Mississippian Mission Canyon carbonates are the most prolific Paleozoic reservoir in the Wyoming Overthrust belt. At Whitney Canyon-Carter Creek field, the Mission Canyon Formation holds recoverable reserves of 240 million bbl of oil equivalent. Production comes from a 350-ft (107 m) gross interval of shallow-water shelf carbonates. Capping the reservoir interval is a 300-ft (91 m) section of anhydrite and tight dolomite that represents sabkha deposits that prograded seaward (westward) over the shelf carbonates.

Production from the shelf sequence comes exclusively from sucrosic dolomites that are interbedded with tight limestones and tight crystalline dolomites. Examination of cores spanning the entire reservoir interval reveals that it is composed of a series of nine shallowing-upward sequences, reflecting a history of progradational events across the Mississippian Wyoming shelf. A single complete sequence averages 40 ft (12 m) in thickness and grades upward from open-marine through restricted-marine to intertidal and/or supratidal environments. Open-marine units are predominantly fossiliferous grainstones and packstones--rocks containing little or no carbonate mud. The overlying restricted-marine and intertidal and/or supratidal units are primarily mud-supported carbonates. Petrographic evi ence indicates that carbonate mud was dolomitized preferentially relative to grains. The best reservoir-quality dolomite, therefore, usually occurs in the mud-rich, upper portions of the shallowing-upward sequences, and tight intervals separating the porous zones generally represent grain-supported, open-marine units.

Such facies characteristically are continuous for great distances along depositional strike. At Whitney Canyon-Carter Creek field, individual porous zones can be correlated for more than 12 mi (19 km) across the

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field. This understanding of facies-controlled porosity development has application both in regional exploration and in field development.

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