Composition, Distribution, and Origin of Thermal, Microbial, and Biodegraded Natural Gas Dissolved in the Trinity Group Aquifer, Southern Parker County, Texas

The Twin Mountains Formation (Trinity Group) has moderate aquifer capacity in southern Parker County where it lies unconformably on Pennsylvanian gas-bearing Stawn strata. The Trinity aquifer contains a modest amount of dissolved natural gas that has migrated into it over geological time across the unconformity. We studied the molecular and isotopic composition of gas samples collected from two horizontal gas wells (Butler 1H; Teal 1H) completed in the deeper Barnett Formation, and also examined headspace and solution gas samples obtained from 25 nearby water wells. We then compared those data to the composition of a large number of gas samples produced from the Barnett Formation and Pennsylvanian reservoirs elsewhere in the Fort Worth Basin (FWB). The Butler and Teal wells produce thermal gas whose composition (C₁/C₂=6; N₂=1.2 mol%; CO₂=0.45 mol%; δ¹³C C₁ = -46.5 per mil) is very similar to the composition of natural gas produced from the Barnett Formation in several other wells in this part of the FWB. Barnett gas samples produced by the Butler and Teal wells contain isotopically-light N (δ¹⁵N = -3 to -4 per mil). A gas sample obtained from the annulus of the Butler well, in contrast, is a mixture of air; thermal gas from Pennsylvanian strata to which the vertical wellbore is open; and microbial gas. The thermal and/or microbial gas components are mixed with more N₂ and isotopically-lighter N than the gas produced from the Barnett Formation. An annulus gas sample obtained from the Teal well is a mixture of only air and microbial gas. All of the gas samples obtained from the water wells are mixtures of air and thermal gas (except that one water well contains microbial gas as well as thermal gas). The C₁/C₂ ratios in natural gas from the water wells (which are higher than this ratio in Barnett gas samples) are influenced by two processes: (1) the preferential metabolism of ethane by microbes (demonstrated by the C isotopic composition of C₂, which is heavier than -25 per mil in eight dissolved gas samples); and (2) the higher solubility of methane than ethane in water. Biodegradation has not significantly altered the C isotopic composition of C₁ in the dissolved gas. The areal distribution of dissolved biodegraded gas and microbial gas supports the conclusion derived from independent geological and petroleum engineering data that Strawn gas sands (not the Butler or Teal wells) are the source of the natural gas dissolved in the Trinity aquifer in this area.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California