Thermal Maturity of the Mancos Shale within the Uinta Basin, Utah and Colorado

The Upper Cretaceous Mancos Shale is an emerging shale-gas play in the Uinta Basin of eastern Utah and western Colorado. This study evaluates the thermal maturity of the Mancos as part of a larger effort to identify prospective areas for shale-gas development. We use trend surface analysis to derive an equation that predicts vitrinite reflectance (Ro) from elevation (MSL) and geographic coordinates (UTM easting, UTM northing). Data used for this analysis include Ro measurements for 297 samples of the Mancos and surrounding strata, collected from 104 outcrops and 59 drill holes. Reflectance values range from 0.3 to 2.4% (median 0.7); most were compiled from the published literature but 24 are new measurements from unpublished theses, company records, or the authors’ files. Regression analysis with these data gave an eight-variable, second-order equation with an adjusted r2 of 0.91 and a standard error of 0.10% Ro; one non-significant variable (t-statistic < 2) was ignored. Besides showing where the Mancos Shale is within the gas window, the model indicates greater surface erosion toward the southern part of the basin. Locations where the model fails to accurately predict vitrinite reflectance are also considered. For example, at the northern edge of the basin, predicted reflectance is higher than measured below a marginal thrust fault, but lower than measured at the surface above the thrust. This is consistent with at least some thrust movement after maximum burial. Additionally, locations in the center of the basin where predicted reflectance does not agree with the measured values are consistent with enhanced heat flow within overpressured intervals manifested by irregular (kinky) down-hole reflectance gradients.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California