Frannie Field lies at the far northern end of the Bighorn Basin, near the small town of Frannie on the Park and Big Horn County line and only a few miles from the Montana border. Its principal reservoir is the Pennsylvanian Tensleep sandstone, the formation that carries most of the basin’s oil, and the field has been produced under injection for the better part of a century. In recent years Frannie has drawn attention for a different reason: as a candidate for recovering oil from the residual zone beneath the main pay.
Frannie was mapped and drilled in the late 1920s, in the same northern Bighorn Basin push that developed Garland, Byron, and Elk Basin. Early geological work on the field, including a published account of its structure and oil possibilities at the end of that decade, treated it as a conventional anticlinal prospect on a structure clear enough to map at the surface. The results justified the interest, and the Tensleep proved substantial enough that the field became a fixture of the basin rather than one of the short-lived shows that surrounded it.
Continental Oil Company later published a detailed geological and Tensleep reservoir summary of Frannie, which is the sort of document a company writes when it is planning to manage a reservoir for the long run rather than simply produce it. Unitization and water injection followed the same path taken across the northern basin, converting a set of competing leases into a single engineered reservoir. Frannie settled into the pattern that describes most of these fields today: low rates, very slow decline, and decades of steady payment to a long list of royalty owners.
Interest revived in the 2010s around the residual oil zone — the interval beneath the conventional oil-water contact where oil remains at low saturation after natural water sweep through geologic time. Work published through the Enhanced Oil Recovery Institute at the University of Wyoming used reservoir simulation to investigate exploiting the Tensleep residual zone at Frannie with carbon dioxide. That work is a study rather than an operating project, and owners should not read it as a commitment to flood the field.
Frannie is an anticlinal trap on the northern rim of the Bighorn Basin, with the Pennsylvanian Tensleep sandstone as its main reservoir and Paleozoic carbonates including the Madison in the section beneath. The Tensleep here is the same eolian sandstone that produces across the basin: good porosity, but permeability that varies sharply between cross-bedded units and dolomite-cemented intervals. Below the conventional oil column lies a residual oil zone, a swept interval holding oil at saturations too low to flow to a well under ordinary drive but potentially recoverable with carbon dioxide.
Water injection has been the working recovery method at Frannie, maintaining pressure and sweeping the Tensleep across decades of unitized operation. The forward-looking question here is the residual oil zone. Reservoir simulation work published through Wyoming’s Enhanced Oil Recovery Institute examined whether CO2 could commercially recover oil from the swept interval beneath the main Tensleep pay — a target that does not appear in conventional reserve bookings at all. Whether that becomes a project depends on CO2 supply, price, and operator appetite, none of which a mineral owner controls or should assume.
Frannie interests carry the two-county wrinkle common to fields on the Park and Big Horn line: leases, probate filings, and recording can land in either county, and inherited interests frequently need curative work in both. The production is late-life waterflood output — small, steady, and slow to decline — with an unbooked residual-zone possibility that no responsible buyer will pay full value for today. We buy minerals and royalties in Park and Big Horn Counties and provide free written offers.
County-level well data, production charts, and selling guides for the counties this field spans:
It is the interval below a field’s oil-water contact where natural water movement over geologic time has swept the oil down to a saturation too low to flow under ordinary production. Carbon dioxide can sometimes recover it, which is why the Frannie Tensleep residual zone has been studied.
Possibly both. Frannie straddles the Park and Big Horn County line, so deeds, probates, and affidavits may need to be recorded in each county depending on where the tract sits. We handle that curative work at our expense when we buy an interest.
The published work is simulation-based research into whether the Tensleep residual oil zone could be exploited with CO2, not an announcement of a flood. Treat it as upside that may never be developed rather than as a reason to expect higher checks.