Structural Partitioning as an Explanation for Reservoir Performance at Anschutz Ranch East--Reservoir Heterogeneity by Tectonism

Helen Lewis

Anschutz Ranch East is a large, asymmetric anticline in the thrust belt of northeastern Utah and western Wyoming. The reservoir unit, which contains large hydrocarbon reserves, is the Nugget Sandstone, a porous aeolian unit of Jurassic age. Operation of this reservoir has always been complex, pressure maintenance having been required from the beginning of production. Geologic investigations have been conducted to provide information about reservoir heterogeneities, one of the difficulties in the operation of this field.

Sedimentology studies have demonstrated a stacking of porous dune deposits with interdune porosity and permeability barriers. This particular heterogeneity precludes easy vertical communication of reservoir fluids, adding to the difficulties in planning and executing efficient production and injection programs. Additional complications result from the combination of the moderate initial porosity in the Nugget and pressure/temperature conditions during tectonism. The resulting deformation mechanisms are typified by compactive modes with accompanying reductions in porosity and permeability, except in the overturned forelimb where the degree of deformation has progressed to the stage of becoming dilatant. Thus, this particular "fractured" reservoir has actually been degraded by the fract ring rather than enhanced.

Fortunately, structural methods allow for prediction of the patterns of deformation, including division of the 1,000-ft thick Nugget into several layer-bounded mechanical units, with layer-normal flow barriers within these units and compartmental faults offsetting the fold crest. Use of modern structural concepts has allowed for improved prediction of pressures, fluid types, and connectivity of injectors and producers, with an overall increase in efficiency of reservoir management.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.