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Geologic Factors Controlling Production in the Codell Sandstone, Wattenberg Field, Colorado


The Upper Cretaceous Codell Sandstone is a major pay in the giant Wattenberg Field of the Denver Basin. Vertical well completions in the Codell date back to 1981. The vertical wells have a history of successful hydraulic refracturing. New horizontal wells (2011 to P) with initial production of 100 to 700 BOPD (GOR ~10,000 cf/bbl) indicate substantial remaining reserves in the formation. Geologic factors important for production include: proximity to thermally mature source beds; thickness; geothermal gradients; pressure gradients; fault bounded reservoir compartments; gas-oil ratios; sufficient reservoir quality (phi-h). The Codell in Wattenberg is characterized by low porosity (<10%) and permeability (< 0.1 md). The Codell is 5 to 20 ft thick across the Wattenberg Field and has formation pressure gradients that range from 0.45 to 0.66 psi/ft. Geothermal gradients range from 1.8 to 2.5oF/100 ft. The highest GORs in the field correspond to the highest geothermal gradients. The sandstone is very fine to fine grained and bioturbated. Thin (< one ft thick) hummocky cross stratified beds are present in the Codell. Depositional environment is interpreted to be a shallow marine shelf setting. Clay content within the pay interval is approximately 20% and consists of 40–45% mixed layer illite-smectite, 30–40% illite, 10–30% chlorite, and up to 7% glauconite. The Codell is a low-resistivity, low-contrast pay. The fault-bounded reservoir compartments form mainly from a well-developed polygonal fault system. Polygons are generally about 1.5 square miles in size. The orientation of the polygons is influenced by pre-existing basement fault systems. The Codell unconformably overlies the Fairport chalk member of the Carlile Formation and is unconformably overlain by either the Juana Lopez or the Fort Hays Limestone Member of the Niobrara Formation.