ISOTOPIC ANALYSES OF GASES FROM LOW-BTU LEADVILLE LIMESTONE FIELDS, PARADOX BASIN, UTAH

The Mississippian Leadville Limestone of the Paradox Basin contains several low-BTU natural gas reservoirs that contain high concentrations of CO₂ and N₂ gas. Compositional analyses (11 gas samples) and isotopic analyses (3 gas samples) from the Lisbon and Lightning Draw fields (San Juan County, Utah) were performed to determine the source and history of the hydrocarbon and non-hydrocarbon phases.

Concentrations of the gases vary markedly: CH₄ concentrations ranged from 23-40 mol% (with C₂₊ ranging from 9-17 mol%); CO₂ ranged from 24-36 mol%; N₂ ranged from 13-30 mol%; He ranged from 0.5-1.6 mol%; and H₂S ranged from 0-1.3 mol%.  Hydrocarbon gases were analyzed for dD and d¹³C; the isotopic composition indicates limited alteration (microbial degradation, thermochemical sulfate reduction (TSR)) and/or non-homogeneity (multiple kerogen sources). The data are consistent with a hydrocarbon source, or sources, that had d¹³C values between 22 and 27‰ (VPDB).

The d¹³C of the CO₂ ranged from -5.3 to -11.1‰ (VPDB). This range of d¹³C values suggests that the CO₂ is derived from both organic (TSR) and inorganic sources (thermal decomposition of carbonates or magmatic degassing) as CO₂ derived from organic sources tend to be less enriched with respect to ¹³C relative to CO₂ derived from non-organic sources. Circumstantial evidence that TSR occurred includes anhydrite dissolution and sulfide mineralization, as well as measurable concentrations of H₂S in most samples. The concentrations and d¹³C values of CO₂ are inversely correlated, which suggests that the higher concentrations of CO₂ are the result of the mixing of CO₂ derived from non-organic and organic sources, whereas the lower concentrations of CO₂ are more predominantly non-organic in origin. These results are consistent with the reservoirs being charged with a uniform concentration of CO₂ from a non-organic source and variable concentrations of organically-derived CO₂ due to differences in local generation. The N₂:Ar ratios range from 172 to 327, greater than the atmospheric ratio of 84, and d¹⁵N values of N₂ range from -0.4 to 1.7‰ (Air) suggesting that N₂ was likely not from an atmospheric source. Forthcoming noble gas isotopic analyses should to help to constrain the source or sources of the non-hydrocarbon gases within the Leadville fields.