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

Abstract

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. 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 /per thousand/ CDT and 10.0 to 14.3 /per thousand/ SMOW, respectively, suggesting precipitation from Late Permian brines and later recrystallization and redistribution accompanied by oxidation of bacterial sulfide, minor mixing with older or youngermore » sulfate, equilibration of isotopically heavier formation waters, or a combination thereof.« less