New reserves in an old field, the Niobrara/ Codell resource plays in the Wattenberg Field, Denver Basin, Colorado

The Niobrara Formation and Codell Sandstone are one of nine horizons that are productive in the giant Wattenberg Field area (GWA) of Colorado. GWA covers approximately 3200 square miles. The field was discovered in 1970 (J Sandstone) and the first significant Codell production from vertical wells was established in 1981 followed by Niobrara production in 1986. Horizontal Niobrara/Codell drilling began in the field in 2009. Wattenberg straddles the Denver Basin synclinal axis and is regarded as a basin-centre petroleum accumulation. The Niobrara/Codell is overpressured and drilling depths are 6200 to 8200 ft. The Wattenberg area is a ‘hot spot’ or positive temperature anomaly. Temperature gradients range from 16 to 18oF/1000 ft on the edges of the field to about 28 to 29oF/1000ft in high GOR areas. The Niobrara consists of four limestone (chalk) units and three intervening marl intervals. The lower limestone is named the Fort Hays and the overlying units are grouped together as the Smoky Hill member. The chalk units are referred to in descending order as the A, B, C, and Fort Hays. Erosional unconformities exist at the top and base of the Niobrara. The upper unconformity removes the upper chalk bed in some areas of the Wattenberg Field. The B and C chalks are the main focus of horizontal drilling by opera­tors in the field. The underlying Codell Sandstone/Fort Hays is also targeted with horizontal wells. Recent horizontal completions in the Niobrara have initial production of approximately 100 to 700 BOPD with a GOR of 500 to 10,000 cu ft per barrel. Estimated ultimate recovery per well is greater than 300,000 BOE.