Vitrinite reflectance of Cretaceous coals and thermal maturity of the Niobrara Formation, DJ basin, Colorado, USA

Abstract

The Niobrara Formation is a major source rock and unconventional reservoir in the DJ basin, and an understanding of its thermal maturity profile is critical for exploration and exploitation of the resource. Previous thermal maturity studies in the Denver Basin have been limited by small sample sizes, have drawn on vitrinite of questionable quality, or have relied on pseudo-measurements and/or inferior proxies to measured vitrinite reflectance. As a consequence, a reliable picture of Niobrara maturity in the DJ basin has been lacking. To fill this substantial “data gap,” we provide new vitrinite reflectance measurements for nearly 200 wells, representing the first truly exhaustive vitrinite reflectance database for the Cretaceous section of the DJ basin of Colorado, Wyoming, and Nebraska. Whole and slabbed cores from over 320 wells were examined for coaly material suitable for reflectance analysis. The present study examines coal collected from Cretaceous fluvio-deltaic sandstones (namely the J- and D-Sandstones), where it typically occurs as local, thin, isolated, and often subtle partings of coal. This is desirable because the macerals from such occurrences are large enough to identify under the microscope; in contrast, the identities of macerals that occur in marine shales and marls (e.g., Niobrara) are commonly ambiguous. A previously published depth-vs-thermal maturity profile for the DJ basin was used to project the vitrinite reflectance to the middle of the Niobrara. More than 180 new vitrinite reflectance data points have helped to both expand and refine the maturity profile of the DJ Basin. The new data have revealed localized and previously-undocumented high and low temperature anomalies.

AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014