Diagenesis of the San Andres Formation in the Seminole Unit in the Central Basin Platform, West Texas

Abstract

The cumulative oil production of the San Andres Formation in the Permian Basin of Texas and New Mexico is over ten billion bbls. These reservoirs have the potential for significant addition to these cumulative volume through implementation of CO₂ EOR in Residual Oil Zones underlying the existing fields. Previous researches were dominated by depositional models and facies analysis. This paper presents the results of a preliminary diagenetic study on the San Andres Formation in the Seminole Unit from the Northeast edge of Central Basin Platform. Dolomitization and anhydrite cementation were the two dominant diagenetic phenomena impacting the reservoir. The first and dominant phase of dolomitization commonly associated with anhydrite and elevated δ¹⁸O values, support they were formed by brine reflux. Evaporation of seawater resulted in the varying salinity from normal seawater to brine close to gypsum saturation, and the change of δ¹⁸O values in dolomites. Some spheroidal dolomite cement precipitated probably related to compaction and organic materials. Anhydrite precipitation, cementation, and replacement, probably occurred from deposition and persist in the entire paragenetic sequence. The result of fluid inclusion analysis is consistent with anhydrite cements forming at temperatures of ~50 to 75 °C. Multi-stage bacterial sulfate reduction (BSR) has probably occurred and was responsible for pyrite and calcite replaced anhydrite, as well as the negative shift of δ¹³C both in bulk dolomite and replacive calcite. This study confirms the importance of diagenesis, includes dolomitizaiton, sulfate emplacement, and BSR, on a sulfate-enriched dolomite reservoirs development.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018