Abstract: Middle Guadalupian Coastal Carbonate Complex--CO₃-SO₄ Diagenesis and Timing of Hydrocarbon Migration

Lawrence R. Baria

The North McElroy field (Crane and Upton Counties, Texas) produces from the middle Guadalupian (Permian) Grayburg Formation. The field is astride the Central Basin platform--Midland basin shelf break. As a result of its paleogeographic position, the Grayburg reservoir includes facies belonging to various peritidal environments. Normal carbonate and sulfate diagenesis within each facies naturally has provided a distinct delineation of the various depositional environments; however, the preferred migration of hydrocarbons into some facies has amplified these mineralogic and textural distinctions. A temporal as well as spatial paragenetic scheme follows.

In the high intertidal and sabkha environments of the platform, early dissolution and vadose calcite cementation preceded a period of replacement and cementation by dolomite, anhydrite, and lutecite. Permeability in this facies apparently was reduced to nil very early in the sediment's history, retarding any subsequent diagenetic processes.

In the lower intertidal and subtidal environments, strata characterized by high original permeabilities (oolite shoals and reefs) have been preferentially cemented by anhydrite during early diagenesis, while those of initially low permeability (lagoon and shelf muds) have been dolomitized. As a general consequence of this prehydrocarbon stage of diagenesis the best reservoir facies are now those with the worst original permeability.

Secondary dolomitization of the reservoir probably is related to increased salinities of Ochoan-aged basin brines. Subsequent oil migration probably occurred during late Paleozoic and early Mesozoic basin-margin fracturing and downwarping.

With the migration of hydrocarbons and associated waters of a lower-than-connate salinity into the reservoir, a reequilibration between host mineralogies and pore fluids has occurred. Anhydrite has inverted to gypsum at the expense of reservoir pore space. Bacterial related reduction of sulfates has further produced a fracture filling suite of minerals consisting of calcite, sulfur, and pyrite.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA