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The most important mineral resource activity in Colorado during the past decade has been the discovery and development of the Wattenberg gas field. Located north of Denver near the axis of the Denver basin, Wattenberg is estimated to have reserves of 1.3 tcf in the tight J Sandstone reservoir over an area of 600,000 acres (2,400 km2), at depths between 7,600 and 8,400 ft (2,310 and 2,560 m). Net pay thickness varies from 10 to 50 ft (3 to 15 m), porosity ranges from 8 to 12%, and permeability varies from 0.05 to 0.0005 md.
The J is interpreted as a fluvial-deltaic sandstone with the principal production from widespread delta-front sandstone. Drilling for gas in the Cretaceous J Sandstone has resulted in multiple pays in overlying strata. The Spindle field, situated in the southwest portion of the Wattenberg field, produces from two marine bar complexes (Hygiene and Terry) in the middle portion of the Pierre Shale. Since 1971, total production is in excess of 36 million bbl of oil and 164 bcf of gas from depths of 4,000 to 5,000 ft (1,220 to 1,525 m).
For the past 2 years, the Codell Sandstone, approximately 500 ft (152 m) stratigraphically above the J Sandstone, has been developed as a new petroleum-producing zone. More than 100 discoveries have been made within and marginal to the outlined Wattenberg field area. The Codell is a tight bioturbated marine-shelf sandstone generally without a central-bar facies. Net pay thickness ranges between 3 and 25 ft (0.9 and 7.6 m), porosities range between 8 and 24% (with the average 10-12%), and permeabilities less than 0.5 md. Because of rapid decline in production and economic uncertainties, potential reserves from the Codell are unknown.
All petroleum accumulations in the Wattenberg area are regarded as stratigraphic traps. Variation in pay thickness and reservoir quality is related to the original environments of deposition and paleostructure which locally influenced unconformities, fracturing and diagenesis.
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