Thermal Maturity of Bowie Coals in Southern Portion of Piceance Basin, Colorado: ABSTRACT

Abstract

Although the thermal maturity of coals is determined primarily by age and depth of burial, shallow Cretaceous coals in the Piceance basin of Colorado display anomalously high thermal maturities, indicating that age and present depth of burial are not adequate to explain their high thermal maturities. The objective of this study was to investigate the physical factors that may have contributed to the high thermal maturity of the Piceance basin coals. Vitrinite reflectance (Ro), which increases with coal rank and provides a quantitative index of thermal maturity, was used to measure the thermal maturity of the coals. An examination of published Ro data and Ro analyses of coal samples from mines, outcrops, and well cuttings indicated that most of the thermal maturity of these coals is due to the thickness of overburden that covered the coals in Pliocene time. A plot of vitrinite reflectance versus reconstructed depth of burial during Pliocene time shows a reasonably good fit, however, some Ro values are too high to be explained solely by depth of burial. A map of Ro was constructed using data obtained by subtracting predicted Ro values from actual values. When compared with a geothermal gradient map, most of the positive Ro residuals correlated with that were related to known igneous intrusives. Some anomalous hot spots may be related to unknown buried intrusives. A residual Ro map, modified on the basis of the geothermal gradient map, was combined with the preliminary Ro map (based on reconstructed Pliocene depth of burial) to show that the resultant Ro is due to depth of burial and geothermal hot spots. A final thermal maturity map was used to select areas in the Piceance basin that are most likely to contain thermally mature coals. The same procedure can be used to predict the thermal maturity of coals in other basins. End_of_Article - Last_Page 857------------