| Description |
Subsurface petrologic study and burial history reconstruction of the Lower Permian White Rim Sandstone in the Tar Sand triangle on the western edge of the Paradox Basin in southeastern Utah suggest that oil migrated into White Rim reservoirs after significant burial during the early Tertiary. Primary(?) oil-bearing and hydrous, two-phase inclusions that have homogenization temperatures averaging 83°C (minimum temperature of formation) are present in growth zones in authigenic dolomite. Although this temperature is close to the temperature (85°-90°C) of the White Rim at maximum burial in Cretaceous to middle Tertiary time, it may not reflect depth of burial because dolomite may have precipitated from warm oil-bearing brines that migrated upward into cooler, shallower White Rim strata. Geologic evidence, including paragenetic relations between oil and early calcite cement, suggests, however, that migration did not take place until near maximum burial. The presence of "dead" oil in the White Rim on the east side of the Green River in Canyonlands National Park constrains oil migration to the period before uplift and dissection of the Colorado Plateau, which began in Oligocene to Miocene time. Therefore, the Cretaceous to middle Tertiary interval is the most favorable time for oil migration into the Tar Sand triangle. At the time of deposition, the White Rim Sandstone was pink due to iron-stained detrital illite-smectite and early diagenetic(?) ferric oxyhydroxide grain rims. After some burial, oil migrated through the sandstones, and organic acids formed during redox reactions. Reduction of iron in grain rims caused bleaching of the sandstone. The organic acids dissolved early poikilotopic calcite cement, creating pores in which the oil accumulated. From the middle Tertiary to the present, many late diagenetic alterations have been associated with the biodegradation and water washing of oil due to the infiltration of meteoric water into the White Rim during uplift of the Colorado Plateau. These alterations include formation of widespread secondary porosity, pyrite, quartz overgrowths (second stage), ferroan dolomite, calcite, and hematite. Movement of some oil out of reservoirs occurred at this time in response to Laramide tectonism. Stratigraphic, hydrologic, and tectonic indicators suggest that the source rock(s) was to the west of the Tar Sand triangle. Because of the estimated enormous size (30-40 billion barrels) of the original accumulation, oil must have migrated along faults and unconformities as well as through permeable beds. Possible source rocks include the Late Proterozoic Chuar Group, the Mississippian Chainman Shale and equivalents, the Middle Pennsylvanian Paradox Formation, and the Lower Permian Kaibab Limestone and Phosphoria Formation. Because of the large distance between some of these formations in eastern Nevada and western Utah and the Tar Sand triangle, a major impetus, such as west-to-east thrusting during the Sevier orogeny, may have hydrodynamically driven the oil eastward toward the Tar Sand triangle. |
