Abstract

South Casper Creek (SCC) field, located approximately 24 mi west of Casper, Wyoming, on the Casper arch produces low-gravity oil from the Pennsylvanian-Permian Tensleep Sandstone. Production from parts of the field is currently being amphlified by steam flooding. To assist the enhanced oil recovery program, a detailed sedimentologic and stratigraphic evaluation of available drill cores (N = 10) and recent well logs was undertaken in 1989. These efforts included interpretation of depositional facies, definition of facies-controlled reservoir flow units and flow barriers, evaluation of lithofacies controls on porosity and permeability, and delineation of small-scale structural features in the reservoir interval. This paper summarizes the results of Unocal's recent efforts.

Eolian lithofacies in the Sandstone member (informal) of the Tensleep are the main producing intervals at SCC field. The Sandstone member, which averages about 160 ft thick, is subdivided into five major flow units separated by four flow barriers or baffles. Flow units are dominated by eolian sandstone, whereas flow barriers and baffles are mainly interdune or erg-margin (marine-transition) deposits. Within the flow units, eolian stratification is dominated by wind-ripple modified grain-flow lamination (58%), followed by wind-ripple lamination (30%), grain-flow lamination (11%), and grain-fall lamination (< 1%). Sets of eolian cross-strata range in thickness from 0.1 to 29.2 ft, and average 1.8 ft. Dipmeter and Formation MicroScanner (FMS) data indicate a south to southwest paleowind.

Flow units have porosity, horizontal permeability, and vertical permeability ranges of 20.7-21.6%, 638-750 md, and 468-491 md, respectively, whereas flow barriers have ranges of 11.0-17.9%, 22-259 md, and 14-117 md, respectively. Grain-flow sandstone has the best average porosity and horizontal permeability (24.4% and 1,580 md), followed by wind-rippled grain-flow sandstone (22.1% and 806 md) and wind-rippled sandstone (18.3% and 293 md). Directional permeability studies of oriented core suggest maximum transmissivity along a northwest-southeast trend.

Fractures and small-displacement faults are common in the reservoir interval at SCC field. Dipmeter and FMS data for these features show considerable variations in azimuth and dip magnitude depending on well location. In the main steam-flood area the reservoir interval has a trimodal fracture pattern. Although incompletely understood, it is probable that fractures play an important role in the migration of fluids at SCC field.