RELATIONSHIP BETWEEN DEFORMATION, DIAGENESIS AND RESERVOIR QUALITY IN THE TENSLEEP SANDSTONE AT ALCOVA RESERVOIR, CENTRAL WYOMING, USA
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 f decreases by 50%, K decreases by 103, 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 on 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.