A Burial, Thermal, and Hydrocarbon Source Rock Evaluation of Lower Cretaceous Rocks in the Southern Moxa Arch Area, Utah and Wyoming

Abstract The production of oil from the Lower Cretaceous Dakota Sandstone at the south end of the Moxa arch immediately north of the Uinta Mountains in Utah and Wyoming is unusual because of the depth of the occurrence (>15,000 ft) and the structurally low position of the accumulation. The area is also notable because of the presence of abnormally low subsurface temperatures and low levels of thermal maturity. The thermal gradient is about 1.1°F/100 ft and the top of the "oil window" (0.6% vitrinite reflectance) occurs at a depth of about 15,500 ft in the Dakota Sandstone. The accumulation and preservation of oil in this area are largely due to an unusual burial and thermal history. Several published studies have demonstrated that the evolution of the Moxa arch predates the Laramide orogeny. Regional thickness variations of some of the Cretaceous units in this region indicate that during Early to Late Cretaceous time, the south end of the Moxa arch was structurally high relative to areas immediately to the north. In latest Cretaceous or Paleocene time, thrust faulting along the north flank of the Uinta Mountains was initiated and the southern end of the Moxa arch began to be structurally depressed by the overriding Uinta Mountains. The Dakota was subsequently buried to over 17,000 ft. The observed low level of thermal maturity in the Dakota requires a prolonged period of low temperature, comparable to the present thermal gradient. We conclude that the present thermal gradient came into existence during or shortly after the faulting that occurred along the north flank of the Uinta Mountains. The faults and fractures, thus formed, provided pathways into the deep basin for the circulation of cold meteoric water that effectively lowered the thermal gradient. Organic geochemical analyses of oils produced from Dakota reservoirs at Bridger Lake and nearby fields on the south end of the Moxa arch show that the saturated hydrocarbon distributions of these oils are similar to those of oils derived from Lower Cretaceous source rocks in other Rocky Mountain basins. Preliminary geochemical analyses (extraction, gas chromatography) of the Lower Cretaceous Mowry Shale north of Bridger Lake field indicate that it is a probable source rock for these oils. The Mowry Shale in this area is thermally mature and is correlative with the oils, based on the distribution of saturated hydrocarbons. Within the context of these burial, thermal, and source rock evaluations, we suggest that during Late Cretaceous time, oil originating from a northern source in the Lower Cretaceous Mowry Shale migrated southward along the Moxa arch to structurally higher areas where it accumulated in stratigraphic traps in the Dakota. Following the structural depression of the south end of the Moxa arch and subsequent deep burial of the Dakota, preservation of the oil was facilitated by the development of an abnormally low thermal gradient, precluding in-reservoir thermal destruction of the oil. Thus, the occurrence of oil at the south end of the Moxa arch is largely due to the temporal relationships between hydrocarbon generation, migration, and accumulation and the burial and thermal history of Lower Cretaceous rocks in this region.