The search for a source rock for the giant Tar Sand triangle accumulation,southeastern Utah

A large proportion (about 36%) of the world's oil resource is contained in accumulations of heavy oil or tar. In these large deposits of degraded oil, the oil in place represents only a fraction of what was present at the tim e of accumulation. In many of these deposits, the source of the oil is unkn own, and the oil is thought to have migrated over long distances to the res ervoirs. The Tar Sand triangle in southeastern Utah contains the largest ta r sand accumulation in the United States, with 6.3 billion bbl of heavy oil estimated to be in place. The deposit is thought to have originally contai ned 13-16 billion bbl prior to the biodegradation, water washing, and erosi on that have taken place since the middle-late Tertiary. The source of the oil is unknown. The tar is primarily contained within the Lower Permian White Rim Sandstone , but extends into permeable parts of overlying and underlying beds. Oil is interpreted to have migrated into the White Rim sometime during the Tertia ry when the formation was at a depth of approximately 3500 m. This conclusi on is based on integration of fluid inclusion analysis, time-temperature re construction, and apatite fission-track modeling for the White Rim Sandston e. Homogenization temperatures cluster around 85-90 degrees C for primary f luid inclusions in authigenic, nonferroan dolomite in the White Rim. The fl uid inclusions are associated with fluorescent oil-bearing inclusions, indi cating that dolomite precipitation was coeval with oil migration. Burial re construction suggests that the White Rim Sandstone reached its maximum buri al depth from 60 to 24 Ma, and that maximum burial was followed by unroofin g from 24 to 0 Ma. Time-temperature modeling indicates that the formation e xperienced temperatures of 85-90 degrees C from about 35 to 40 Ma during ma ximum burial. Maximum formation temperatures of about 105-110 degrees C wer e reached at about 24 Ma, just prior to unroofing. Thermal modeling is used to examine the history of potential source rocks f or the White Rim oil. The most attractive potential sources for White Rim o il include beds within one or more of the following formations: the Protero zoic Chuar Group, which is present in the subsurface southwest of the Tar S and triangle; the Mississippian Delle Phosphatic Member of the Deseret Lime stone and equivalent formations, the Permian Kaibab Limestone, the Sinbad L imestone Member of the Triassic Moenkopi Formation, and the Jurassic Arapie n Shale, Twin Creek Limestone, and Carmel Formation, which are present west of the Tar Sand triangle; the Pennsylvanian Paradox Formation in the Parad ox basin east of the Tar Sand triangle; and the Permian Park City Formation northwest of the Tar Sand triangle. Each formation has a high total organi c carbon content and is distributed over a wide enough geographic area to h ave provided a huge volume of oil. Source beds in all of the formations rea ched thermal maturity at times prior to or during the time that migration i nto the White Rim is interpreted to have occurred. Based on all available d ata, the most Likely source for the Tar Sand triangle appears to be the Mis sissippian Delle Phosphatic Member of the Deseret Limestone. Secondary migr ation out of the Delle is interpreted to have occurred during the Cretaceou s, during Sevier thrusting. Subsequent tertiary migration into the Tar Sand triangle reservoir is interpreted to have occurred later, during middle Te rtiary Laramide deformation.