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 (R_o), 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 R_o data and R_o 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 R_o values are too high to be explained solely by depth of burial. A map of R_o "residuals" was constructed using data obtained by subtracting predicted R_o values from actual values. When compared with a geothermal gradient map, most of the positive R_o residuals correlated with "hot spots" that were related to known igneous intrusives. Some anomalous hot spots may be related to unknown buried intrusives. A residual R_o map, modified on the basis of the geothermal gradient map, was combined with the preliminary R_o map (based on reconstructed Pliocene depth of burial) to show that the resultant R_o 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.

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