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Faulted Column Heights

Abstract

Column heights in faulted traps are thought to be controlled by juxtaposition of reservoirs against top seals and/or capillary effects of the fault-rocks themselves. The latter has received considerable attention in respect of the relationship of the shale gouge ratio (SGR) to measured column heights. There is good evidence to support the argument that SGR is a useful predictor of column height/fault-sealing capacity despite the fact that SGR is simply an average of wall-rock shale volume fraction. SGR was never intended as a precise description of grain-scale mixing in the fault gouge but a generic, upscaled, measure of the fault zone composition. Indeed, outcrop studies show there is considerable variation both in the morphology of fault rocks and the mechanisms that lead to them beyond the simple idea of SGR. In binary sand/shale sequences, fault rocks are often composed of discrete shale smears, quantified by the shale smear factor (SSF). Field observations supplemented by sandbox experiments show that smears tend to be continuous in cross-section from footwall to hanging wall at low values of SSF but become breached (forming holes) as displacement increases. The critical value of SSF at which breaching occurs depends on the strength/brittleness of the clay/shale layer and the confining stress at which fault displacement occurs. In this case the distribution of smears and holes may directly control column height in a way analogous to juxtaposition. However, the location of holes is not predictable and we address this variation using the probabilistic shale smear factor (PSSF). Convolution of stratigraphy with shale smear architecture introduces a spectrum of complexity. We illustrate this with examples from the Brent Province and Tertiary shallow-marine clastics. For multiple beds and probabilistic smear breaching, holes are more likely at lower SGR. However, these holes may themselves have limited seal capacity due to cataclasis and/or cementation. We identify two column-controlling end-members: (i) smear holes which directly control the OWC, with an across-fault leak point at the base of the column, and (ii) fault-rock leak points, which support an underlying column and may leak oil or gas from within the column. The case studies provide evidence for both end-members. Despite its mechanistic simplicity, as an empirical attribute, SGR provides a pragmatic probabilistic fault seal predictor in sand-shale sequences for this range of fault behaviour.