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The Richey field was discovered in July, 1951, by the Shell Oil Company No. 1 Northern Pacific, SE. NW. NW. Sec. 19, T. 23 N., R. 50 E., Dawson County, Montana, with completion in the Charles formation of Mississippian age. Subsequent wells also established production in the Mississippian Mission Canyon formation. This was the first commercial oil discovery in the Montana portion of the Williston Basin.
Regionally, the Richey field is situated on the west flank of the Williston Basin paralleling the Calf Creek-Weldon lineament and is believed to be tectonically related to it. The local structure is a northeasterly trending anticline with a normal fault parallel with the northwest flank, and a transverse fault across its south end. A structural closure of approximately 100 feet is indicated. The oil accumulation is controlled primarily by structure, but modified by a decrease in porosity along the southeast flank.
The Charles reservoirs contain the bulk of the oil reserves at Richey and consist of three zones of fractured limestone and dolomite with intercrystalline porosity having an aggregate average net pay thickness of about 35 feet. The Mission Canyon reservoir consists of very finely crystalline to dense limestone in which porosity occurs exclusively as fractures. There is an average net pay thickness of 15 feet in this unit. An effective water drive is present in all reservoirs.
In general, the production has been characterized by a high initial potential, followed by an early and abnormally high water cut. It is indicated that with the established practice of commingling the Charles and Mission Canyon reservoirs, the highly vertically fractured Mission Canyon zone is produced beyond its maximum efficient rate of flow, resulting in a rapid water coning which suppresses oil production from the Charles zone. It is indicated the ultimate oil recovery will be considerably greater if the reservoirs are produced separately, and the Mission Canyon at a low rate.
Development is on an 80-acre pattern. There are presently 12 producing wells in the field. Cumulative production to December 1, 1959, was 1,239,000 barrels of oil. Current monthly production is about 20,000 barrels of oil. Approximately 1,700 acres are considered proved productive with an estimated recoverable reserve of 5,000,000 barrels of oil.
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