During deposition of the Upper Cretaceous Austin Chalk and Taylor Marl, "serpentine plugs" formed by submarine volcanic activity along major fault zones in the ancient Gulf of Mexico. After eruption, palagonite-tuff mounds, formed by the hydration of basaltic glass, localized deposition of shoal-water carbonates on topographic highs.

The serpentine plugs occur along an arcuate belt extending approximately 250 mi (400 km) from Milam County southwestward to Zavala County, Texas.

Hydrocarbon traps in and around serpentine plugs have yielded approximately 47 million bbl of oil and significant quantities of natural gas from altered volcanic tuff and associated shoal-water carbonates. Shallower production is from overlying sedimentary rocks structurally influenced by volcanic plugs.

Entrapment of hydrocarbons occurs as: (1) stratigraphic traps within porous zones of carbonate units; (2) stratigraphic traps within porous volcanic tuff (serpentine); (3) structural traps within overlying sands; and (4) traps within high-porosity fracture zones. Exploration for plugs should be concentrated along existing fault zones by either magnetic or seismic surveys.

Analysis of the plugs suggests that there are at least 3 distinctive groups: (1) a southern group characterized by well-developed and productive marginal sands; (2) a middle group characterized by hydrocarbon-saturated biocalcarenite and reworked volcaniclastic facies; and (3) a northern group in which the dominant hydrocarbon saturation is in the volcanics themselves. Each of these groups appears to reflect a somewhat different geologic history.

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