Abstract

The Bighorn Basin, located in north-central Wyoming and south-central Montana, is an asymmetric intermontane basin of the Rocky Mountain foreland. It is delineated by Laramide uplifts including the Owl Creek Mountains to the south, Absaroka Volcanic Plateau and Beartooth Mountains to the west, Big Horn Mountains to the east, and the Nye-Bowler Lineament trend to the north in Montana.

More than 2.7 billion barrels recoverable oil (BBO) and 1.8 trillion cubic feet of natural gas (TCFG) have been discovered since the first fields, Garland and Greybull, were discovered in 1906 and 1907, respectively. Most production has been from basin-margin structures such as Oregon Basin, Elk Basin, Hamilton Dome, Grass Creek and Garland anticlines. Primary reservoirs are the Pennsylvanian Tensleep Sandstone and Permian Phosphoria Formation. The largest stratigraphically-controlled field is Cottonwood Creek, discovered in 1953, with production primarily from the Phosphoria Formation.

In the Bighorn Basin, both structural and stratigraphic traps occur in Paleozoic and Cretaceous source rock-reservoir systems. Structural plays include basin margin subthrust, basin margin anticline, deep basin structure, and sub-Absaroka structure plays. Principal stratigraphic plays include Phosphoria pinchout, Tensleep paleotopography, and Greybull-Cloverly-Muddy Sandstone stratigraphic plays.

Other plays and potential plays have trapping possibilities and in some cases have yielded small amounts of oil or gas. They include the Bighorn-Darby wedge-edge pinchouts, Flathead and Lander equivalent sandstone pinchouts, Madison Limestone karst-related stratigraphic traps, and Darwin-Amsden discontinuous sandstones. Also, stratigraphic traps occur in sandstones of the Triassic and Jurassic, the Cretaceous Cody and Frontier formations, and in shallow Upper Cretaceous and Tertiary strata. In the basin center, unconventional gas resources may occur in Cretaceous and Tertiary low-permeability (tight-gas) sandstone reservoirs.