Southern Piceance Basin Model—Cozzette, Corcoran and Rollins Sandstones
Published:January 01, 1986
Charles A. Brown, Thomas M. Smagala, Gary R. Haefele, 1986. "Southern Piceance Basin Model—Cozzette, Corcoran and Rollins Sandstones", Geology of Tight Gas Reservoirs, Charles W. Spencer, Richard E. Mast
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The Corcoran, Cozzette, and Rollins sandstones of Late Cretaceous Campanian age were deposited in a shoreline and shelf environment. Subsequent diagenesis caused quartz overgrowths and calcite cementation and abundant amounts of pore–filling authigenic clays. Today the original conventional, intergranular porosity has been reduced to primarily microporosity with extremely low permeability; hence, the Corcoran, Cozzette, and Rollins are classified as tight gas sands.
Structural trapping of gas accounts for most of the discovered reserves to date. However, the future production from the Corcoran, Cozzette, and Rollins sandstones in the southern Piceance basin will be found in tight gas–bearing sandstones present in the nearshore and offshore strandline facies. Data support the presence of a basin–centered gas trap with a dynamic updip flow of gas out of the basin. Pressure equilibrium of water upstructure toward the outcrop, very small capillaries, and continuous gas generation and migration provide the setting for dynamic capillary–pressure trapping (diagenetic trapping). Development of this type of trapping model requires the existence of a thermally mature basin with sufficient gas source beds adjacent to blanket reservoir beds. Primary gas flow occurs along the path of least resistance (the shoreline trend), but this migration route is also controlled by diagenetic degradation of the reservoir. In the gas leg of the accumulation, the blanket sandstone is perceived to be a system of gas–filled pores with interconnecting, water–filled capillaries.
The most significant implication of this trapping model is that major amounts of additional gas resources are found in the basin. These undeveloped resources would lie downdip of the Plateau and Shire Gulch fields, toward the center of the basin, and along the trends of better sand reservoirs.
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Geology of Tight Gas Reservoirs
Tight gas reservoirs occur in low-permeability, gas-bearing formations that are present to some extent in all gas-producing basins worldwide. This is the first volume to bring together data on tight reservoirs for a variety of basins and different geologic settings. The papers in this volume discuss characteristics of some of the most significant tight gas areas in the United States; however, these data are equally applicable to many other recognized and unrecognized tight gas provinces in other nations. In general, tight reservoirs in the United States are grouped into tight gas sandstones and eastern Devonian shales. The Devonian shale sequences are dominantly marine shale but include some siltstone and sandstone. Tight gas sandstone formations of other than Devonian age are present throughout the United States and consist primarily of fluvial and marine sandstones and siltstones. In addition, gas also occurs in low-permeability marine carbonate reservoirs. The 14 papers in this volume cover such topics as: coal-bed methane and tight gas sands interrelationships; gas-bearing shales in the Appalachian basin; exploration and development of hydrocarbons from low-permeability chalks; and geologic characterization of low-permeability gas reservoirs.