GEOCHEMICAL AND NOBLE GAS INDICATORS OF THE ORIGIN OF CARBON DIOXIDE GAS ACCUMULATIONS IN THE GREATER PERMIAN BASIN

The Permian Basin of west Texas and southeastern New Mexico has numerous Paleozoic natural gas reservoirs with high concentrations (> 5 mol%) of natural carbon dioxide (CO₂). These high-CO₂ gas fields are the focus of a U.S. Geological Survey research initiative on CO₂-enhanced oil recovery, natural CO₂ resources, and analogues for anthropogenic CO₂ storage. To better understand the source of the CO₂ in these reservoirs, compositional and isotopic analysis of free and dissolved gases, including noble gas isotopes, were conducted on gas samples collected from 23 producing petroleum wells located across a four-county area, as well as water and gas samples collected from hot springs located along the southwestern outer perimeter of the Permian Basin in the Rio Grande River valley.

Results indicate that CO₂ concentration in the sampled reservoirs and hot springs ranges from 0.4 to 90.6 percent with the highest concentrations occurring in the southwestern basin near the Marathon-Ouachita Thrust Belt. Values of δ¹³C-CH₄ (methane) for the gas reservoirs range from -43.5 to -35.3 ‰, while the hot spring values range from +16.3 to +24.3 ‰. These data suggest two active CH₄ systems; CH₄ in gas reservoirs is of thermogenic-organic origin whereas CH₄ in hot springs is complex and may be partially magmatic (abiogenic) sourced. Values of δ¹³C-CO₂ for all samples range from -10.3 to -1.96 ‰, with hot springs generally associated with more negative values of δ¹³C-CO₂ than reservoir samples. ³He/⁴He ratios of collected gases and dissolved water samples range from 0.018 to 0.87 R/Ra (relative to ³He/⁴He ratio in air), indicating a mix of sources: crustal-sourced He (< 0.02 R/Ra) and associated CO₂ for samples collected in more interior parts of the basin, and magmatic-sourced He (> 0.8 R/Ra) and CO₂ in southwestern regions of the basin and at hot spring locations. The CO₂/³He ratios are variable and range from 2.04 x 10⁷ to 4.12 x 10¹⁰. These initial results and interpretations differ from previous studies, and suggest a complex mix of magmatic and crustal sources for the CO₂ in the sampled areas. Likely sources of the magmatic gases may be related to post-Paleozoic volcanism and subsequent gas migration along deep-seated faults and lineaments.