An elongated, strongly asymmetric Laramide trough between the Bighorn Mountains and the Black Hills. A 5,000 ft column of Cretaceous marine shales and shelf sandstones holds stacked oil reservoirs, now developed with horizontal wells from the Parkman and Sussex down through the Niobrara, Turner, Frontier and Mowry.
Laramide uplift of the Bighorn Mountains, Casper Arch and Laramie Mountains downwarped an asymmetric trough about 250 miles long and 100 miles wide. Beds on the western margin dip steeply to overturned against the Bighorns, while the eastern flank rises gently toward the Black Hills.
The Cretaceous Western Interior Seaway laid down thick marine shales (Mowry, Niobrara, Steele) interleaved with shelf-bar and deltaic sandstones that prograded eastward from the rising west: Frontier, Turner, Shannon, Sussex, Parkman and Teapot. The older Muddy records deltaic channels, tidal flats and barrier islands as sea level rose and fell.
The Niobrara and Mowry shales are both source rocks and horizontal reservoirs. The Niobrara carries oil-prone Type II kerogen; the deeper Mowry is mixed Type II and III and more gas and condensate prone, so the shallower sands and Niobrara are the oilier targets.
The basin supplies most of Wyoming's oil. Operators stack wells through several horizons from the same pads, and results in the Niobrara and Turner have made the Powder River one of the Rockies' leading oil plays.
Diagram 1. A generalized block from the gentle Black Hills flank to the Bighorn Mountain front, showing the asymmetric trough, the steep western flank and the stacked horizontal targets. Select a unit for its lithology, thickness and reservoir data. Units in navy are TRP targets.
Diagram 2. Tertiary and Cretaceous section in the central Powder River Basin, with lithology, thickness and depth data and each unit's role as source, reservoir or seal.
Campbell and Converse counties. Stacked Parkman, Sussex, Shannon, Turner, Niobrara, Frontier and Mowry laterals. Anschutz holds about 450,000 net acres; EOG, Continental, Devon and Occidental are also active.
More than 725 named fields, from Salt Creek (1889) to Minnelusa and Muddy stratigraphic traps. Many older units sit above newer horizontal targets, so legacy minerals can gain value.
Laramie County, bounded by the Laramie Range and the Hartville uplift. More than 30 named fields and about 10% of recent Wyoming oil, from the same Niobrara and Codell targets described on our Colorado page.
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