RESERVOIR HETEROGENEITY CHARACTERIZATION OF THE BIGHORN DOLOMITE FOR CO₂ GEOSEQUESTRATION, MOXA ARCH, SE WYOMING

Accurate and thorough geologic site characterization is crucial to choosing and engineering a location for long-term subsurface storage of carbon dioxide. Characteristics of a viable site include large reservoir capacity, permeability values that allow reasonable injection rates, geomechanical and geochemical conditions that will trap the CO₂ without severely occluding porosity, and an effective structural or stratigraphic seal. As federal and state governments incorporate these characteristics into regulations for geologic sequestration of CO₂, geoscientists are working to assess these attributes and conduct preliminary modeling necessary for permitting. Unlike subsurface models designed to predict potential hydrocarbon reservoirs, CO2 geosequestration models must correctly forecast storage capability and predict leakage risk for thousands of years. It is therefore imperative that models consider micro, local, and regional variations in the potential injection zone and seals.

The importance of considering these scale variations is exemplified by the Moxa Arch of southwestern Wyoming, where the Ordovician Bighorn Dolomite is a potential storage reservoir. The Bighorn Dolomite displays centimeter-scale porosity variations from the dolomitization process and subsequent compaction. Compaction and folding worked to chemically and mechanically deform the reservoir, resulting in a complex stylolite and fracture network. Porosity differences, stylolites, and fractures combine to form baffles and barriers to flow, and may be responsible for reservoir compartmentalization at the micro to local scale. The overlying potential seal, the Darby Formation, further complicates the issue because it represents an onlapping series of sub- to peritidal dolomitized sandstone, siltstone, and claystone. The lithologic variation in this seal is significant and should be characterized within a regional stratigraphic framework. Only with detailed analyses of multi-scale heterogeneities can reservoirs such as the Bighorn Dolomite be confidently utilized for long-term CO₂ storage.