The Lone Star Hydrocarbon Producers from Ancient Volcanoes and Serpentine Plugs

Serpentine and volcanoes in Texas? Yes, against popular belief, the Texas terrain once featured active volcanoes and was idyllic for prehistoric lifeforms. Only 200 volcanoes are dormant today; the last eruption occurred in the Trans Pecos region about 30 million years ago. There are numerous ruins of volcanoes that were formerly present in the shallow oceans about 80 million years ago in South and Central Texas. Pilot Knob in Travis County serves as one such location, and there are other others close to Uvalde, according to Linda McCall of the University of Texas.

In contrast to how Pilot Knob appears today when it was a towering titan 80 million years ago, it was a low-lying hill. In southern Texas, it was covered by a warm, shallow sea during the Cretaceous period. Layers of limestone and green volcanic rock (basalt) were formed over millions of years of dormancy and activity cycles when seawater crept into the volcano's throat, came into contact with hot magma, and erupted.

The installation of "serpentine plugs" was made possible by Texas' early marine heritage; Texas is covered with these plugs for a distance of 250 miles (402 kilometers). All of the plugs, which are believed to date from the Late Cretaceous and produce oil (more than 50 MMBO), whether exposed to the surface or buried at depths of 5,000 feet or more, are tightly related to the plugs or are indirectly related to the plugs by generating oil from deposits impacted by structures. Serpentine plugs are pyroclastic in origin and comprise weathered palagonite tuff from volcanic eruptions.

Thirty-five oil fields have used serpentine as a source of production, and 144 of these fields generated significant amounts of oil that were intimately related to the presence of serpentine. Seven oil fields that produce serpentine produce between one and ten million barrels apiece, for a total of 31 million barrels by 1965.

A water well was dug in 1915 in a serpentine-like, heterogeneously changed igneous rock in Williamson County, about one mile east of Thrall. Knowing that this was an isolated body of igneous rocks sitting in a portion of Upper Cretaceous sedimentary strata assumed to be intrusive in origin, so named an igneous or serpentine plug, came from the development of the Thrall oil field later on.

Further analysis of a core from wells in these fields by petrographers revealed that these rocks are a complex assemblage of hydrate minerals created by altering basaltic rocks and that while serpentine was present in various concentrations, it did not predominate.

Another thing to consider is that these rocks' texture and structure suggested that they were pyroclastic, the product of extrusive volcanic activity, with little to no evidence of intrusion activity.

Currently, serpentine plugs from submarine volcanoes near the end of the Austin Chalk Formation's deposition are found in South Texas. These plugs are part of a belt that extends southeast from the outcrops of the early Taylor and Anacacho formations and roughly parallels the Balcones Fault Zone. Between 15 and 40 miles along the Gulf Coast regional dip, behind the outcrops of Eocene rocks halfway to Claiborne formations.

In contrast to other igneous rock reservoirs, which often rely on open fractures rather than matrix pores for productivity, the igneous rocks in south and central Texas have been found to contain hydrocarbons. The geologically traditional standard bearers for hydrocarbon systems are sandstones and limestones, but igneous rocks are beginning to make inroads into the possibility of extended oil recovery to meet the rising energy needs of the continental United States and its West Texas Intermediate (WTI) supply market.

Gratitude to Dr. Susan Nash for enabling me the ability to receive research articles with topics on petroleum production from igneous based rocks and serpentine plugs in Texas.

Referenced articles:

Loucks, R. M. (2022). Textures, Mineralogy, and Reservoir Properties of an Altered Mafic Tuff Core from the Upper Cretaceous of Central Texas. GCAGS Journal, 15.

Simmons, K. A. (1967). A Primer on Serpentine Plugs in South Texas. Around the Society , 13.