ABSTRACT: Thermal Maturity of Upper Cretaceous Coals, San Juan Basin, New Mexico and Colorado

James E. Fassett, Vito F. Nuccio

The earliest published basinwide study of thermal maturity of a Western Interior Laramide basin was based on Upper Cretaceous coals in the San Juan basin of northwestern New Mexico and southwestern Colorado. That study concentrated on analyzing the percentage of fixed carbon and volatile matter of 65 Fruitland Formation coal samples collected in the 1960s. Proximate analyses of coal samples, mainly, from well cuttings but supplemented by a few samples from near-surface coal mines and outcropping coal beds, were conducted by the U.S. Bureau of Mines, Pittsburgh, Pennsylvania. Fixed-carbon values were shown to range from 46% to 72% by volume and generally increased toward the basin's present structural axis near the New

Mexico-Colorado State line. In the 1980s, splits of some of the same Fruitland coal samples used earlier for determining fixed carbon percentages, along with a few new well samples, were analyzed for vitrinite reflectance (R_m). This sparse data set of 19 samples showed R_m values ranging from 0.46% to 1.45%, and more or less agreeing with the fixed-carbon pattern indicating increasing thermal maturity with increasing depth of burial toward the structural axis of the basin. Work in progress involves obtaining vitrinite reflectance values of coal samples from drill cuttings of hundreds of wells throughout the basin. Most of these samples are from the Fruitland Formation, but some are from various levels in the Menefee Formation (Mesaverde Group). This vertical distr bution of samples will yield thermal maturity profiles. Preliminary results indicate that vitrinite reflectance values are generally higher than those previously reported but do correspond to the structure of the San Juan basin and do correlate fairly well with fixed-carbon values. This study of closely spaced R_m data may also reveal the presence of local thermal-maturity anomalies that could shed new light on the paleo-heat flow in the basin.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990