The Codell Sandstone Resource Play of the Northern Denver Basin

Abstract

The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of new oil and gas production is in the deeper part of the basin between the Silo and Wattenberg fields of Wyoming and Colorado, respectively. Thickness of the Codell averages 15 to 20 ft in this area. The cause of the low resistivity is clay and pyrite content. Cores of the Codell illustrate that the sandstone is low permeability, low porosity, bioturbated and reworked finegrained marine shelf sandstone. The Codell is the upper member of the Carlile Formation and unconformably overlies the lower Carlile shales or the Greenhorn and is unconformably overlain by the Niobrara Formation. The Niobrara, lower Carlile, and Greenhorn formations are important source beds for the Codell in the Denver Basin. All are regarded as Type II sapropelic source rocks. Keys to new production are source rock maturity, horizontal drilling, and multistage fracture stimulation. Sweet spot areas coincide with high heat flow, high gas-oil ratios, overpressuring, and natural fracture development. INTRODUCTION The mid-Turonian Codell Sandstone Member of the Carlile Formation has long been a focus as an exploration and development target in the Denver Basin. The Denver Basin is a large asymmetric basin located in northeast Colorado, southeast Wyoming and northwest Nebraska (Figures 1, 2). Most of the production from the Codell Sandstone comes from the Wattenberg field. Drilling activity in the Wattenberg field targeting the Codell commenced in 1981. The drilling depths for the Codell in Wattenberg range from 4000 to 8200 ft. Older Codell fields occur southeast of the Hartville uplift. The fields illustrate the presence of oil and gas in the Codell but the area is generally regarded as uneconomic. A two well field (Chugsprings) was drilled in 1975. The Codell was productive from depths of 3500 to 4600 ft. A two well field (Springer) was drilled in 1974 and 1987 and found Codell production at depths of 6476 to 6530 ft. Both the Chugsprings and Springer fields also produce significant amounts of water. New wells are being drilling for the Codell north and northeast of Wattenberg Field (northern Weld County, Colorado) and west and southwest of Silo Field (Laramie County, Wyoming). This area of new production suggests the occurrence of a large accumulation. Initial production (IP’s) as high as 944 BOPD has been reported from wells in southeast Wyoming. This area will be further discussed in this paper. URTeC 2014 Page 2869