Transgressive-Regressive Cycle Characterization of the Jurassic Ellis Group, Wyoming and Montana: A Model for Predicting Reservoir Facies in Northern Gulf Coast Salt Basins

William C. Parcell¹, Andrew L. Haner¹, Manwika Ploynoi¹, and Ernest A. Mancini^2
1Department of Geology, 1845 Fairmount Ave., Box 27, Wichita State University, Wichita, Kansas 67260; [email protected]
²Center for Sedimentary Basin Studies, Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487

Examination of Mesozoic strata in Wyoming and Montana aids in the development of a transgressive-regressive (T-R) sequence stratigraphic predictive model, which is used to improve hydrocarbon reservoir imaging, detection, and delineation in the Mississippi Interior Salt Basin (MISB) and the North Louisiana Salt Basin (NLSB). The hydrocarbon volume of these basins ranks in the top 8% of the most petroliferous basins in the world, yet 2-3 billion barrels of oil are estimated to remain in these basins. Data from well-exposed Jurassic units in Wyoming and Montana provide an opportunity to view the lateral extent, vertical changes, geometries, and nature of the physical bounding surfaces in the field; elements critical to the formulation and application of a T-R cycle model.

The Jurassic of Wyoming and Montana record at least four major T-R cycles associated with the development of a retro-arc foreland basin related to subduction of the proto-Pacific Ocean. With the onset of subduction during the Middle Jurassic, a foredeep developed in present-day Utah and eastern Idaho depositing the Twin Creek Formation. To the east of this trough, along the western edge of a cratonic forebulge (present-day Wyoming and Montana), a ramp margin deposited the Ellis Group. In this setting, transgressive episodes favored intensified chemical sedimentation resulting in thick carbonate and evaporite units deposited in subtidal to peritidal environments. Regressive periods deposited supratidal redbeds and sabkha evaporites. Regionally significant unconformities can be recognized by lag deposits and leaching at the boundaries separating major T-R cycles.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana