Reservoir Characteristics of Ancient Fluvial Deposits with Emphasis on Rocky Mountain and Mid Continent Regions
Frank G. Ethridge, 1985. "Reservoir Characteristics of Ancient Fluvial Deposits with Emphasis on Rocky Mountain and Mid Continent Regions", Recognition of Fluvial Depositional Systems and their Resource Potential, Romeo M. Flores, Frank G. Ethridge, Andrew D. Miall, William E. Galloway, Thomas D. Fouch
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Fluvial depositional systems often constitute significant host for hydrocarbons. In the Frio Depositional Systems in Texas, the Gueydan fluvial system has the highest whole rock yield factor of all Frio Systems (Galloway, et al., 1982) and in Prudhoe Bay field, Alaska, the largest oil field in North America, braided stream deposits host the principal producing reservoir. Interest in hydrocarbon-bearing, fluvial depositional systems remains high, especially with regard to the Cretaceous in the Powder River Basin, Wyoming, the Tuscaloosa in Mississippi and the Pennsylvanian in the D-J Basin, Colorado. Because of the diverse nature of fluvial deposits, the characteristics of hydrocarbon-bearing fluvial reservoirs are variable. At one extreme are braided river deposits, which have abundant potential reservoir rocks but lack seals, and at the other extreme are fine-grained meander belt deposits with small- to moderate-size reservoirs isolated in mudrock (Galloway and Hobday, 1983).
In this chapter we will review the characteristics of point bar, braided stream and fan delta reservoirs and data pertaining to the recognition of paleotectonics, unconforrnities and hydrocarbon-bearing valley fill sequences. Special emphasis will be placed on an evaluation of fluvial hydrocarbon-bearing reservoirs in the Rocky Mountains and the Mid-Continent.
Point bar deposits, which result from deposition on the concave bank of a meandering stream during falling river stage following flooding, constitute the principal reservoirs in fine-grained meander belt systems (Fig. 1). Point bar deposits are recognized in vertical sequences by an erosional base, an overall decrease upward in the scale of sedimentary structures from trough cross-beds and