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Cody Energy, Inc., Denver, CO

Abstract: Modern Analogs to Ancient Hydrocarbon-related Hypogenic Caves in West Texas and New Mexico

In West Texas and New Mexico, hypogenic caves were dissolved by sulfuric acid derived from hydrogen sulfide (H2S) generated in the Delaware basin. Although hydrogen sulfide is a product of volcanism, it also forms from the biological reduction of sulfate in the presence of hydrocarbons. Sulfur in biologically catalyzed H2S is characteristically enriched in d32S.

In the Guadalupe Mountains, H2S migrated to the water table where it degassed into the atmosphere and dissolved into oxygenated water coating bedrock surfaces. As H2S dissolved, it oxidized to sulfur and sulfuric acid. Where bedrock is limestone, this acid reacted with calcium carbonate and gypsum precipitated. Where bedrock is non-calcareous, sulfur may be preserved. This process created an oxidizing environment that mobilized elements such as uranium and altered montmorillonite clay to endellite. Some evidence for this mode of cave development is the presence of sulfur and sulfate minerals enriched in d32S, endellite clay, and oxides of uranium and vanadium (tyuyamunite). In the Guadalupe Mountains this process is no longer active.

There are examples of currently forming hypogenic caves in petroleum-bearing sedimentary basins worldwide. Possible modern analogs to Guadalupe caves are in the Big Horn basin, Wyoming, USA, near the Black Sea, Romania, and in the southern part of the Campeche Embayment, Tabasco, Mexico. Other petroleum-rich areas with possible hypogenic caves are Saudi Arabia and the Balearic Islands of the western Mediterranean basin.

In frontier basins, caves sulfate minerals enriched in d32S and evidence of a strong oxidizing environment may indicate the presence of hydrocarbons.

AAPG Search and Discovery Article #90918©1999 AAPG Southwest Section Meeting, Abilene, Texas