ABSTRACT: Petroleum Geology of the Cretaceous Niobrara Formation, Colorado

BILLO, S. M., King Saud University, Riyadh, Saudi Arabia

The Niobrara Formation in Boulder County, Colorado, consists of alternating cycles of marine limestones and calcareous shales or marlstones. The formation has two members, the Fort Hays Limestone Member at the base and the Smoky Hill Member at the top. The depositional environment of the Fort Hays Member seemingly approximated a neritic open-marine outer shelf with occasional storms and long-shore currents and eddies. The Fort Hays depositional cycles are correlative regionally across much of Colorado, Kansas, and northeastern New Mexico. Widespread bentonite seams were used as key beds for correlation. Both the Fort Hays Limestone and the Smoky Hill Members of Niobrara Formation have produced oil in the Denver basin at depths varying from shallow in fields adjacent to the Front Range uplift to 6000 ft in the deeper parts of the basin. The frequent occurrence of large Laramide structures within the maturation environment may have generated fracture venues for confined migration through the indigenous Niobrara oil reservoirs. Regional thickness variations in groups of shale-limestone couplets were associated with the structural elements that affect Fort Hays deposition in relation to Late Cretaceous reactivation of the transcontinental arch in Colorado.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)