Fault seal prediction: the gouge ratio method

Abstract

Abstract The reduction in pore throat size attributable to the enhanced clay content in the fault rock between two juxtaposed reservoir bodies is able to produce an effective seal to hydrocarbon migration. The gouge ratio, when applied to sand-shale sequences, indicates the proportion of phyllosilicate material that is expected to be incorporated into the fault rock, hence it provides a measure of the seal capacity. Gouge ratios for faulted reservoir sequences are computed from data that are routinely available from fields at the appraisal or production stage. We present a repeatable methodology that maximizes the use of seismically based geometric data and well-based stratigraphic and compositional data. The resulting gouge ratio calculations can be used comparatively at the scale of individual faults, fields or basins. We examine an example fault from the Oseberg Syd Field that is known to seal and we show that a threshold shale gouge ratio of 18% will support a cross-fault pressure difference of c. 8 bar.