INVESTIGATION OF A CRETACEOUS VOLCANIC CENTER IN MEDINA COUNTY, TEXAS VIA MAGNETIC & GRAVITY SURVEYING

In 2015 a driller encountered basalt at depths of 300 to 360 feet while attempting to drill a water well into the Edwards Formation on a property in Medina County, Texas. The circulation pit revealed oil shows appearing to originate from near the igneous lithology. The association of hydrocarbons with the colloquially named “serpentine plugs” has been a long documented phenomenon, but this area of the volcanic trend has undergone limited exploration. In 2001, an aerial magnetic survey conducted by the USGS, in association with mapping the Edwards Aquifer formation, revealed widespread clusters of hundreds of previously undocumented localized magnetic anomalies potentially associated with igneous formations in Uvalde and Medina Counties, Texas. The trend contains submarine volcanic mounds formed during the Late Cretaceous that span northeast to southwest from Williamson County to Zavala County, Texas.

It has been proposed that these igneous bodies play multiple roles in a petroleum system serving as stratigraphic traps, structural antiforms, and as reservoirs themselves. While the USGS aerial survey revealed a far greater number of magnetic anomalies than previously expected, only a handful of the anomalies were able to be confirmed by reference to known outcrops. In order to make informed decisions on the hydrocarbon potential of these igneous bodies, geoscientists should acquire an accurate model of the location, morphology, and volume of the igneous bodies in question. I propose integrating a high resolution surface magnetic survey with gravity measurements. Rock properties such as magnetic susceptibility and density measured from a nearby volcanic outcrop analog will help to constrain a subsurface model of the volcanic formation at the Medina County locality.