitivity of across-fault connectivity to fault zone structure. In general fault zones have multiple slip surfaces separating volumes of mildly to intensely deformed rock, with chaotic spatial variation in structure of the fault zone. We question whether it is even theoretically possible to characterise deterministically, complex fault zone architectures of subsurface faults, given the limited data available. Empirical risking methods implicitly take account of the unpredictable complexities in fault zone structure which inevitably are present. The way forward is by further refinement of current empirical methods, i.e., by more detailed characterisation of sub-surface faults to allow more objective comparison of target faults and faults of known sealing behaviour.
Acknowledgements This research was part funded by the EC Joule II Reservoir Engineering Project (Contract No. JOU2CT92-0182) and the EC Joule III PUNQ Project (Contract No. JOF3-CT95-0006). The lithological sequence in Fig. 5 was interpreted by Kees Geel from a well log provided by Norsk Hydro. We are grateful to Barry Murphy and Nick Lindsay for fieldwork leading to the production of Fig. 8. We thank Marie Eeles for help in preparing the manuscript. Peter Keller and Andreas Koestler are thanked for their helpful reviews of the manuscript.
References Antonellini, M. and Aydin, A. 1994. Effect of faulting on fluid flow in porous sandstones: petrophysical properties. Am. Assoc. Pet. Geol. Bull., 78: 355-377. Barnett, J.A.M., Mortimer, J., Rippon, J., Walsh, J.J. and Watterson, J. 1987. Displacement geometry in the volume containing a single normal fault. Am. Assoc. Pet. Geol. Bull., 71: 925-937. Bentley, M.R. and Barry, J.J. 1991. Representation of fault sealing in a reservoir simulation: Cormorant block IV UK North Sea. In: 66th Ann. Tech. Conf. and Exhibition of the Soc. Pet. Engineers, Dallas TX, pp. 119-126. Blenkinsop, T.G. 1989. Thickness-displacement relationships for deformation zones: discussion. J. Struct. Geol., 11: 1051-1054. Bouvier, J.D., Kaars-Sijpesteijn, C.H., Kluesner, D.F., Onyejekwe, C.C. and Van Der Pal, R.C. 1989. Three-dimensional seismic interpretation and fault sealing investigations, Nun River Field, Nigeria. Am. Assoc. Pet. Geol. Bull., 73: 1397-1414. Childs, C., Watterson, J. and Walsh, J.J. 1996. A model for the structure and development of fault zones. J. Geol. Soc. London, 153: 337-340. Engelder, T. 1978. Aspects of asperity-surface interaction and surface damage of rocks during experimental frictional sliding. Pure Appl. Geophys., 116: 705-716. Evans, J.P. 1990. Thickness-displacement relationships for fault zones. J. Struct. Geol., 12: 1061-1065. Fristad, T., Groth, A., Yielding, G. and Freeman, B. 1997. Quantitative fault seal prediction: a case study from Oseberg Syd. In: P. Mr and A.G. Koestler (Editors), Hydrocarbon Seals:
C. Childs, J.J. Walsh and J. Watterson
72 Importance for Exploration and Production. Norwegian Petroleum Society (NPF), Special Publication No. 7. Elsevier, Singapore, pp. 107-124. Gibson, R.G. 1994. Fault zones seals in siliciclastic strata of the Columbus Basin, offshore Trinidad. Am. Assoc. Pet. Geol. Bull., 78: 1372-1385. Griffiths, P.S. 1980. Box-fault systems and ramps: atypical associations of structures from the eastern shoulder of the Kenya Rift. Geol. Mag., 117: 579-586. Huggins, P., Watterson, J., Walsh, J.J. and Childs, C. 1995. Relay zone geometry and displacement transfer between normal faults recorded in coal-mine plans. J. Struct. Geol., 17: 1741-1755. Hull, J. 1988. Thickness-displacement relationships for deformation zones. J. Struct. Geol., 10: 431-435. Knipe, R.J. 1992. Faulting processes and fault seal. In: R.M. Larsen, H. Brekke, B.T. Larsen and E. Talleras (Editors), Structural and Tectonic Modelling and its Application to Petroleum Geology. Norwegian Petroleum Society Special Publication (NPF), Special Publication 1. Elsevier, Amsterdam, pp. 325-342. Knott, S.D. 1993. Fault seal analysis in the North Sea. Am. Assoc. Pet. Geol. Bull., 77: 778-792. Koestler, A.G. and Ehrmann, W.U. 1991. Description of brittle extensional features in chalk on the crest of a salt ridge (NW Germany). In: A.M. Roberts, G. Yielding and B. Freeman (Editors), The Geometry of Normal Faults, Special Publication 56. Geol. Soc. London, pp. 113-123. Larsen, P.-H. 1988. Relay structures in a Lower Permian basementinvolved extension system, East Greenland. J. Struct. Geol., 10: 3-8. Lehner, F.K. and Pilaar, W.F. 1997. On a mechanism of clay smear emplacement in synsedimentary normal faults. In: P. Mr Pedersen and A.G. Koestler (Editors), Hydrocarbon seals: Importance for Exploration and Production. Norwegian Petroleum Society (NPF), Special Publication 7. Elsevier, Singapore, pp. 39-50.
C. CHILDS J.J. WALSH J. WATTERSON
Lindsay, N.G., Murphy, F.C., Walsh, J.J. and Watterson, J. 1993. Outcrop studies of shale smears on fault surfaces. In: S. Hint and A.D. Bryant (Editors), The Geological Modelling of Hydrocarbon Reservoirs and Outcrop Analogues. Int. Assoc. Sediment., 15: 113-123. Otsuki, K. 1978. On the relationship between the width of shear zone and the displacement along fault. J. Geol. Soc. Jpn., 84:661-669. Peacock, D.C.P. and Sanderson, D.J. 1991. Displacements, segment linkage and relay ramps in normal fault zones. J. Struct. Geol., 13: 721-733. Peacock, D.C.P. and Sanderson, D.J. 1994. Geometry and development of relay ramps in normal fault systems. Am. Assoc. Pet. Geol. Bull., 78: 147-165. Robertson, E.C. 1983. Relationship of fault displacement to gouge and breccia thickness. Soc. Mining Eng., Am. Inst. Mining Eng. Trans., 35: 1426-1432. Segall, P. and Pollard, D.D. 1983. Joint formation in granitic rock of the Sierra Nevada. Am. Assoc. Pet. Geol. Bull., 94: 563-575. Smith, D.A. 1980. Sealing and non-sealing faults in Louisiana Gulf Coast salt basin. Am. Assoc. Pet. Geol. Bull., 64: 145-172. Stewart, I.S. and Hancock, P.L. 1991. Scales of structural heterogeneity with neotectonic normal fault zones in the Aegean region. J. Struct. Geol., 13:191-204. Weber, K.J., Mandl, G., Pilaar, W.F., Lehner, F. and Precious, R.G. 1978. The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures. In: 10th Ann. Offshore Technology Conf., Soc. Pet. Eng., Paper No. 3356, pp. 2643-2653. Wolf, R. 1985. Tiefentektonik des linksneiderrheinischen Steinkohlengebietes. In: G. Drozdzewski, H. Engel, R. Wolf and V. Wrede (Editors), Beitr~ige zur Tiefentektonik westdeutscher Steinkolhlenlagerst~itten, Geologisches Landesamt NordrheinWestfalen, Krefeld.
Fault Analysis Group, Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK Fault Analysis Group, Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK Fault Analysis Group, Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK