Abstract

The thermal effects of rapid sedimentation and uplift have been modeled in an attempt to quantify the timing and magnitude of hydrocarbon generation from Cretaceous and Permian source rocks in the Bighorn Basin, Wyoming.

Using published ages of formations and thicknesses from well logs, burial histories have been constructed for the Fee 1-22 and Red Butte Federal A-1 wells in the Bighorn Basin. The burial histories include quantitative estimates of the amount of Tertiary erosion for each well. Three hydrocarbon source intervals in each well—the Fort Union Formation / Lance Formation boundary, the Mowry Shale, and the Phosphoria Formation—are investigated in the study. For each interval, temperature histories are calculated using four different methods: (1) a time-invariant geothermal gradient as determined from bottom-hole temperatures; (2) a constant regional heat-flow value with time-invariant thermal conductivities assigned to each formation; (3) a constant heat-flow value with thermal conductivities that change through time due to rock compaction; and (4) a one-dimensional finite difference model that incorporates regional heat-flow, rock thermal diffusivities (which change through time due to rock compaction), and the thermal effects of rapid sedimentation and uplift.

Hydrocarbon maturation for each interval is calculated using published geochemical rate constants and the four temperature histories. Results show that the constant geothermal gradient calculations give the lowest maturation values. The rapid sedimentation model indicates that hydrocarbons matured earlier in the burial history and to a greater extent when compared to the constant gradient model. The modeling shows that mature hydrocarbons exist at shallow depths (1700 m (5,580 ft)) in the Bighorn Basin. This conclusion confirms that the lower Fort Union / Lance boundary is a valid exploration target in the Bighorn Basin.