ORIGIN AND LONGEVITY OF CARBON DIOXIDE IN THE MOXA ARCH, GREEN RIVER BASIN, WYOMING, U.S.A

Several Paleozoic units (Pennsylvanian Tensleep, Mississippian Madison Limestone, and Ordovician Bighorn Dolomite) within the Greater Green River basin (GGRB) in western Wyoming contain significant proportions of CH₄ and CO₂. The Moxa arch is a 200-km long, basement-involved structure within the GGRB that formed during the Campanian to Eocene, and is the largest gas-bearing structure in the basin. Within the arch, the Madison and Bighorn formations contain several hundred TCF of gas filled to an apparent fault-dependent spill point. The majority of the gas is carbon dioxide (65-95%), but also includes methane (0-23%), hydrogen sulfide (0.5-6%), nitrogen (6-11%), and helium (~0.5%). The origin of this gas, and its prolonged existence within the Moxa arch trap, speak to the potential stability of geologic carbon sequestration.

The joint occurrence of hydrocarbons and dissolved sulfate from anhydrite at high temperatures (> 210°C) resulted in thermochemical sulfate reduction (TSR) and is the origin of the H₂S in the Paleozoic reservoirs. Evidence confirming TSR reactions include the replacement of nodular anhydrite by calcite containing fluid inclusions with high H₂S contents (6-38 mole %) and heavy d³⁴S isotopic values in pyrobitumen of +18.9 ± 3.9 per mil that are consistent with inferred values for native Mississippian sulfate. Interestingly, CO₂ from the Madison Formation at Labarge field has d¹³C compositions of about -4.4 per mil, irrespective of the concentration of CO₂. This signature is inconsistent with TSR as the sole source of CO₂, and instead implies that the majority of it is mantle-derived. This is also supported by the CO₂/³He ratio of the gases of approximately 9 x 10⁸, similar to the MORB range of 1 x 10⁹ – 1 x 10¹⁰ (Marty and Jambon, 1987). ³He/⁴He ratios trend around 1.2x10^-7, suggesting that most of the helium is radiogenic and derived from the crust underlying the sedimentary basin. Fluid inclusion evidence suggests that CO₂, as a species, has been trapped within the Moxa since maximum burial (during the Paleogene). Estimates of the age of the gas accumulation, based on steady state accumulation of ⁴He within the Paleozoic reservoirs, implies longevity of the gas accumulation and excellent seal integrity. This bodes well as a natural analog “sequestration system” for potential geologic storage of CO₂.