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Stratigraphy and Reservoir Geology of San Andres Dolomite, Yates Field, West Texas

D. H. Craig, M. J. Heymans, D. H. Mruk, P. D. Crevello

Production from the San Andres dolomite, principal reservoir unit in the Yates field, is controlled by structure and by stratigraphic variation in depositional diagenetic facies, resulting in subdivision of the field into eastside and westside areas. In the east, the upper San Andres is dominated by peloidal, fusulinid, and algal carbonate sand and gravel shoals that were deposited as an easterly prograding platform margin sequence. Carbonate lithologies in the west Yates are typically peloidal lime mudstones and wackestones with a restricted biota of dasyclad algae and gastropods, and thin interbeds of gray-green clay shale--all deposited in intertidal and lagoonal environments.

Subaerial exposure of the San Andres sediments as a cluster of low-relief islands resulted in development of caves, cave sediments, and banded cave cements, and created a network of open fractures by promoting brecciation and collapse. Diagenetic processes that acted on the carbonate platform sequence, both enhancing and reducing matrix porosity and permeability, include early calcite cementation, dolomitization, physical compaction, stylolitization, and burial cementation. Most dolomitization of the upper San Andres postdates the San Andres unconformity and probably resulted from seepage refluxion.

Two general porosity groups comprise most of the matrix porosity in the San Andres Formation: (1) compact or dense vuggy and moldic dolomite (west Yates), and (2) sucrosic dolomite with fine molds and fine to coarse intergrain and intercrystal pores (east Yates). Most of the reservoir storage in the San Andres of the Yates field is in sucrosic dolomite. Fractures, vugs, and caves have greatly enhanced drainage from the highly porous reservoir rock matrix and have contributed to the remarkable high productivities of many wells drilled in the field during its early history.

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