Dolomite, Gypsum, and Anhydrite in Permian McElroy Field, Central Basin Platform: Genetic and Spatial Relationships to Facies Tracts, Cyclicity, Structure, and Pay Zones?

Hans G. Machel, Susan A. Longacre

The Permian McElroy field at the eastern margin of the Central Basin platform is part of the giant P.J.W.D.M. field complex. Oil production occurs mainly from the Grayburg Formation, which consists of at least four and possibly five internal sedimentary cycles. Trapping is facilitated by a combination of structure (asymmetrical anticline) and a seal of fine-grained peritidal and supratidal dolostones plugged and overlain by gypsum and anhydrite. Although most oil is located close to the top of the structure, the gross pay zone appears to be subdivided into smaller irregularly shaped pods. The geometry and degree of interconnection of these pods are difficult to predict and may be related to the internal cyclicity as well as to diagenesis and structure.

Dolomitization largely enhanced and emplacement of sulfates largely reduced the reservoir quality. Petrographic and geochemical data from approximately 30 cores place some constraints on the timing of fluid flow during dolomite and sulfate formation. Stable isotope data of the dolomites scatter from 0 to 6.5^pmil PDB for oxygen and 1.5 to 6.0^pmil PDB for carbon, with strontium concentrations of 35 to 100 ppm. These data suggest subhorizontal fluid flow in at least one part of the field during dolomite formation or recrystallization from gypsum-saturated brines. Sulfur and oxygen isotope data of the sulfates scatter from 10.0 to 12.5^pmil CDT and 10.0 to 14.3^pmil SMOW, respectively, suggesting precipitation from Late Permian brines and later recrystallization and redistribution accom anied by oxidation of bacterial sulfide, minor mixing with older or younger sulfate, equilibration of isotopically heavier formation waters, or a combination thereof. The stable isotope ratios of the gypsum-hydrating water is -1.0 to 3.5^pmil SMOW for oxygen and -35 to -50^pmil SMOW for deuterium, suggesting hydration of anhydrite by evaporated ground water and/or by formation waters that had been isotopically enriched by the surrounding country rocks. The relationship of these interpretations to cyclicity and structure is under further investigation.

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