Abstract: Transpressional Gas-Prone Paleogene Basins of the Northern Rocky Mountain Region, a Model for International Exploration

William J. Perry, Jr., Romeo M. Flores, D.J. Nichols

Timing, structural style, and sedimentology of transpressional Rocky Mountain Foreland basins are identified in order to determine the sequence of deformation and subsequent generation, migration, and entrapment of hydrocarbons. Late Cretaceous eastward progradation of the Laramide deformation front from SW Montana and NW Wyoming reached the Colorado Front Range by 69 Ma, following initial growth of the Wind River uplift. Laramide deformation then spread northeastward from the Granite Mountains-Shirley Mountains uplift in south-central Wyoming to the Owl Creek uplift-Casper arch and Black Hills in late Paleocene time, creating first the Hanna, then Wind River, and finally Powder River basins, partitioning these basins from an earlier continuous foreland basin with minor welts.

The Hanna basin, south of the Shirley Mountains, contains approximately 12,000 ft of nonmarine Paleocene rocks. The latter is subdivided into the Ferris Formation (P₁-P₃), early to early middle Paleocene, as thick as 5,500 ft, and the overlying Hanna Formation (P₃-P₆), early middle to late Paleocene, as thick as 6,500 ft. Paleocene rocks onlap the northern end of the Late Cretaceous Front Range-Laramie uplift to the east. Remarkably, west to east thorough-going drainage was maintained throughout the Paleocene. A southwestern source during P₁-P₃ time was replaced by a northern source in later Paleocene (P₄P₅) time as the Granite Mountains-Shirley Mountains uplift began to rise. Significant hydrocarb ns have not yet been discovered in this basin; however, coal-bed methane is present and conventional natural gas may have been generated and trapped beneath the north-bounding thrust system.

The larger Wind River basin to the north exhibits a similar structural pattern. The Owl Creek thrust system to the north and northeast began to grow in late middle Paleocene (P₄) and developed significant structural relief in late Paleocene (P₄) time. Middle Paleocene (P₃) or older conglomerates occur along the western and southern margins of the basin. Lacustrine deposition (Waltman Member of Fort Union) was initiated by P₄-P₅ time as the growing Casper arch-Owl Creek uplift dammed flow-through drainage. Burial and thrust loading generated hydrocarbons that migrated updip into sandstone reservoirs from deeply buried Waltman source rock.

Comparison of the tectonics and sedimentation of the Hanna and Wind River transpressional basins provides clues as to differences in hydrocarbon productivities.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994