A geologic deconstruction of one of the world's largest natural accumulations of CO2, Moxa arch, southwestern Wyoming

Abstract

Geologic sequestration of anthropogenic carbon dioxide (CO2) is one of the most promising approaches to safely and effectively reduce emissions of CO2 created through the oxidation of fossil fuels. Methods used by the petroleum industry in the characterization of hydrocarbon accumulations can be used to assess potential subsurface traps for sequestration purposes. In this article, we use these approaches to evaluate the characteristics of a naturally occurring accumulation of CO2 in western Wyoming. The Moxa arch is a 200-km (124-mi)-long basement-involved anticline. The Mississippian Madison Formation and the Ordovician Bighorn Dolomite contain the most CO2 within the structure. Relict anhydrite in these and other Paleozoic units was an important factor in evolving hydrocarbons into CO2 through inorganic thermal sulfate reduction and, more importantly, in creating a seal to hold large columns of buoyant gas. Fluid-inclusion data sets have been particularly useful in understanding the sealing characteristics of the units within the Moxa arch and affirming that the Devonian Jefferson, Mississippian Amsden, and Triassic Dinwoody and Woodside formations have been very effective seals. Existing pressure data reveal that the two gas columns in the Madison and Bighorn formations lie on a similar gradient and share a common gas-water contact, yet are likely not in hydraulic communication. Currently, all available data suggest that both reservoirs share a fault-dependent spill point. By reconciling the spill points of the gas in the Madison and Bighorn reservoirs, their compositions, their initial and current pressures, their seal, and the uncertainties associated with injection of CO2 can be identified and potentially derisked with additional information. If the Madison and Bighorn are filled to their fault-dependent spill point, it implies that additional storage capacity in the reservoir can only be obtained by production of the original gas column. This uncertainty may be abated if data from future drilling demonstrates that neither the Madison Formation nor the Bighorn Dolomite have a fault-dependent spill point, suggesting that these structures are underfilled with respect to their closure and possess additional storage capacity.