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Abstract
Gulf Coast Association of Geological Societies Transactions
Volume 54 (2004)
EXTENDED ABSTRACT: Bacteria-Petroleum
Reactions; Salt Dome Cap Rock Genesis Compared with Similar Processes from
Permian Outcrops in West Texas
Lock, Brian E.,1 Walker
Fife, Ashley,1,2 and Anderson, Elizabeth1
ABSTRACT
Gulf Coast salt domes commonly develop a cap rock of secondary
limestone over residual anhydrite where salt penetrates close to the surface.
Anhydrite is common in domal salt. It is somewhat less soluble in fresh 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 sulfate-reducing 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, and with native sulfur,
H2S gas and patches of extremely coarse secondary, hydrothermal?
selenite gypsum. Selenite crystals may be several feet in diameter.
Some outcrops appear to represent a calcitized
facies of the Salado Formation, which over-lies the Castile Formation. The
Salado secondary carbonates include botryoidal cements, which suggest that
early bacterial precipitation may have taken place at or near the very late
Permian sea floor. Methane seepage is suggested to have occurred over a geologically
long time interval.
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