Fracture Distributions in the Tensleep-Equivalent Casper Sandstone at Flat Top Anticline, Wyoming: Implications for Reservoirs

Lorenz, John C.¹ and Peigui Yin^2
1Enhanced Oil Recovery Institute, University of Wyoming, Laramie, WY
²University of Wyoming, Laramie, WY

     Natural fractures and fracture corridors are pervasive within the Tensleep-equivalent Casper sandstones that crop out at Flat Top Anticline, six miles north of Medicine Bow in southeastern Wyoming. The pattern of oil-staining in the sandstones is not influenced by the distribution of fractures, but ornamentation and the sequence of fracture-face coatings, consisting of both mineralization and micro-gouge, suggest extension fractures that were commonly reactivated in shear to create low-permeability planes. Less oil is present in the reduced pore space immediately adjacent to the sheared fractures, and such features would compartmentalize and control flow patterns in similar reservoirs. The fracture system includes an older fracture set that strikes NNE-SSE, oblique to the ENE-WSW axis of the anticline, and a younger fracture set that is nearly parallel to that axis. Fracture strikes and spacings vary with position around the anticline. More widely spaced fracture swarms mark fault zones that trend sub-parallel to the older set of fractures. The two-set fracture system was the compound response of poorly cemented Casper strata to initiation and growth of the anticline near the toe of the hanging wall of a thrust fault. NNE-directed horizontal compression (oblique to the present anticlinal axis) caused initial uplift and the older set of extension fractures; extension during folding formed the younger set of fractures parallel to the hinge of the fold. Both sets of fractures were sheared during continued NNE translation of the thrust plate. Initial anhydrite fracture cementation has converted to calcite in the near-surface environment.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah