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

D. Deming, D. S. Chapman

During the last three years, we 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. Average present-day geothermal gradients are 24 °C/km at the Pineview field and 23 °C/km at the Anschutz Ranch, Cave Creek, and Anschutz Ranch East fields. Present-day regional heat flow in this part of the thrust belt is 60 mW/m² (± 10-15%).

Present-day heat flow, vitrinite-reflectance data, and measured thermal properties of geologic units are used to constrain the thermal history. Temperature as a function of time and depth is calculated by a finite-difference model that accommodates depositional, thrusting, and erosional history, sediment compaction with concommitant porosity changes, temperature-dependent thermal properties, and radioactive heat generation. 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/m², 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 this 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.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988