Abstract

The Hanna basin is one of the world’s deeper intracratonic depressions. It contains exceptionally thick sequences of mature, hydrocarbon-rich Eocene through Paleozoic sediments, and has the requisite structural and depositional history to become a major petroleum province.

Stratigraphic traps exist within the deeper central parts of the basin in low permeability, possibly overpressured Eocene, Paleocene, and Upper Cretaceous rocks. The Eocene-Paleocene Hanna and Ferris Formations consist of up to 20,000 ft (6,100 m) of organically rich lacustrine shales, coals, and fluviatile sandstones. The Upper Cretaceous Medicine Bow, Lewis, and Mesaverde formations consist of up to 10,000 ft (3,050 m) of marine and nonmarine dark, organic-rich shales that enclose many stacked hydrocarbon-bearing sandstones.

Structural prospecting should be most fruitful around the edges of the basin where Laramide flank structures exist. Deformation of the Hanna basin sediment package into its 30-mi (50-km) wide by 8-mi (13-km) deep present configuration should have produced out-of-the-basin thrusts terminating in closed anticlines. Strata along the northern margin of the basin, located on the southward-displaced Emigrant Trail-Bradley Peak-Shirley thrust complex, were crumpled into anticlinal folds such as O’Brien Springs and Horseshoe Ridge. Oil and gas ranging in age from Pennsylvanian to Upper Cretaceous have been found in these structures.

Only 7 wells have been drilled in the deeper part of the Hanna basin. Two of these tested gas at commercial rates from Upper Cretaceous rocks at depths from 10,000-12,000 ft (3,050-3,660 m). Sparse drilling along the basin flanks has revealed structurally trapped oil and gas at depths from 3,000-7,000 ft (915-2,130 m). The encouragement from the few wells drilled indicates that a more concerted exploratory effort in the Hanna basin is justified.

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