Print this pageAbstract: Middle Guadalupian Coastal Carbonate Complex--Stratigraphy and Environmental Reconstruction
Open shelf, shelf margin, patch reef, oolite shoal, exposed flats, beach and bar, intertidal flat, and sabkha are the major environments that generated the shallow-marine to supratidal carbonate and evaporite deposits of the middle Permian Grayburg Formation. The onlap-offlap sequence of Grayburg carbonate and evaporite deposits is reservoir for hydrocarbons at the North McElroy field, on the eastern edge of the Central Basin platform in Crane and Upton Counties, Texas.
The depositional sequence begins with the transgression of open-marine waters from the Midland basin across an exposed and eroded carbonate surface. The open-shelf environment was conducive to the accumulation of thick, widespread fusulinid biomicrites. Several facies developed when the sediments accumulated to wavebase; algal and bryozoan biomicrites accumulated on the higher energy shelf margin; in the more protected and restricted areas, shelly micrites and desiccated micrites accumulated on intertidal and supratidal mudflats. Deeper, open-marine waters returned to much of the area, depositing another blanket of fusulinid biomicrite which in turn was blanketed by a cross-bedded oolite.
The upper third of the Grayburg records many responses to a variety of conditions related to shallowing of water and progradation of the shoreline. None of these facies is widespread. The more common facies are burrowed and desiccated micrites of the exposed flats, subtidal intramicrites and biomicrites, cross-bedded intraclastic oolite from shallow-water bars and beaches, and a few sponge/algal patch reefs. This sequence is punctuated with sabkha-evaporitic carbonate rocks. The laterally equivalent anhydritic clastic rocks of the Queen Formation blanket the Grayburg carbonate deposits.
The best reservoir units are in facies that originally were impermeable lime muds; epigenetic dolomitization of these micrites and biomicrites enhanced the organization of secondary porosity and permeability. In contrast, porosity in the most attractive primary reservoir units (reef, oolite, some of the beach and bar) was filled with diagenetic sulfate deposits.
AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA