Comparative Burial and Thermal History of Lower Upper Cretaceous Strata, Powder River Basin, Wyoming

Vito F. Nuccio

Burial histories were reconstructed for three localities in the Powder River basin (PRB), Wyoming. Thermal maturity of lower Upper Cretaceous source rocks was determined by vitrinite reflectance (R_m) and time-temperature index (TTI) modeling, producing independent estimates for timing of the oil window (0.55-1.35% R_m).

In the northwestern PRB, lower Upper Cretaceous rocks were buried to about 12,500 ft and achieved a thermal maturity of 0.50% to 0.56% at maximum burial, 10 Ma, based on measured R_m. TTI modeling suggests a slightly higher thermal maturity, with an R_m equivalent of approximately 0.75%, placing the source rocks at the beginning of the oil window 30 Ma.

In the southwestern PRB, lower Upper Cretaceous rocks have been buried to about 15,000 ft and achieved thermal maturities between 0.66% and 0.75% about 10 Ma based on measured R_m; therefore, petroleum generation may have begun slightly earlier. TTI modeling estimates an R_m equivalent of 1.10%, placing the beginning of the oil window at 45 Ma.

In the northeastern PRB, lower Upper Cretaceous rocks have been buried only to approximately 5,500 ft. Measured R_m and TTI modeling indicate a thermal maturity for lower Upper Cretaceous rocks between 0.45% and 0.50% R_m, too low for petroleum generation.

The higher R_m values determined by the TTI models may be due to overestimation of maximum burial depth and/or paleogeothermal gradients. The two independent maturity indicators do, however, constrain fairly narrowly the onset of petroleum generation.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.