STORING CARBON DIOXIDE; NATURE’S LESSONS FROM THE MCELMO DOME FIELD, SOUTHWESTERN COLORADO

New research into the sequence stratigraphy and diagenetic evolution of the Mississippian Leadville Limestone reservoir in the McElmo Dome field in Southwestern Colorado is providing insights in to one of the largest naturally occurring subsurface accumulations of carbon dioxide in North America. Reservoir and potential storage space is controlled primarily by facies selective dissolution, dolomitization, and prolonged sub-aerial exposure, which vary in three dimensions within a sequence stratigraphic framework. Combined outcrop and core studies indicate that the Leadville Limestone built into the Antler foreland basin in multiple fourth order sequences which thickened to the northwest. These Mississippian (Tournasian /Osagean) carbonates are underlain by a sequence boundary on the regionally karsted and eroded unconformity on the top of the Late Devonian (Famennian) Ouray Limestone (Armstrong and Mamet, 1976). In the McElmo Dome region, basal dolomitic boundstone and grainstone-dominated sequence sets backstep to the east, and are overlain by deeper water bryozoan-rich dolo-wackestone facies during the third order transgressive systems tract. In these dolomites intercrystalline and moldic porosity form the best reservoirs with porosities as high as 30% and permeability up to 200 millidarcies (Cappa and Rice, 1996). Progradational crinoidal packstones and grainstones overlie the muddy dolomites and mark the transition to the highstand system tract. Syntaxial calcite cements in these crinoid-rich facies occlude primary intergranular porosity, making this portion of the sequence a very low quality reservoir, except where it has been subjected to hydrothermal fluids along faults and fractures. Reservoir quality in the upper sequences of the Leadville Limestone is also diminished by the extensive re-cementation associated with prolonged late Mississippian sub-aerial exposure, karst formation, and infilling by early Pennsylvanian Molas Formation siltstones. Lessons learned from studying this naturally occurring carbon dioxide reservoir in carbonates are that storage capacity and compartmentalization may be facies selective, and they are further influenced by alteration at sequence boundaries.