Abstract: Composition and Origin of Oils and Associated Gases in Pennsylvanian, Permian, and Jurassic Rocks, SE Powder River Basin: A Potential New Oil and Gas Play

CLAYTON, J. L.; T. S. AHLBRANDT, M. TUTTLE, A. WARDEN, F. CARSON, and S. MONK

We report chemical and isotopic composition (delta{l3}C; delta{34}S) of oils and associated gases produced from the Pennsylvanian Leo Sandstones, the Permian Minnelusa Formation, the Jurassic Canyon Springs Member of the Sundance Formation, and Cretaceous Dakota and Muddy Sandstones. The purposes of these analyses were to determine the thermal history, origin, and migration histories of the oils and gases. Delta{34}S values of Leo whole oils are between +7 and -23 o/oo and are easily distinguished from Minnelusa oils which have a very limited range of delta{34}S values (-5 plus or minus 2 o/oo). Delta{34}S(H[2]S) for Leo and Minnelusa gases are from +24 to -18o/oo and +6 to -13o/oo, respectively. Leo hydrocarbon gases are relatively {13}C-depleted, indicating generation at low temperatures and short migration distances, and are easily distinguished from more {13}C-rich gases associated with Minnelusa and Cretaceous oils. An important finding is that gases in Leo and Canyon Springs reservoirs at Lance Creek and Red Bird Fields, respectively, are derived from Cretaceous source rocks. Although oil produced from the Leo at Lance Creek is also from Cretaceous source rocks, the oil produced from the Canyon Springs is derived from Leo shales. Therefore, the Cretaceous gas migrated into the Canyon Springs at Red Bird Field independently of Leo oil emplacement. Further, gas reported previously in very shallow Leo reservoirs at Old Woman Anticline may also be of Cretaceous origin. Thrust faulting, relative movement to the west, would have provided migration routes for Cretaceous oil and gas to charge overlying older reservoirs on basin margin structures. These results indicate the possibility of a new oil and gas play in the southeastern Powder River Basin where Cretaceous oil and gas has migrated into older reservoirs. An advantage of this play is that oil and gas from Cretaceous source rocks contain negligible amounts of sulfur, whereas high sulfur content is typical for Leo oils and gases (H[2]S content up to about 48 %).

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado