The Wyoming carbon underground storage project: Geologic characterization of the Moxa Arch and Rock Springs Uplift

Abstract The state of Wyoming, in the northwestern United States, produces 40% of the nation’s coal, most of which is transported out of the state. The remainder is used at power plants within Wyoming to generate approximately 7% of U.S. electricity. Carbon capture and storage from these power stations could significantly reduce U.S. carbon emissions. Wyoming statutes and rules proposed by the U.S. Environmental Protection Agency and Wyoming Department of Environmental Quality regarding subsurface carbon storage require that CO2 injection must not affect established or potential drinking water aquifers, oil and gas fields, or other mineral estates. Wyoming has several potential large-scale geologic carbon storage reservoirs that meet these criteria, in the form of saline aquifers in regional basins and uplifts. The Wyoming Carbon Underground Storage Project has recently been funded by the U.S. Department of Energy and the state of Wyoming to 1) assess the CO2 storage potential of two possible locations in southwestern Wyoming: The Moxa Arch and the Rock Springs Uplift, 2) develop a system for displaced fluid management, 3) plan monitoring and verification activities, and 4) design infrastructure in preparation for geologic carbon sequestration. The Wyoming Carbon Underground Storage Project represents collaboration between the University of Wyoming, the Wyoming State Geologic Survey, ExxonMobil Corporation, Los Alamos National Laboratory, and Baker Hughes Incorporated. The authors are involved primarily in the geologic stratigraphic and structural characterization of the Moxa Arch and Rock Springs Uplift. The Moxa Arch is an anticline that trends from the Uinta Mountains, 200 km north-northwest to the eastern front of the Wyoming fold-and-thrust belt. Potential storage reservoirs on this large geologic structure include the Jurassic Nugget Sandstone, the Mississippian Madison Limestone, and the Ordovician Bighorn Dolomite. The Nugget Sandstone is a heterogeneous and anisotropic eolian deposit that has been extensively exploited for oil and gas at certain locations on the Moxa Arch, which complicates its usage as a repository for carbon dioxide. The Madison Limestone is a proven storage reservoir; ExxonMobil has been injecting CO2 (up to 25 MMCFD) and H2S (up to 65 MMCFD) into the Madison Limestone on the Moxa Arch for seven years at the Shute Creek Gas Plant. The Bighorn Dolomite is stratigraphically complex with large variations in porosity and permeability due to primary burrowing and repeated dolomitization and dedolomitization. Depending on location on the anticline, the Nugget Sandstone lies 3 to 6.5 km below the surface (-1 to -4.5 km subsea), and the Bighorn Dolomite and Madison Limestone range from 4.5 to 8 km below the surface (-2.5 to -6 km subsea). The Moxa Arch is structurally uncomplicated; it is a basement-involved anticline formed by a west-vergent Late Cretaceous-age thrust fault, with gently-dipping limbs (0 to 5 degrees). Leakage risk is extremely low because impermeable evaporite (anhydrite) intervals overly the potential reservoirs, and preliminary interpretation of seismic data reveal that few faults exist other than the main thrust. The Naughton Power Plant, a 707 MW coal-fired power station emitting up to 6 Mt of CO2 per year, lies 30 km west of the crest of the anticline. The Rock Springs Uplift, 100 km east of the Moxa Arch, extends 80 km north from the Wyoming-Utah border. The target storage reservoirs are the Pennsylvanian Weber Sandstone (correlative to the Tensleep Sandstone) and the Mississippian Madison Limestone. The Weber Sandstone exhibits wide variations in reservoir properties (porosity and permeability) due to dune/interdune/intradune facies changes, and appears to have experienced local secondary diagenesis that further reduced porosity. The Madison Limestone is expected to have similar reservoir properties to its lithologic correlative on the Moxa Arch. The Weber Sandstone and Madison Limestone range in depth from 2 to 6 km below the surface (0.3 to -4.5 km subsea), depending on location on the anticline. The Rock Springs Uplift offers challenges for structural analysis. Like the Moxa Arch, the Uplift was formed by a Late Cretaceous-age west-vergent basement-involved reverse fault, but the limbs of the fold are at steeper dips (approximately 15 degrees on the west limb, shallower on the east limb), and these limbs are cut at depth by additional reverse faults. In a hydrocarbon field on the southeastern flank of the uplift there is a possibility that condensate is migrating from the Weber Sandstone along one of these reverse faults, suggesting that the trap is breached. In addition, a series of east-west trending normal faults cut Cretaceous shales at the surface, possibly with throws that exceed the thickness of the uppermost regional seal. It is necessary to determine if these east-west faults also compromise the Triassic units that could provide a seal above the Weber Sandstone. The Jim Bridger Power Plant (coal-fired) is located on the east flank of the Rock Springs Uplift; it has 2200 MW capacity and emits up to 18 Mt of CO2 per year. Future U.S. energy demands will draw heavily on Wyoming’s coal-fired power plants, and the state is taking steps to sequester the produced carbon. Wyoming hosts several large geologic traps that if properly risked and evaluated have promise as long-term, stable repositories for anthropogenic carbon dioxide. Based upon our preliminary assessment of the multiple clastic and carbonate receiving formations in the Moxa Arch and Rock Springs Uplift, and the experience of successful injection at ExxonMobil’s Shute Creek Gas Plant, these geologic structures in southwestern Wyoming are among the most promising large CO2 geologic storage sites in the United States.