2014 Rocky Mountain Section AAPG Annual Meeting

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Noble Gas Geochemistry at the La Barge Platform, Wyoming, USA: CO2 Source and Potential Total Petroleum System Investigation Tool


Initial analyses of noble gas isotopes, major element isotopes, and gas composition data from high carbon dioxide (CO2) natural gas samples at the La Barge Platform, Wyoming, USA, indicate that the CO2 has a strong crustal signature. This finding is in line with geochemical, mineralogical, and fluid inclusion analyses that suggest that inorganic thermochemical sulfate reduction (TSR) is the source of La Barge's CO2. Isotopes of noble gases released from hydrocarbons during TSR carry the isotopic signature of the air-saturated sea water and crustal environment of the hydrocarbon source rock. Helium isotope ratios reported relative to their concentrations in air ranged from 0.077 to 0.085 (R/Ra); these values as well as those of neon and argon indicate crustal origin. La Barge Platform is the largest source of CO2 used in enhanced oil recovery (EOR) at oil and gas fields across the northern Rocky Mountains. The field is located in southwestern Wyoming, west of the towns of Big Piney and La Barge. The field produces natural gas (21% average) and CO2 (65% average) from the Mississippian-age Madison Limestone. In this initial round of sampling, three gas samples were collected from 14,500 to 16,500 feet of depth at three different well heads. These results are an initial effort towards describing the total CO2 system at La Barge; this includes source, reservoir extent, connectivity, and differentiability or fingerprinting. CO2 generated from TSR is chemically related to the source hydrocarbon and its total petroleum system. Noble gas isotope analyses may be another tool in defining extents and tracing migration in total petroleum systems and associated CO2 systems.