Abstract

Five techniques of determining thermal maturity, using the same core samples of the Upper Cretaceous Steele Member of the Cody Shale (at the level of the Shannon Sandstone Beds) from the Powder River basin, Wyoming, indicate different levels of maturation and timing of petroleum generation. Three methods were direct-measurement techniques (vitrinite reflectance, and Tmax and production indices from Rock-Eval pyrolysis), and two were predictive modeling techniques (time-temperature index and kinetic modeling).

Mean random vitrinite reflectance measurements of 0.50-0.70 percent, Tmax values of 431-444 °C, and production indices of 0.10-0.40 indicate that the Steele samples are immature to mature with respect to petroleum generation. If these values record maximum thermal maturity, then hydrocarbon generation began shortly before or at maximum burial, which occurred at 10 Ma.

Time-temperature index and kinetic modeling techniques predict higher levels of maturity than the direct-measurement techniques and thus an earlier timing for petroleum generation. The time-temperature index model indicates the Steele Member began to generate petroleum about 47 Ma. Two kinetic models were run: a type II kerogen model because the Steele is a marine shale, and a type III kerogen model because Rock-Eval pyrolysis analyses indicated that the Steele contains a mixture of both type II and III kerogen. Type II kinetic modeling indicates that the Steele began to generate oil as early as 40 Ma, whereas type III kerogen kinetic modeling indicates some gas generation by maximum burial and temperature at 10 Ma.

If only one technique were used, it could lead to an erroneous determination of the level of thermal maturation and timing of petroleum generation. Because all thermal maturation techniques, whether measured or modeled, have innate problems, the “true” maturity and timing of petroleum is probably somewhere between the measured and modeled results. This problem stresses the importance of using as many techniques as possible to determine the thermal maturity and timing of petroleum generation of a source rock.