Southern Piceance Basin Model--Cozzette, Corcoran and Rollins Sandstones

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.