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Facies Control of Reservoir Quality in Grayburg Formation, Dune Field, Crane County, Texas

Don G. Bebout

The lower Guadalupian Grayburg carbonates of Dune field were deposited along the edge of the Central Basin platform on the west side of the Midland basin. Dune field lies on the northeast side of a low-relief structure that dips gently into the basin.

In the Mobil University Unit 15/16 of Dune field, the Grayburg reservoir section has been divided vertically into three parts based on carbonate facies recognized in whole cores. Predictable average porosities (^phgr) and geometric mean permeabilities (k) associated with these facies vary widely between four orders of magnitude. Fusulinid wackestone (^phgr = 7%, k = 0.2 md), the most widely distributed facies studied, composes the lower unit in all cores. The middle unit is represented by sponge-algal framestone (^phgr = 8%, k = 0.15 md) in the western third and crinoid packstone/grainstone (^phgr = 11%, k = 1.2 md) in the eastern two-thirds of the area studied. The upper unit contains an upward-shoaling succession from bottom to top of fusulinid wackestone, pellet grainstone (^phgr = 9%, k = 0.5 md), ooid grainstone (^phgr = 5%, k = 0.05 md), and pisolite grainstone (^phgr = 4%, k = 0.02 md). This sequence is interpreted to represent a progradational sequence from shallow-water subtidal to arid tidal-flat environments. Siltstone beds occur throughout the upper unit but are more numerous and thicker toward the top.

Moldic, vuggy, interparticle, and intercrystalline porosities occur in the Grayburg dolomites. Although diagenesis of the San Andres/Grayburg carbonates (including massive dolomitization, and anhydrite and gypsum replacement and cementation) is extensive, the primary depositional fabrics still exert some control over the distribution, type, and amount of porosity and permeability. Permeability is highest (geometric mean = 0.5-1.2 md) where interparticle porosity is developed in the grainstones and packstones; it is generally lower in the wackestone facies (geometric mean = 0.2 md) where moldic and vugular porosities are common.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.