Thermal Maturity Patterns and Time-Temperature Modeling of Cretaceous and Tertiary Rocks, San Juan Basin, Colorado and New Mexico: ABSTRACT

Abstract

Mean random vitrinite reflectance (R{sub m}) values of coal beds in the Upper Cretaceous Fruitland Formation in the San Juan basin of Colorado and New Mexico range from 0.42% in the southern part of the basin to 1.54% in the northern part. R{sub m} profiles of wells drilled through approximately 8,000 ft of cretaceous and Tertiary rocks in the area of high thermal maturity are commonly nonlinear, with three or more segments. R{sub m} gradients through the coal-bearing part of the Fruitland are high, ranging from 1.0 to 1.5%/1,000 ft and 0.30%/1,000 ft, respectively. These gradients most likely represent the combined effects of contrasting thermal conductivities associated with lithologic variations and differences in heat transfer processes (conductive vs. convective). Time-temperature modeling of a nonlinear vitrinite reflectance well profile in the area of high thermal maturity indicates that the present heat flow of 96 m W/m{sup 2} is insufficient to account for the measured levels of thermal maturity. Furthermore, in order to match the nonlinear R{sub m} well profile, it was necessary to assign artificially high thermal conductivity values to nearly all of the stratigraphic units penetrated in the well. The authors interpret the unreasonably high thermal conductivity values as evidencemore » of convective heat transfer and conclude that the high level of thermal maturity in the northern part of the San Juan basin is due to a deep, localized source of high heat flow in conjunction with convective heat transfer associated with a strong component of vertically directed fluid flow.« less