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.

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