The Pitchfork oil field, discovered in 1930, is on a steeply dipping, surface-anticlinal structure along the west flank of the Big Horn basin, Park County, Wyoming. The discovery well was completed at a total depth of 3,903 ft. in the Tensleep Sandstone. Field production today is primarily from the Tensleep, though a small amount of production is from the Permian Phosphoria Formation. One well was drilled in 1962 to test older formations, and reached a total depth of 7,766 ft. in the Precambrian.
The original oil found in the discovery well was 18° API. Because of the lack of demand for such oil, poor transportation, and low prices, the well was shut in until 1944. Development since 1944 has been sporadic.
The Tensleep is 250–275 ft. thick, and consists of porous, very fine- to fine-grained, heavily oil-saturated sandstone bodies separated by tight, slightly oil-saturated sandy dolomite and dolomite lenses. Within this active water-drive field, there is 130–150 ft. of net pay having permeability in excess of 25 md. and porosity in excess of 10%. As many as 5–6 zones in the field are characterized by such values. However, as development progressed, it became apparent that “pods” of oil were not being drained effectively by updip wells because of the irregular permeability and porosity.
High water cuts are found in low-permeability sandstone beds where the highly viscous oil is by-passed, and in high-permeability fractured zones where communication is established early with down-dip, higher water saturations.
The upper part of the Phosphoria Formation contributes 15–20 b/d of oil from 3 dually completed wells. The 30 ft. of crystalline, vuggy, fossiliferous, heavy-oil-saturated Phosphoria dolomite has a porosity range of 14 to 23%, but low permeability reduces the effectiveness of the water drive and results in low fluid recovery.
Development drilling continues within the Pitchfork Unit in the oil-water transition zone because the economic productive limits are undefined.
As of January 1, 1966, 37 wells were producing at the rate of 103,000 bbls. of oil per month from 720 acres on a 23-acre-spacing pattern. Cumulative production to January 1, 1966, was 8,600,000 bbls. of oil and 30,000,000 bbls. of water.