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Thick sequences of marine rocks consisting of thin (1 to 5 cm) composite bedsets of sandstone and shale constitute a major part of the Cretaceous sedimentary prism that accumulated in the Western Interior. Today, these rocks include significant source beds and are locally important reservoirs for natural gas. Because of their large areal extent, immense volume, and ubiquitous gas content, sequences of composite sand-shale bedsets constitute an important potential resource of hydrocarbons throughout the Rocky Mountains region--an energy frontier.
The composite bedsets contain the reservoirs, source beds, and seals. The shale layers serve as both source beds and seals, and very thin (1 to few cm) wavy and lenticular beds of sandstone make up the reservoir rocks. Because these lithologies are associated generally with marine regressions, most of the constituent organic matter is terrigenous and the strata are gas-prone. The composition of the gas varies, depending on the degree of thermal maturity, from biogenic methane in Upper Cretaceous strata of the northern Great Plains of the United States and Canada, to supermature dry gas in the Mancos Shale in parts of the Piceance Creek basin in Colorado.
The reservoirs in these rocks do not require structural closure for gas entrapment, but successful commercial production generally requires fracture stimulation. In general, the reservoirs are characterized by high irreducible water saturation, high ratios of horizontal to vertical permeability, and sensitivity to water-based fluids. Field boundaries are determined by economic factors. The maximum depth of production is determined ultimately by reservoir porosity which, in turn, is the product of two distinct porosity loss trends. The shale component undergoes almost complete loss of effective porosity during early compaction, although products of organic matter transformations, shale dewatering, and clay diagenesis continue to be expelled into adjacent sandstone layers throughout mos of the subsidence history of the basin. The porosity trend in the sandstone layers is controlled mainly by cementation byproducts of shale diagenesis and by the original sand composition. The overall reservoir porosity reflects the ratio of sandstone to shale in the sequence and the extent of porosity loss in the sandstone layers. Generally, porosity loss increases with increasing depth and maturity and is irreversible and predictable.
Thus far, production from these rocks has been mainly a result of accidental discovery during exploration for more conventional reservoirs and subsequent field development. Successful exploitation of the enormous gas resources in these strata will require intentional exploration programs.
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