Charles E. Barker, Bonnie L. Crysdale
The burial history of this fractured Niobrara Limestone reservoir and source rock offers a setting for studying the stabilization of thermal maturity because soon after peak temperature of approximately 100°C was reached, exhumation lowered temperature to about 60-70°C. Vitrinite reflectance (Rm = 0.6-0.7%) and published clay mineralogy data from the Niobrara Limestone indicate that peak paleotemperature was approximately 100°C. Fluid inclusion data also indicate oil migration occurred at 100°C. Burial history reconstruction indicates 100°C was reached in the Niobrara Limestone only during maximum burial, which occurred at 70 Ma and 8000 ft depth. However, erosion beginning at 70 Ma and continuing until 50 Ma removed over 3000 ft of rock. This dep h of erosion agrees with an Rm of 0.4% measured in surface samples of the Pierre Shale. The exhumation of the reservoir decreased temperature by about 30°C to near the corrected bottom-hole temperature of 50-70°C. Lopatin time-temperature index (TTI) analysis suggests the Niobrara Limestone as a source rock, matured to the oil generation stage (TTI = 10) about 25 Ma, significantly later than maximum burial, and after exhumation caused cooling. The Lopatin TTI method in this case seems to overestimate the influence of heating time. If time is an important factor, thermal maturity should continue to increase after peak burial and temperature so that vitrinite reflectance will not be comparable to peak paleotemperatures estimated from geothermometers set at near-peak temperature a d those estimated from burial history reconstruction. The agreement between geothermometry and the burial history reconstruction in Berthoud State 4 suggests that the influence of heating time must be small. The elapsed time available at near peak temperatures (about 1-5 m.y.) was sufficient to allow stabilization of thermal maturation in this case.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990