A SEQUESTRATION SYSTEMS APPROACH TO GEOLOGIC CHARACTERIZATION OF CO2 STORAGE SITES: EXAMPLES FROM THE GREEN RIVER BASIN, WYOMING

An analysis of naturally occurring accumulations of CO₂ can provide insight into the geologic characteristics necessary to safely contain anthropogenic CO₂, including its trap, reservoir, and seal. The Moxa Arch (MA), a 200 km-long anticline located in the western Green River Basin, currently contains over 100 TCF of CO₂, and its distribution can be understood by a description of the reservoirs and seals contained within this large trap.

The Mississippian Madison Limestone and Ordovician Bighorn Dolomite are the two major CO₂ reservoirs within the MA. Anhydrites within the Lower Devonian Jefferson Formation appear to segregate gas in the Bighorn Dolomite from the Madison Limestone. The Upper Mississippian Amsden Formation overlies the Madison and also serves as an effective seal. In addition, the Pennsylvanian Weber and Permian Phosphoria formations also contain comparatively small accumulations of CO₂. Evaporite beds (anhydrite and minor halite) within the Triassic Dinwoody Formation and Woodside Shale are very effective seals that isolate the CO₂ within the Paleozoic intervals from the overlying reservoir units, a point reinforced by fluid inclusion volatile (FIV) datasets. The Bighorn and Madison reservoirs are hypothesized to be filled to a single fault-dependent spill point along the western edge of the structure.

The Rock Springs Uplift (RSU), located in the central Green River Basin, is a north-south striking, broad anticline cut by several late stage normal faults. It is currently being studied as a potential site for storing anthropogenic CO₂. The prospective reservoirs in this trap include the Madison and Weber/Tensleep, which like the MA, store accumulations of naturally occurring CO₂; exact volumes are uncertain. The Amsden/Morgan Formation is the suspected top seal for the Madison Limestone. The top seal for the Weber/Tensleep is hypothesized from preliminary FIV work to be the Triassic Dinwoody and Red Peak formations. The CO₂ accumulations and their extents appear to be controlled by faults with throws, which in at least one case, exceeds the thickness of these seals. More work is needed to understand the fill-and-spill relationships that isolated the CO₂ traps on the RSU to determine the feasibility of long-term storage at this site.