Abstract

Oil is sourced and produced in the Berthoud oil field from the fractured Upper Cretaceous Niobrara Formation. The well-defined peak burial and temperature history of this formation offers a unique setting for studying the stabilization of thermal maturity. Vitrinite reflectance (R_m = 0.6-0.7%) and published clay mineralogy data from the Niobrara Formation indicate that peak paleotemperature was approximately 100 °C. Burial history reconstruction indicates 100 °C was reached in the Niobrara Formation only during maximum burial, at 70 Ma and 2,438 m (8,000 ft) depth. Fluid inclusion data indicate oil migration also occurred near 100 °C. However, erosion of more than 914 m (3,000 ft) of Tertiary and Upper Cretaceous overburden beginning at 70 Ma and continuing until 50 Ma, lowered the temperature by about 30-50 °C to near the present-day measured reservoir temperature of 50-70 °C. This estimate of erosion agrees with a surface R_m value of 0.4% measured in samples of Upper Cetaceous Pierre Shale that overlie Berthoud field. Lopatin time-temperature index (TTI) analysis suggests the Niobrara Formation source rock matured to the oil generation stage (TTI=10) about 25 Ma, significantly later than maximum burial and after erosion caused cooling. The Lopatin TTI method in this case seems to overestimate the influence of heating time.