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Monitoring the Active Migration and Biodegradation of Natural Gas in the Trinity Group Aquifer at the Silverado Development in Southern Parker County, Texas


The Lower Cretaceous Trinity Group aquifer contains natural gas generated by the Barnett Formation that migrated over geologic time into the aquifer from the underlying Pennsylvanian Strawn Group across an angular unconformity. Thermal gas dissolved in water samples collected from that aquifer generally is severely biodegraded except where water wells either occur above – or inadvertently penetrate – gassy Strawn strata. We describe changes in the abundance and the molecular and C isotopic composition of the natural gas dissolved in water samples collected periodically from all accessible wells in the Silverado Development between December 2010 and December 2012. Initially the gas dissolved in only six out of 22 water wells contained >20 mol% methane, and the presence of isotopically-heavy ethane (> −24 per mil) in most water samples proved that natural gas in the aquifer generally was severely biodegraded. By May 2012, the amount of dissolved methane had increased substantially at one well location from ∼26 mol% to ∼63 mol%. The C isotopic composition of ethane indicates dissolved natural gas was less biodegraded at two other well locations than it was in December 2010, while ethane was more biodegraded than previously measured at a different well location. We also observed changes in the amount of dissolved methane and the C isotopic composition of ethane in water samples collected from several wells in August 2012 and December 2012. During the 24–month period that we studied water samples collected from the Trinity Aquifer in the Silverado Development, the amount and/or the C isotopic composition of HC gas compounds dissolved in water changed in ∼20% of the wells. At one well location, the dissolved natural gas systematically became more biodegraded every time water was collected. But at two other well locations, the data indicate that a pulse of fresh natural gas migrated into the Trinity Aquifer across the unconformity between December 2010 and May 2012, and by December 2012 subsequently was biodegraded by microbes in that aquifer. Temporal and spatial changes in the amount and composition of natural gas dissolved in water samples demonstrate that thermal gas in the Strawn Group episodically migrates vertically across an unconformity into the Trinity Aquifer. These data reinforce our prior conclusion that two nearby horizontal production wells completed in the Barnett Formation are not the source of the natural gas dissolved in the Trinity Aquifer.