Thermal Histories and Hydrocarbon Generation: Example from Utah-Wyoming Thrust Belt

Abstract

The thermal history of the Utah-Wyoming thrust belt in the vicinity of the Pineview oil and gas field is studied with a detailed thermal model. The model incorporates a heterogeneous thermal conductivity structure, radioactive heat generation in the crust, sediment compaction, variable basal heat flow through time, and thermal effects of sedimentation, erosion, and thrust faulting. The analysis is constrained by 206 corrected bottom-hole temperatures, 660 thermal conductivity measurements, and 10 new vitrinite reflectance measurements. Present surface heat flow in this part of the Utah-Wyoming thrust belt is estimated to be 60 (± 8) mW/m2. Surface heat flow in the Late Cretaceous (about 70 Ma) is inferred to have been 38 (± 8) mW/m2 from an lysis of vitrinite reflectance data. About half of the apparent difference between ancient and present surface heat flow may be attributed to transient thermal effects of sedimentation, erosion, and thrusting. The hypothesis of a constant background thermal state in this part of the thrust belt for the last 70 m.y. may not be ruled out. However, thermal history simulations in which the background thermal state changes through time better match constraints imposed by estimated present surface heat flow and vitrinite reflectance data. Application of the thermal history analysis to hydrocarbon generation is illustrated with a kinetic model of organic maturation. Oil and gas at the Pineview field may have been generated in two stages: first stage just before and during thrusting, and the second stage during Cenozoic warming.