Clay smears

Four mechanisms have been suggested to explain how faults provide seals. The most frequent case is that of clay smear and juxtaposition (Fig. 5.8)... 

Clay smear soft clay, often of marine origin, is smeared into the fault plane during movement and provides an effective seal. 

Figure 5.8 Fault seal as a result of clay smear and juxtaposition...

Surprising little has been published on the detailed physical properties or microstructural evolution of fault rocks in hydrocarbon reservoirs. Recent papers which have begun to address this aspect of sealing include analysis of clay smears (Knipe 1992a, 1994 Berg and Avery, 1995), cataclasites from pure sandstones (Pitman, 1981 Underhill and Woodcock, 1987 Antonellini and Aydin, 1994 Fowles and Burley, 1994), and deformation in impure sandstones... 

Phyllosilicate/clay smears These are developed by the deformation of material with high concentrations of phyllosilicates. The type and continuity of the clay smear (Fig. 2b) depends on the architecture or proximity of the shale units, the lithifi-cation state of the phyllosilicate rich units at the time of deformation and the amount of shearing or fault offset. 

Seal analysis based on the assumption that juxtaposition analysis (i.e., construction of Allan diagrams, clay smear assessment and leaking sand/sand contacts) would only have been successful in -40% of the cases studied. 

Clay smearing is the critical sealing mechanism in only -35% of cases. 

Jev, B.I., Kaars-Sijpesteijn, C.H., Peters, M.P.A.M., Watts, N.L. and Wilkie, J.T. 1993. Akaso field, Nigeria use of integrated 3-D seismic, fault slicing, clay smearing, and RFT pressure data on fault trapping and dynamic leakage. Am. Assoc. Pet. Geol. Bull., 77 1389-1404. 

The emplacement of clay smears in synsedimentary normal faults inferences from field observations near Frechen, Germany... 

Background and methods. This paper reports on an outcrop study of clay smears in synsedimentary normal faults that were exposed in the open-cast lignite mines at Frechen near Cologne, Germany. The observations made are interpreted in terms of a mechanism of clay smear emplacement. 

Results and conclusions The fault zones at Frechen contain clay fillings of up to 1 m in thickness, derived from extremely plastic shale source beds and smeared out over distances as much as 70 m in dip direction. The generation of substantial smears requires slow fault displacement rates and sufficient shale ductility. When a thick shale source bed is traversed by a normal fault, it is first flexed and eventually disrupted by a pull-apart mechanism that creates room for the emplacement of thick clay smears. Simple theoretical considerations suggest that the source bed thickness to some power n + 1 > 2 may be a key parameter in the ranking of seal quality. The length of continuous smears increases with source bed thickness, but will ultimately be controlled by the smearing process. The latter remains to be investigated. 

The last observation could indeed have major practical implications as a guiding principle in assessing fault trap prospects. It is therefore of some interest that Lindsay et al. (1993) have observed clay smears in tectonic faults that affected a Westphalian sand/shale sequences after lithification. These smears were apparently formed by abrasion of indurated shales. In this process, the surface of a sandstone becomes coated by a thin veneer of abraded material in much the same way as the surface of sandpaper. This veneer may run continuously along polished slip surfaces, but - as Lindsay et al. have documented in their study - with increasing fault displacement and de-... 

Plate 1. (a) Max Rudolph fault (Frechen mine) continuous clay smear over approx. 70 m throw, visible as a thin streak between sand and down-thrown coal (Height of exposure approx. 40 m). (b) Earlier exposure of same fault, showing continuity of clay smear in strike direction over approx. 400 m. (c) Detail of fault zone with compact clay smear (approx. 30 cm thick) and fault-parallel minor D-shear in upthrown block. 

Plate 2. (a) Clay smear in minor shear, (b) Sample of clay smear (approx. 5 cm thick) fdling fault zone of Max Rudolph fault (Frechen mine). Sigmoidal shear pattern became visible upon drying. 

Plate 3. (a) Clay smear and sets of R- and conjugate R -sheais in near-surface position on upthrown and downthrown blocks of Max Rudolph fault (F). (b) Detail of (a) showing R-shears offset by later R -shears and truncation of shears by fault. 

The clay smears found in the major fault zones form a continuous band which is gradually thinning away from the source bed (see Plates 1 and 3). The smears consists of remarkably pure clay material. At places these smears can reach a thickness of approximately 1 m, but thicknesses of the order of 10-20 cm are more common. Along portions of the Max Rudolph fault, a continuous smear exists over the... 

Fig. 2 summarizes in a schematic fashion the main features associated with the continuous clay smears that were observed in the major fault zones of the Frechen mines. It is reproduced from Weber et al. (1978), again to provide a synthesis of the most pertinent observations and a convenient reference in the subsequent discussion. 

Mechanism of clay smear emplacement suggested by observations... 

The overall picture emerging in the course of this field study was that clay smears are formed consistently on all scales in the Frechen exposures, where they represent a universal phenomenon. This would suggest that the conditions necessary for the emplacement of clay smears in minor shear bands are the same as those met along the major faults. Among the factors that are likely to determine the thickness, length and continuity of a smear, the thickness of the source bed and the fault throw are readily identified in the field. The requirement that the shales possess the necessary plasticity is also clearly met, i.e., the shale source bed material may be characterized as highly plastic, fat clay in accordance with the standard soil mechanics classification (Bowles, 1984). 

Fig. 1. Merging of two clay smears to form layered smear (cf. Plate 3b).

As discussed by Weber et al. (1978), the same mechanism appears to have operated in experiments performed by Mandl et al. (1977), during which continuous clay smears were produced in a ring-shear apparatus by extruding material from a sheared-off clay band. In these experiments, the much smaller difference between the fault-parallel and the fault-normal stress within the plastic clay, as compared with that in the sand, must have caused the observed extrusion. 

Pull-apart mechanism of clay-smear emplacement... 

A continuous supply of clay material from the source bed to the shear zone can be maintained, given the necessary plastic properties of the material, if a driving horizontal stress difference between distant parts of the source bed and the immediate vicinity of the fault can be maintained. This requires some mechanism of horizontal stress relief to operate at the fault intersection of a source bed. Preferably, the same mechanism should be capable of resolving the space problem implied by the emplacement of massive clay smears. 

Such a mechanism was in fact conceived at some stage in the course of this study, in the first place as a way to overcome the space problem kinematically. Thus we invented the pull-apart mechanism shown in Fig. 3. On traversing a shale source bed, a normal fault only has to be offset in the direction of the downthrown block in order to make room for the emplacement of a clay smear. Evidently, this mechanism... 

Our observations thus suggest the following main stages in the development of a continuous clay smear long a major fault (cf. Fig. 4). (a) A newly deposited, highly plastic shale first forms a monocline in re-... 

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