Pennsylvanian Petroleum System of the Deep Paradox Basin Fold and Fault Belt, Colorado and Utah

Larry Rasmussen¹ and Donald L. Rasmussen^2
1Whiting Oil & Gas Corp, Denver, CO
²Paradox Basin Data, Longmont, CO

A revived exploration frontier in the Paradox Basin is the deep Fold and Fault Belt adjacent to the Uncompahgre Uplift, an area characterized by extremely thick evaporites deposited during the Pennsylvanian, and complexly deformed by deposition of thick siliciclastics from Late Pennsylvanian to Early Triassic. Plays include a Permian-Pennsylvanian marginal siliciclastics gas play, a fractured interbed (shale) gas play, and the many structural and stratigraphic features related to halokinesis which remain essentially unexplored. Play analysis benefits from a renewed look at the associated Pennsylvanian Petroleum System. This study expands on previous work, focusing primarily on the thermal maturity and timing of generation and expulsion using high-resolution data and forward kinetic modeling tools. Onset of hydrocarbon generation for the organic-rich source rocks is highly variable, starting earlier (Late Pennsylvanian) closest to the Uncompahgre Uplift and migrating progressively westward during the Permian and Triassic. Hydrocarbon expulsion began during the Early Permian and continued as late as the Early Cretaceous. On the shelf outside of the deep Fold and Fault Belt, late generation and expulsion occurred during the Late Cretaceous to Miocene. For all source rocks, onset of hydrocarbon generation corresponds with a calculated thermal maturity of 0.46%Roc, and the maximum rate of hydrocarbon generation corresponds with a calculated thermal maturity of 0.8%Roc. The influence of thick Pennsylvanian to Triassic siliciclastic deposition and contemporaneous salt movement on the thermal history and burial history is important and helps explain the temporal variability of the Pennsylvanian Petroleum System. Salt movement played the most important role, albeit indirectly, in early source rock generation and expulsion in the deep Fold and Fault Belt. Salt withdrawal related to thick siliciclastic wedges shed from the ancestral Uncompahgre Uplift resulted in rapid burial and thermal maturation of source rocks. Uplift of the Colorado Plateau removed as much as 9,000 feet of overburden since the Late Miocene and modeling indicates that this uplift and subsequent erosion was sufficient to stop generation and expulsion in the deep Paradox Fold and Fault Belt and surrounding shelf and basin margins.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado