Recognition and Significance of Primary and Recycled Kerogen in Upper Cretaceous Source Rocks, Denver and Powder River Basins, USA
Christopher D. Laughrey, Jack D. Beuthin, Jackie Holt, and Wayne Knowles
Petroleum source beds in the Denver and Powder River basins include the Mowry Shale, the Frontier and Greenhorn Formations, and the Niobrara Formation. These sediments were deposited during transgression and basin deepening, and have relatively high total organic carbon comprised mostly of hydrogen-rich type II or mixed-type II/III kerogen. Bituminite is the principal maceral. Inertinite, semi-fusinite, solid bitumen, sporinite, alginite, and vitrinite occur in variable, but subordinate, quantities. Optical parameters consistently yield bimodal thermal maturation results suggesting the presence of primary and recycled kerogen. Recycled kerogens are more mature than primary macerals. Vitrinite occurs as finely comminuted dark gray particles dispersed throughout mineral matrix. Primary vitrinite is uncommon to rare and sporadic. It consists of relatively large Telocollinite particles. Primary vitrinite reflectance values yield accurate thermal maturation estimates corroborated by several ancillary maturity parameters. Recycled vitrinite, derived from the erosion of older sedimentary rocks, consists of small, angular to irregularly shaped and broken Telocollinite particles, finely dispersed Vitrodetrinite debris, and Collodetrinite. Reflectance measured on recycled Telocollinite consistently indicates peak/late oil generation. Alginite and sporinite also occur as primary and recycled kerogen. Respective fluorescence and TAI measurements made on primary alginite and sporinite yield maturation estimates compatible with those derived from other thermal indicators. Fluorescence and TAI results from recycled alginite and sporinite, however, do not agree with other maturity indicators. TAI and fluorescence of recycled kerogen do match the reflectance of recycled Telocollinite. We postulate that recycled kerogen was derived from older source beds that were buried to depths adequate for peak oil-generation, then uplifted and eroded along fronts of the Sevier Orogeny thrust system. Recognition of primary and recycled kerogen is critical for accurate resource assessment in these rocks, particularly with regard to determining the type and thermal maturity of source beds.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013