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Facies and Diagenesis of Grayburg-San Andres Dolomite Reservoirs, Central Basin Platform, Permian Basin

Paul M. Harris, Raymond A. Garber

Porous dolomites of the Grayburg and San Andres Formations (Permian, Guadalupian) form major hydrocarbon reservoirs on the Central Basin platform of the Permian basin. Wireline-log and core studies reveal a shallowing-upward carbonate shelf sequence, from 250 to 300 ft thick, in each of five fields that lie along 55 mi of the eastern edge of the platform. From south to north, the fields are McElroy, Dune, Waddell, Goldsmith, and Triple N. The sequence consists of (1) a basal unit of open shelf dolomudstones and bioturbated fusulinid dolowackestones/packstones; (2) a middle unit of shallow shelf dolomudstones and non-skeletal skeletal dolopackstones/grainstones; and (3) an upper unit of arid coastline dolomudstones, quartz siltstones, and evaporites that grade into the overlying Queen Formation. Reservoir zones averaging greater than 9% intercrystalline and moldic porosity and 10-md permeability are best developed in the middle unit of the depositional sequence.

Diagenesis occurred in two stages. Stage 1, during deposition and earliest burial, involved submarine cementation, pervasive dolomitization, leaching of grains, minor dolomite cementation, and sulfate cementation and replacement; and stage 2, near maximum burial depth and during subsequent uplift, included fracturing, anhydrite cementation, minor calcite replacement of anhydrite, and gypsum and kaolinite formation. The pervasive dolomite is commonly finely crystalline, anhedral to subhedral, and inclusion rich, and it preserves the original depositional texture. Stable carbon and oxygen isotopic analyses of both whole rock and microsamples, and cathodoluminescence suggest a near-surface origin for the dolomite textures and fairly uniform fluid composition during dolomite formation: ^d r13C values average +5.52 ^pmil PDB (ranging from +3.35 to +7.08 ^pmil), and ^dgr18O values average +2.69 ^pmil PDB (ranging from -1.70 to +5.35 ^pmil); dolomites luminesce a dull orange-brown, with only minor bright orange rims found lining some pores.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.