MARINE MICROBIAL CARBONATE FACIES, FABRICS, AND PETROLEUM RESERVOIRS IN UTAH

Recent discoveries in Early Cretaceous lacustrine microbial carbonates of the deepwater offshore of Brazil (pre-salt Santos Basin reservoirs) as well as other large oil deposits in microbialites reveal the global scale and economic importance of these distinctive reservoirs. Evaluation of the various microbial fabrics and facies, associated petrophysical properties, diagenesis, and bounding surfaces are critical to understanding these reservoirs. Utah has well-documented examples of lacustrine microbialites in the Eocene Green River Formation within the Uinta Basin of northeastern Utah. In addition, Great Salt Lake is a modern hypersaline lake, actively forming microbialites.

A survey of carbonate cores from active Utah oil fields also reveals a variety of previously undocumented marine microbial fabrics, associated carbonate grains, pore types, and reservoir characteristics. The reservoirs, fields, and geologic locations, respectively, are the (1) Jurassic Twin Creek Limestone, Pineview field, thrust belt, (2) Permian Kaibab Limestone-Triassic Moenkopi Formation, Upper Valley field, Kaiparowits Basin, (3) Pennsylvanian Paradox Formation, Greater Aneth field, Paradox Basin, and (4) Mississippian Leadville Limestone, Lisbon field, Paradox Basin. Like their lacustrine counterparts, these marine rocks display a wide variety of stromatolitic and thrombolitic growth forms, and possible leolites in mud mounds; associated carbonate grains include ooids, peloids, and oncoids. Porosity has developed in many of the microbial fabrics: intercrystalline, dissolution, interparticle, and extensive microporosity. Microbial dolomite/dolomitization have enhanced porosity and permeability development.

These marine microbial carbonates represent undocumented zones within existing Utah oil and gas fields. They could also become the drilling targets for new potential hydrocarbon plays in Utah and the Rocky Mountain region. These publically available cores at the Utah Core Research Center serve as production-scale analogs for comparison of marine microbialites to the more common freshwater lacustrine microbial reservoirs. This offers the opportunity to better identify marine microbialites and their hydrocarbon potential in the Rockies and elsewhere in the world.