Grass Creek Field, in Hot Springs County on the southern side of the Bighorn Basin, is one of the five fields the Wyoming State Historical Society credits with turning the state from a ranching economy into an industrial one. Oil was found here in June 1914, production climbed to a peak by 1918, and a company town of several hundred people grew up around the dome. More than a century later the field is still producing, sustained by injection into the Phosphoria and Tensleep rather than by anything the original drillers would recognize.
Shows and shallow tests were known around Grass Creek before 1914, and sources differ on how early the first oil was found — some accounts point to 1907 or 1908. The documented turning point is June 1914, when oil was discovered in commercial quantity and development moved quickly. Roughly three hundred and fifty wells were eventually drilled to the upper oil sands at depths of only eight hundred to fifteen hundred feet, and production rose to its maximum by 1918 as wartime demand pulled every barrel Wyoming could deliver.
Two companies dominated the field: the Ohio Oil Company, which later became Marathon, and Midwest Refining. Their control gave Grass Creek the infrastructure that scattered independent operators could not build — pipelines, camp housing, and refining outlets — and the town of Grass Creek grew to roughly five hundred residents. Cumulative production through the mid-1950s placed it among Wyoming’s largest fields to that date. The corporate lineage matters to owners because the old lease forms and unit agreements still in force were drafted by those companies.
The deeper Paleozoic section is what carried Grass Creek into the modern era. The Permian Phosphoria and Pennsylvanian Tensleep became the primary reservoirs, produced under unitized operation with injection to maintain pressure, and engineering studies have examined alternating gas and water injection in the Phosphoria-Tensleep as a way to improve sweep. Merit Energy has been associated with operations at Grass Creek in recent years, part of the same consolidation of mature Bighorn Basin floods that took in Oregon Basin and Hamilton Dome.
Grass Creek is a domal structure on the southwestern flank of the Bighorn Basin, a fold pronounced enough that the first wells found oil at less than fifteen hundred feet. The shallow Cretaceous sands carried the early boom; the durable production came from the Permian Phosphoria and Pennsylvanian Tensleep beneath. Those two units are the workhorses of the entire basin, a carbonate and an eolian sandstone with reservoir quality that changes markedly over short distances. Well performance across the dome accordingly varies more with position on the structure and local rock quality than with depth alone.
Injection has sustained Grass Creek for most of its life. Waterflooding restored and held pressure in the unitized Phosphoria and Tensleep, and technical work on the field examined alternating gas and water injection as a means of improving sweep through reservoirs whose permeability varies unevenly. In practice this means the field behaves as an engineered system rather than as a set of independent wells, and the pace of future production depends on the operator’s injection program. For royalty owners, that translates into low rates that persist for decades instead of falling away.
A Grass Creek royalty is one of the longest-running income streams in Wyoming — a century of payment history on interests that have typically passed through three or four generations. The practical problems are title rather than geology: old lease forms, unit agreements owners have never read, and heirs scattered across the country holding decimals nobody has reconciled. The production itself is flat, injection-supported, and long-lived, which is valued very differently from shale. We buy minerals and royalties in Hot Springs County and provide free written offers.
County-level well data, production charts, and selling guides for the counties this field spans:
Oil was discovered in commercial quantity in June 1914, though some accounts place earlier shows in 1907 or 1908. Development moved fast after 1914 and the field reached its production peak in 1918.
The Ohio Oil Company, later Marathon, and Midwest Refining controlled most of the field during its early decades. Merit Energy has been associated with operations in recent years, following the pattern of consolidation across mature Bighorn Basin waterfloods.
Because injection-supported carbonate and sandstone reservoirs decline very slowly once they have matured. That flatness is the defining feature of these Wyoming royalties, and it is the reason they should be valued on remaining life rather than on a multiple of last year’s income.