Basin-wide evaluation of the uppermost Green River Formation's oil-shale resource, Uinta Basin, Utah and Colorado

reportDue to the recent increase in crude oil prices and concerns over diminishing conventional reserves, the Utah Geological Survey has reexamined the Uinta Basin's oil-shale resource, primarily in the Mahogany zone of the Green River Formation. Past assessments, the first conducted in 1964 and subsequent studies continuing through the early 1980s, concentrated on the Eocene Green River Formation's Mahogany zone in the southeastern part of the Uinta Basin, and were limited in the amount of drill hole data available at the time. We have broadened the investigation to include the entire Uinta Basin, taking advantage of the hundreds of geophysical logs from oil and gas wells drilled over the last two decades. We created conversion equations by correlating available Fischer assays with corresponding density and sonic measurements as a way to predict oil yield from geophysical logs. In addition to the core-based Fischer assays obtained from 107 wells drilled specifically for oil shale, 186 oil and gas wells with oil yields calculated from digitized bulk density or sonic logs were used to create a basin-wide picture of the oil-shale resource in the Uinta Basin. These widespread data were used to map oil-shale thickness and richness and create isopach maps delineating oil yields of 15, 25, 35, and 50 gallons of shale oil per ton (GPT) of rock. Thicknesses were centered around the extremely rich Mahogany bed of the Mahogany zone (R-7) within the Parachute Creek Member of the Green River Formation. From these isopach maps, new basin-wide resource numbers were calculated for each richness grade. In addition, oil-shale resource numbers were adjusted according to different sets of constraints, including resources less than 3000 feet deep, resources located on specific landownership categories, and resources associated with conventional oil and gas fields. The thickest and richest oil-shale zones are located in central Uintah County in T. 8 S. to T. 12 S. and R. 20 E. to R. 25 E., Salt Lake Base Line and Meridian. Overburden in these areas ranges from zero at the outcrop in the east, to almost 4000 feet farther to the northwest. Acontinuous interval of oil shale averaging 50 GPT contains an in-place oil resource of 31 billion barrels in a zone ranging up to 20 feet thick. Where the 50 GPT interval is at least 5 feet thick and less than 3000 feet deep, the in-place resource drops to 26 billion barrels. An interval averaging 35 GPT, with a maximum thickness of 55 feet, contains an in-place oil resource of 76 billion barrels. Where this interval is at least 5 feet thick and less than 3000 feet deep, the total in-place resource drops to 61 billion barrels. The 25 GPT zone and the 15 GPT zone contain unconstrained resources of 147 billion barrels and 292 billion barrels, respectively. The maximum thickness of 25 GPT rock is about 130 feet, whereas the maximum thickness of 15 GPT rock is about 500 feet. Where these two intervals are at least 5 feet thick and less than 3000 feet deep, the 25 GPT resource drops to 111 billion barrels and the 15 GPT resource drops to 228 billion barrels. The 25 GPT resource calculated for U.S. Bureau of Land Management (BLM) lands that could be considered for commercial oil-shale leasing is approximately 69 billion barrels, roughly 50% of Utah's total oil-shale resource. The remaining resource is located on tribal (20%), private (16%), state trust (9%), U.S. Forest Service (3%), and protected land (2%) such as state wildlife reserves, national wildlife refuges, state sovereign lands, and state parks. Furthermore, approximately 25% of Utah's oil-shale resource lies within existing oil or gas fields, creating resource conflict issues that will need to be addressed as conventional and unconventional resources are developed. After placing several constraints on Utah's total in-place oil-shale resource, we determined that approximately 77 billion barrels of oil could be considered as a potential economic resource. This estimate is for deposits that are at least 25 GPT; at least 5 feet thick; under less than 3000 feet of cover; not in conflict with current conventional oil and gas resources; and located only on BLM, state, private, and tribal lands