Petroleum generation and migration from the oil shale interval of the Eocene Green River Formation, Uinta Basin, Utah
An examination of the current distribution of oil shale resources combined with paleogeographic reconstructions of the lacustrine oil shale interval in the northwestern depocenter of the Uinta Basin indicates that oil shale source rocks in the area originally contained significantly greater organic richness. Thermal maturation in this deeply buried part of the basin has led to the loss or depletion of a substantial portion of the original petroleum-generating potential of these source rocks. Some of this loss of potential has led to the presence of unconventional petroleum resources, including tar sands and solid bitumens like gilsonite, in the Uinta Basin. In-place oil shale resources of the Eocene Green River Formation were compiled and reported in the recent U.S. Geological Survey assessment. These results have been used to define areas within the oil shale interval along the deep structural trough of the basin that appear to be depleted in terms of Fischer assay oil yields (gallons of oil per ton of rock) and in-place oil shale resource (barrels of oil per acre). The oil shale interval has also been established as the most likely source for gilsonite deposits in the basin and much of the tar sands resource. Petroleum expulsion may have occurred at unusually low degrees of thermal stress due to the very high organic carbon content and hydrogen-richness of the Type I kerogen present in Green River oil shale. In order to examine the possible sources and migration pathways for the tar sands and gilsonite deposits, paleogeographic reconstructions of several oil shale zones in the basin were created. Applying oil yields from core and cuttings samples collected near the edge of the defined ‘depleted area’, we estimate that the reduction in petroleum-generating potential is slightly more than 500 billion barrels. This loss represents nearly 40% of the original oil shale resource and is more than 20 times greater than all of the tar sands (~12 billion barrels) and gilsonite (~10 billion barrels of oil equivalent) deposits in the basin sourced primarily from the oil shale interval. Analysis to develop a first-approximation of the amount of actual generated and expelled petroleum was conducted leading to estimates ranging from 15 to 50 billion barrels. No attempt was made to estimate the amount of oil lost to leakage, erosion and biodegradation, but based on our current understanding of the history of the Uinta Basin, it is expected that such losses could account for much of the discrepancy between the known deposits and estimates of generated oil. Late Eocene uplift of the Uinta Basin exposed the marginal areas of the Green River Formation around much of the basin. Exposure of persistent marginal lacustrine sandstone and stacked fluvial sandstones that connect to the depleted oil shale area may have allowed most of the generated and expelled petroleum to escape. The tar sands deposits around the present-day basin margins can be thought of as erosional remnants of migration pathways that allowed oil to reach the surface.
AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016