Bacteria-Petroleum Reactions; Salt-Dome Cap-Rock Genesis Compared with Similar Processes from Permian Outcrops in West Texas

Brian E. Lock¹, Ashley W. Fife², and Elizabeth Anderson^1
1 Department of Geology, University of Louisiana, Lafayette, Louisiana
² Now at: Core Laboratories, Lafayette, Louisiana

Gulf Coast salt domes commonly develop a cap rock of secondary limestone over residual anhydrite where salt penetrates close to surface. Anhydrite is common in domal salt. It is somewhat less soluble in groundwater than the host halite; hence a concentration remains above salt as the rising salt dissolves. Hydrocarbons are commonly associated with the salt structures, and sulfate-reducing bacteria utilize some of the anhydrite as an oxygen source to enable them to metabolize the oil or gas. Extensive porosity results from the subsequent mineralogical and volumetric changes.

Methanophage bacteria have played a very similar role in association with methane seeps through Permian evaporites in west Texas and New Mexico. Sulfuric acid (from oxidized hydrogen sulfide) is believed to have been the principal agent responsible for creating Carlsbad Caverns and the related Lechugilla cave system. This includes the associated gypsum deposits and stunning cave formations in Lechugilla. The down-dip components of the Permian hydrologic system can be seen in the Gypsum Plains area of Culberson County, Texas. Here, methane seeps, some still active, are associated with calcitized Castile Formation evaporites, complete with characteristic well-preserved disharmonic folds, along with native sulfur, H₂S gas and patches of extremely coarse secondary, possibly hydrothermal, selenite. Selenite crystals may be several feet in diameter.

Some outcrops appear to represent a calcitized Salado Formation that overlies the Castile Formation. This suggests that early bacterial precipitation of botryoidal cements took place at or near the very late Permian sea floor. Methane seepage is suggested to have occurred over a geologically long time interval.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004