Abstract

The Mesaverde Group of the southeastern Piceance Creek Basin consists of 2,000 to 5,000 ft of marine, transitional, and nonmarine strata deposited during and subsequent to withdrawal of the sea from this area in the Late Cretaceous. Mesaverde strata have been subdivided into two formations, the marine and transitional Iles Formation at the base and the nonmarine and transitional Williams Fork Formation at the top. The Iles consists of three members (Corcoran, Cozzette, and Rollins) composed primarily of blanket-like littoral and sublittoral sandstones. The Williams Fork consists of two formal members (Bowie Shale and Paonia Shale) at the base and the Undifferentiated unit at the top. The Bowie and Paonia are coal-bearing sequences formed in delta complex and barrier complex environments. The Undifferentiated unit is primarily of alluvial plain origin.

Large volumes of natural gas are present in the “tight” Corcoran, Cozzette, and Rollins Sandstones of the Iles, and perhaps equally large amounts are trapped in slightly more permeable sandstones of the Williams Fork. Nine gas fields have been named in this part of the basin, and five other unnamed fields or significant discoveries have been made. With continuing drilling, many of these fields are now coalescing to form one large field that measures 30 by 10 mi.