CHARACTERIZATION OF CO2 RESERVOIRS IN THE ABSAROKA THRUST SHEET, IDAHO-WYOMING FOLD-AND-THRUST BELT

Several large faulted anticlines occur in the Absaroka thrust sheet in Bonneville and Caribou Counties, SE Idaho. These structures have been periodically tested for oil and natural gas since the mid-1900s, but they were extensively reevaluated through reflection seismic surveys and additional drilling during the “overthrust exploration boom” of the mid-1970s through mid-1980s. Although no commercial quantities of hydrocarbons were discovered, several wells produced variable quantities of CO₂ from the Jurassic Twin Creek and Nugget Formations during drill-stem testing, as well as from deeper units. CO₂ generation may have occurred as early as the Eocene during intrusion of the Caribou Mountains and related intrusive bodies in the area, enhanced by crustal heating along the eastern Snake River Plain to the north. Therefore, these reservoir rocks and structures may be excellent natural analogues for CO₂ sequestration due to their integrity over long periods of geological time.

This study focuses on outcrop and lab analyses of Jurassic, Triassic and deeper Paleozoic reservoir rocks exposed in the Northern Salt River Range and Big Hole Mountains, WY-ID. The Leeds Creek Member of the Twin Creek Formation consists of blocky, calcite-filled mode-I and mode-III fractures, indicating enhanced fracture-permeability during and immediately after deformation followed by post-kinematic cementation from warm CO₂-rich fluids. The Triassic/Jurassic Nugget Sandstone in the Stewart Peak Culmination is similar to cores from the hydrocarbon-producing region of the thrust belt in SW Wyoming, being bleached, friable, highly porous, and containing hydrocarbon residue; bleaching and bitumen residue may be an indicator of past hydrocarbon migration. The Pennsylvanian Wells Formation is similarly bleached in the area. The Phosphoria and Madison Formations show evidence of late-stage diagenesis involving warm formation fluids (low-temp hydrothermal alteration), including breccia systems, pervasive neomorphic recrystallization (including saddle dolomite), multiple-generation cements and veins, and localized silicification. Primary capping or sealing intervals above the hydrocarbon-CO₂ system include shale and evaporite in the Triassic section and thick shale throughout the Cretaceous succession.