Stratigraphy and Petroleum Potential of Latest Cretaceous Rocks, Bighorn Basin, Wyoming

Abstract

Approximately 3,000 feet of Upper Cretaceous post-Cody non-marine sediments in the southwestern part of the Bighorn basin interfinger toward the north and east with marine shale. In the southwestern part of the basin the stratigraphic section includes the Mesaverde Formation (Gebo Formation of Hewitt), Meeteetse Formation, and Lance Formation. In the eastern part of the basin four formations are readily distinguishable; they are, in ascending order, the Eagle Sandstone, Claggett Shale, Judith River Formation, and Bearpaw Shale. Major marine transgressions are evidenced by the Claggett and Bearpaw Shales and regressions by the near-shore and non-marine sediments of the Eagle, Judith River, and Lance Formations. Isopachous maps, using well information and 14 partial surface sections, show the distribution of each formation. The stratigraphic interval containing the Eagle Sandstone thickens from 200 feet near the Wyoming-Montana border to 800 feet along the southern margin of the basin, but the sandstone content decreases southward as the over-all interval thickens. Eagle sandstones are particularly well developed in the area between Worland and Coon Creek. The Claggett Shale interval is recognizable throughout the eastern half of the basin and reaches a maximum thickness of 275 feet along the eastern margin. The Judith River and Mesaverde Formations thin northward and eastward throughout the basin, and the overlying Bearpaw Shale has a reciprocal relationship ranging in thickness from zero in the southwestern part to 1,000 feet in the northern end. In the southeastern part of the basin the Judith River i divisible into three mappable units: upper sandstones, middle continental deposits, and lower sandstones. Marine and transitional environments that have produced sandstone reservoirs and petroleum accumulations in several other basins in the Rocky Mountains are well represented in the Eagle, Judith River, and Bearpaw of the Bighorn basin. Numerous littoral and neritic sandstones provide a variety of trapping conditions in conjunction with both old and present-day structural features. Incipient development of Laramide structures during Late Cretaceous resulted in thinning of some sediments over some of the major structural features.