Hanna Basin Geology

The Hanna Basin is a small yet anomalously deep (9,144 m; 30,000 ft) intermontaine Laramide-style basin, approximately 64 km (40 mi) east-west and 40 km (25 mi) north-south. The basin is bounded to the north by the Shirley and Seminoe mountains, to the east by Simpson Ridge, to the south by the Medicine Bow Mountains and Park Range, and to the west by the Rawlins Uplift.

The structural development of the Hanna Basin occurred in multiple stages. The Hanna Basin was first isolated from the Greater Green River Basin by the uplift of the Shirley and Granite mountains during the early Paleocene, followed by middle-Paleocene growth of the Sweetwater Uplift. The Medicine Bow Mountains and Rawlins Uplift occurred during the late Paleocene. The Cambrian- through Jurassic-age sedimentary strata that accumulated before the structural development of the Hanna Basin are less than 762 m (2,500 ft) thick.

During the Laramide orogeny, the Hanna Basin was isolated from the surrounding basins and became a closed drainage. This structural configuration resulted in a thick succession of Upper Cretaceous to Lower Paleogene fluvial and lacustrine sedimentary deposits. These fluvial and lacustrine strata account for the bulk of the strata in the basin center, and can be up to 5,791 m (19,000 ft) thick.

Haystack Mountains
Haystack Mountains Formation dipping northeast into the Hanna Basin, Haystack Mountains. [Credit: WSGS]


Conventional oil and gas exploration occurred in the Hanna Basin throughout the 20th century. There are currently 25 named fields in the Hanna Basin, 13 of which are abandoned or shut-in (WSGS oil and gas map). The basin’s most productive oil field, Big Medicine Bow field, has produced from the Frontier Formation, Muddy Sandstone, Cloverly (Dakota) Formation, Morrison Formation, Sundance Formation, and Tensleep Sandstone (WOGCC, 2024). The most productive gas field, Separation Flats field, produced from the Muddy Sandstone but is no longer active. The Hanna Basin has not been extensively explored for undiscovered petroleum accumulations, and there are potential conventional and unconventional undiscovered accumulations (Dyman and Condon, 2007).

Coal mining has been active in the Hanna Basin since 1868 (Flores and others, 1999). Mines operated at the town site of Carbon until 1900, when mining operations moved to the town of Hanna after the railroad was rerouted. Most of the coal extraction in the Hanna as well as Carbon basins (which is separated from the Hanna Basin by the northeast- southwest-trending Saddleback Hills anticline) has been from the Hanna coal field (Pierce, 1996). Coal is primarily mined from the Upper Cretaceous and Paleocene Ferris Formation, as well as the Paleocene Hanna Formation.

Dyman and Condon (2005) define the Hanna-Mesaverde coalbed gas total petroleum system in the Hanna Basin, as including portions of the Mesaverde (Almond), Medicine Bow, Ferris, and Hanna formations. Two coalbed natural gas (CBNG) pilot projects in the basin, established by 2005, have produced very little gas from this petroleum system. The Seminoe Road CBNG pilot project contained 16 wells that produced 1,400 cubic feet of gas per day; the Hanna Draw CBNG pilot project had nine wells that averaged less than 1,000 cubic feet of gas per day (Dyman and Condon, 2005).

Tensleep Sandstone
Tensleep Sandstone, Goose Egg Formation, and Chugwater Formation (foreground), on the flanks of the Seminoe Mountains. [Credit: W.A. Sullivan]

Future Development

Because of the anomalous structure of the Hanna Basin relative to other Laramide basins (that is, it is a small but very deep basin), exploration targets are limited to the flanks of the basin— the basin center is considered too deep for most exploration. There has been very little exploration in the basin over the past few decades, mostly limited to CBNG projects. No drilling projects on federal lands have been proposed in the Hanna Basin.



Rachel Toner, rachel.toner@wyo.gov

Derek Lichtner, derek.lichtner1@wyo.gov