Application of Sequence Stratigraphy to Permian Carbonates and Evaporites, McElroy Field, Central Basin Platform, Texas

Hans G. Machel, Susan A. Longacre

The McElroy field in west Texas is part of a series of Permian subsurface carbonate reservoirs fringing the eastern margin of the Central Basin platform. Traditionally, these reservoirs have been subdivided into three stratigraphic units: San Andres Formation, Grayburg Formation, and Queen Formation. The San Andres and Grayburg Formations apparently form shallowing-upward cycles capped by intertidal or low-supratidal facies. Oil production occurs mainly from the Grayburg Formation. The Queen Formation consists largely of bedded sulfates and siltstones, and forms the reservoir seal.

Applying the concept of sequence stratigraphy to wireline logs and core data refines the previous stratigraphic and geologic interpretations. (1) The Grayburg Formation consists of three internal cycles. (2) Laterally equivalent types of microfacies are similar but not identical during transgression and regression. (3) A paleo-shoreline was present near the eastern margin of the field before the end of Grayburg deposition. (4) Syndepositional downfaulting of the eastern part of the field by several meters occurred during Grayburg deposition. (5) Downfaulting of the eastern part of the field by several tens of meters occurred after Queen deposition.

These findings may help interpret reservoir diagenesis. In particular, pervasive dolomitization and emplacement of sulfates (mainly gypsum and anhydrites) may be related to the internal cyclicity or the faulting events.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.