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Penetrative Strain on the Field Scale: Detrimental to Reservoir Quality?

Abstract

Penetrative strain constitutes the proportion of the deformation that is not accommodated by the development of field-scale structures. In compressive settings, PS is reported to be between 2-30% shortening. At a field scale, slip on minor fault arrays and development of minor folds may accommodate significant PS, changing pore space and connectivity of thinly-bedded units. On the grain scale, PS may be accommodated by intragranular (e.g. twinning) or intergranular (e.g. grain impingement) processes. This study considers the PS accommodated by a cover sequence over a basement-thrust-related, Laramide-age fold in the Bighorn Basin, the Rattlesnake Anticline. Field observations, thin-section analysis and structural modeling are used to characterize the amount and mechanisms of PS accommodation in the exposures of the Shoshone Canyon. Faults on the gently-dipping limb are steep, with a reverse sense of movement, and are regularly spaced. They are interpreted as reactivated fractures from an earlier, regional fracturing event. Reactivation occurred during late tightening of the anticline. Other faults on this limb, with generally lower dip angles and without the regular spacing, are likely to have formed during tightening without reactivating fracture sets. An intense zone of deformation is observed in the cover rocks immediately adjacent to the inferred master fault. Thrust faulting in this zone is either sub-parallel to the master fault, or forms low angle, out-of-the-syncline faults. Most faults contain some fault gouge. Rocks in this zone have also deformed by buckling of the thin, competent carbonate layers and thickness variations in the mudrock interlayers. Cross-section restoration indicates that fault structures accommodate over 10% of the strain in the system, with the remaining PS taken up by grain impingement and twinning. Overall, volume loss by PS accommodation is detrimental to the reservoir quality of unconventional fields. Significant grain impingement contributes strongly to the reduction of pore space, reducing the amount of hydrocarbon that can be trapped. Fault gouge in a mudrock-dominated system tends to reduce the permeability of a fault, and in this example, has led to reduced permeability of an otherwise promising fracture set. Thus, understanding the contribution of PS to the deformation of an unconventional reservoir allows for more realistic estimates of hydrocarbon in place, and improved operating decisions.