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ABSTRACT: Outcrop Analogs for Modeling Heterogeneous Restricted Platform Reservoirs: Grayburg Formation (Guadalupian) of the Guadalupe Mountains and Subsurface Permian Basin

Charles Kerans, H. S. Nance, D. G. Bebout

Restricted platform deposits of the Permian basin have produced more than 10 billion bbl of oil, but because of complex internal facies architecture ultimate recovery rarely exceeds 30% of original oil in place. The first step in locating and exploiting the substantial unrecovered hydrocarbons in this play is developing improved development-oriented (as opposed to exploration-oriented) models of facies/permeability heterogeneity. Quantitative geologic/engineering analysis of outcrops analogous to reservoirs allows characterization of styles of variability and three-dimensional geometry at interwell, reservoir, and play scales beyond that possible using limited subsurface data.

The Guadalupian Grayburg Formation is exposed in the northern Guadalupe Mountains less than 60 mi west of reservoir trends on the Northwest Shelf and Central Basin platform. These outcrops provide a test for the development of outcrop-based reservoir heterogeneity models. A 10-mi dip section of the mixed siliciclastic/carbonate Grayburg of the Guadalupes exhibits inner ramp (4-mi dip width), ramp-crest ooid shoal/tidal flat (4 mi dip width), and outer ramp fusulinid-peloid (2-mi dip width) facies tracts. Maximum facies heterogeneity occurs in upward-shallowing cycles of the ramp crest where porous ooid shoals pass laterally into tight peloid packstones in less than 1000 ft. Siliciclastic sands are intercalated in all facies tracts typically as sheets. However, in the ramp crest they a so occur as 15-ft-thick by 50-ft-wide channels dissecting ooid shoals.

Grayburg reservoirs on the eastern side of the Central Basin platform contain facies tracts and styles of heterogeneity analogous to the outcrop. Production trends from these reservoirs similarly illustrate most variability in grainstone shoal complexes.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990