GEOMETRIC EFFECTS ON FLOW ACROSS FAULTED SEDIMENTARY STRATA – IMPLICATIONS FOR FAULT GOUGE

Fluid flow across faulted clastic sedimentary strata is complicated by two main factors: (1) geometric disruption of permeable beds by fault offset; (2) introduction of low permeability fault gouge. These complexities arise even in the simple case of permeable/impermeable strata common to many petroleum reservoir and groundwater problems. Through a series of simple numerical models, we find a first-order influence of stratigraphic geometries at the fault (i.e. the geometry of permeable/impermeable beds) on flow reduction relative to the unfaulted case. Not only does vertical stratigraphic stacking matter, so too do the lateral dimensions (relative to observation points, typically wells). For example, with good vertical communication between permeable strata in a reservoir, contact between permeable strata across the fault needs to decrease to <10% of the original cross-sectional area to have a significant impact on cross-fault flow. In contrast, poor vertical communication in a stratigraphic sequence results in reduced cross-fault flow with only ca. 50% juxtaposition area.

Shale and cataclastic fault gouge types, both of which reduce permeability of sandstone reservoirs, create comparable effects to the geometries described. Since fault zones are complex, fault gouge distributions and permeabilities variable, and the details of stratigraphic geometries difficult to precisely define at fault surfaces, prediction of the effects of faults on subsurface flow is challenging. We find that describing fault gouge as flow/no-flow entities and distributing them in a fault zone in various ways, we achieve comparable flow results to the purely stratigraphic geometries. Thus combining complex stratigraphic and fault gouge distribution geometries results in non-unique flow behavior. In order to make reasonable predictions of subsurface, cross-fault flow from our models, we strive to treat systems with enough simplicity to maintain accuracy (e.g, simple flow/no-flow units) and search for threshold behavior where small changes in geologic inputs result in different flow predictions.