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Abstract

The most important mineral resource activity in Colorado during the past decade has been the discovery and development of the Wattenberg and adjacent petroleum fields. Located north of Denver across the axis of the Denver basin, the Wattenberg is estimated to have reserves of 1.3 trillion cubic feet (tcf) in the tight J (Muddy) Sandstone (delta front) reservoir over an area of 600,000 acres at depths of 7,600 to 8,400 ft (2,310 to 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 0.005 millidarcys (md) (Matuszczak, 1973, 1976).

Drilling for J gas 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. In 1981 and 1982, the Cod ell Sandstone, approximately 500 ft (152 m) stratigraphically above the J, was developed as a new producing horizon of oil and gas. 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 fades. Net pay thickness ranges from 3 to 2 5 ft (0.9 to 7.6 m). Porosities determined from logs range from 8 to 24%, but the average core porosity is from 10 to 12 % and permeabilities are 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, although unconformities and paleostructure have played a subtle but detectable role.

Variation in thickness and reservoir quality is related to original environmental facies and paleostructure that locallyinfluenced unconformities, fracturing, and diagenesis.

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