Reservoir Properties and Gas Productivity of the Corcoran and Cozzette Tight Sandstones, Colorado
Authors: Finley R.J. 
Published: December 2007

Abstract
The Upper Cretaceous Corcoran and Cozzette Sandstones are fine to very fine grained blanket-geometry, low-permeability barrier and strandplain sandstones that trend northeast across the southern Piceance Creek Basin of Colorado. Development of these tight gas Piceance Creek Basin of Colorado. Development of these tight gas reservoirs has been concentrated in the Shire Gulch-Plateau field area where core and log data show contrasting trends in reservoir properties for sandstones with blocky or with upward-coarsening properties for sandstones with blocky or with upward-coarsening gamma ray log patterns. Lesser reservoir quality is associated with increased amounts of carbonate cement, and with more highly burrowed sandstones containing additional silt. Individual Corcoran or Cozzette gas productivity is difficult to determine because production is commingled and is from more than one depositional unit even within a single sandstone member. Net pay by generic depositional unit offers a means of understanding resource distribution.

Introduction
The Corcoran and Cozzette Sandstones are part of the Upper Cretaceous Mesaverde Group of the southern Piceance Creek Basin of northwestern Colorado. The sandstones are members of the Price River Formation of the Mesaverde, and consist of progradational marginal marine facies that overlie the marine Mancos Shale and underlie the continental facies that make up the bulk of the Mesaverde Group. Together with the overlying Rollins Sandstone, the Corcoran and Cozzette have recently been developed as low-permeability gas reservoirs.

Stratigraphic Framework
The Corcoran and Cozzette have long been recognized as shoreline sandstones, but only recently have data become available on the distribution of specific genetic facies. Palmer and Walton defined shelf, shoreface, tidal inlet, and coastal plain facies from outcrop studies, and Finley and Ladwig defined barrier and strandplain facies in the subsurface. Zapp and Cobban recognized that Campanian-age regressive shorelines, represented by the Corcoran and Cozzette Sandstones in this study, had a northeast trend across the southern Piceance Creek Basin. This shoreline orientation is related to progradation into the Cretaceous interior seaway from the west, including major deltaic progradation at a point along the Colorado-Wyoming border in northwestern Colorado. point along the Colorado-Wyoming border in northwestern Colorado. Within this depositional setting, therefore, the Corcoran, Cozzette, and Rollins Sandstones are intermediate facies located between the marine shelf muds of the Mancos and the fluvial and coal-forming environments within which Mesaverde sediments were primarily deposited. As the latter deposits prograded successively primarily deposited. As the latter deposits prograded successively farther into the seaway with time, marginal marine sandstones, such as the Corcoran and Cozzette, formed a blanket sandstone with good lateral continuity as the leading edge of the Mesaverde sediment package. The sandstones are time transgressive and become younger toward their basinward (southeastern) margins. In the depositionally updip direction (northwestward), the Corcoran and Cozzette grade into continental facies and lose the readily correlatable log signature characteristic of marginal marine blanket sandstones.

Structural Setting
The Piceance Creek Basin is a Late Cretaceous to Early Tertiary structural basin defined by a series of Laramide-age uplifts. The Douglas Creek Arch is a mildly positive feature on the western basin margin that separates the Piceance Creek Basin from the Uinta Basin in Utah. There is little evidence of uplift on the Douglas Creek Arch at the time of Mesaverde deposition, and Laramide structural elements in general had little influence on Cretaceous depositional patterns. The strongly asymmetrical Piceance Creek Basin has a gentle western flank and a steeply dipping eastern flank that lies close to the deep axis of the basin. The Corcoran and Cozzette Sandstones crop out along parts of the western basin margin in the Book Cliffs and along the eastern basin margin as part of the Grand Hogback (Fig. 1). Structure contours on the Cozzette Sandstone show relatively uniform northeast-to-north dip throughout most of the southern part of the basin.