Thermal History of Utah-Wyoming Thrust Belt: Implications for Hydrocarbon Maturation: ABSTRACT

Abstract

During the last three years, the authors have been engaged in collecting and analyzing thermal data from the Pineview, Anschutz Ranch, Cave Creek, and Anschutz Ranch East oil and gas fields in the Utah Wyoming thrust belt. These data include more than 200 corrected bottom-hole temperatures, 660 thermal-conductivity measurements on 15 geologic formations, and 10 vitrinite-reflectance measurements. Preliminary modeling results indicate that a constant thermal state in this part of the thrust belt is incompatible with vitrinite-reflectance data and that near-surface heat flow at the time of major movement on the Absaroka thrust (about 70 m.y.a.) was approximately 40 mW/m/sup 2/, 30% lower than present day. A model involving warming beginning 45 m.y.a. is consistent with vitrinite-reflectance data, present-day heat flow, and geologic and tectonic constraints. This thermal model is used to calculate the temperature history of Cretaceous source rocks, which, in turn, is used as input to a kinetic model of oil and gas generation. Oil and gas generation is predicted to occur in two stages. The first stage is initiated at time of thrusting and lasts from 1 to 5 m.y. Oil-generation rates at the Pineview field are between 1 and 10% converted kerogen per m.y. during thismore » time. The second phase of hydrocarbon generation is associated with Tertiary warming and is characterized by oil-generation rates on the order of 1% converted kerogen per m.y. for the last 30 m.y. These results depend on the validity of empirical models that relate vitrinite reflectance to temperature and time. They are also sensitive to uncertainties in thrusting kinematics, thickness of eroded overburden, ancient surface temperature, thermal properties of eroded formations, and local variations in temperature due to heat refraction.« less