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Relationship Between Reservoir Quality and the Deformation Associated With Anticlinal Folding in the Tensleep Sandstone at Alcova Reservoir, Central Wyoming, USA


Anticlines of Pennsylvanian Tensleep Sandstone are the most prolific oil-producing reservoirs in the Laramide Rockies. While the Tensleep's eolian and mixed carbonate lithologies are typically of excellent reservoir quality, the patterns of local production performance are complex being attributed to diagenetic variations and degree of natural fracturing. Based on our outcrop study we find that deformation-triggered processes leading to cementation and other diagenetic effects occur synchronously with fracturing. These effects must be studied in tandem while characterizing these reservoirs for field development and management. At Alcova Reservoir the Tensleep Sandstone crops out in the gently-dipping homocline of the Sweetwater Arch, herein referred to as the undeformed domain (U), and continuously in an oil field-scale basement-rooted fault-propagation fold (backlimb (B), forelimb (F), and crest (C)). Within the context of these structural positions we compare porosity and permeability, petrographic character, outcrop fracture intensity, and geomechanical properties of samples obtained from the same bed of eolian subarkose sandstone (>75% quartz). We find that there is a systematic evolution of diagenetic and deformational characteristics that consistently tracks in the sequence U->B->F->C. In this sequence porosity decreases by 50%; permeability decreases by 1000x; and fracture intensity, UCS, and Y increase by 6x. The fractures are dominantly strata bound and mineral-free. Compaction bands are common at F and C. In this sequence petrographically, the matrix displays greater degrees of intragranular penetration and pressure solution, fractured grains, and cement overgrowths (mainly qtz). In positions F and C, gouge-bearing shear zones are common in thin section and as deformation bands in outcrop. We conclude that the stress path due to folding produced a complex diagenetic and rheological response resulting in an overall degradation in reservoir matrix properties as the rock became cemented, stronger, and presumably more brittle. However, fracturing from grain- to bed-scale became pervasive which would greatly enhance bulk reservoir performance. These processes are a strong function of structural position, and presumably lithology, which makes it feasible to predict this behavior in the subsurface.