Prospects and Progress in the Green River Formation Oil Shale, Western United States
Published:December 01, 2009
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Alan R. Carroll, 2009. "Prospects and Progress in the Green River Formation Oil Shale, Western United States", Unconventional Energy Resources: Making the Unconventional Conventional, Tim Carr, Tony D’Agostino, William Ambrose, Jack Pashin, Norman C. Rosen
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The Eocene Green River Formation has long been believed to contain the world’s largest commercial oil shale deposits, having a recently estimated in situ resource of 2 trillion barrels of oil. Most of this resource lies within the Piceance Creek basin in northwestern Colorado, but additional oil shale intervals also occur within the Uinta basin in Utah and the greater Green River basin in Wyoming. The smaller reported magnitude of resources in Utah and Wyoming reflects thinner stratigraphic intervals but may also be due in part to more conservative assessment approaches (Utah) or to less complete assessment data (Wyoming).
The Green River Formation represents the deposits of long-lived lakes that occupied several intermontane basins within the broken “Laramide” foreland. Oil shale facies consist dominantly of carbonate-rich mudstone, having organic enrichment reaching up to 60 gallons of oil per ton (Fischer Assay). Lithofacies assemblages record a wide range of depositional conditions that define three major lake basin types. Under-filled lake basins often contain bedded evaporites deposited by hypersaline lakes, and their stratigraphy is dominated by aggradational lake cycles. Identifiable fossils are typically absent, but mudstone facies may be highly enriched in organic matter due to high algal and cyanobacterial productivity. Balancedfill lake basins contain lakes of fluctuating salinity that may reach brackish or fresh water conditions. Rich oil shale deposits and fish fossils are common, and their stratigraphy reflects a mix of aggradational and progradational geometries. Over-filled lake basins contain fresh water lakes, and their stratigraphy is dominated by shoreline progradation processes. Coal and carbonaceous shale are common, often associated with mollusks and other freshwater fauna. Oil shale can be present but is often of relatively low grade.
Volcanic tuff horizons interbedded with lacustrine strata have recently helped to establish an extensive chronostratigraphic framework for the Green River Formation. Radioisotopic dating of these tuffs (at temporal resolution of ∼100 ky) indicates that the Green River Formation spanned more than 8 million years, from <52 ma to >44 ma. Different lake types often occupied adjacent basins at the same time, indicating that fill and spill relationships were as important as climate in determining paleoenvironmental conditions and oil shale quality. Major lake-type transitions appear to have been caused by changes in regional drainage organization. For example, expansion of the Mahogany oil shale across the Piceance Creek and Uinta basins appears to have occurred in response to capture of a mountain river in central Idaho. This river flowed into Lake Gosiute in Wyoming, which in turn spilled into Colorado and Utah.
Large-scale commercial production of Green River Formation shale oil depends on resolving two significant problems: production costs, and potential environmental impact. Both concerns are currently being addressed through the development of new in situ retort techniques. These techniques involve slow heating of oil shale (to temperatures near 700°F), with the aim of directly producing relatively high quality light oil. In contrast to conventional mining and surface retort, asphaltenes and other potentially harmful components are retained in the subsurface. Requirements for process water are also greatly reduced.
At last three distinctly different in situ retorting methods are being developed for use in the Piceance Creek basin. The Shell In Situ Conversion (ICP) process uses vertical heating and production wells, with containment by an outer feeze-wall. Heating is accomplished by an electrical resistance element, and the freeze-wall is maintained by injection of chemical refrigerant into an outer ring of wells spaced approximately 8 ft apart. In contrast, ExxonMobil’s method utilizes horizontal wells and hydraulic fracturing of the oil shale. Heating will be accomplished using a conductive proppant material (calcined petroleum coke), to which an electrical current will be applied. Finally, the American Shale Oil Company (AMSO) plans to use inclined wells, drilled below the stratigraphic level of Piceance Creek basin evaporite minerals. All three methods are currently undergoing field tests.