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The Codell Sandstone Member of the Carlile Shale is a new exploration target for oil and gas in the northern Denver basin. The Codell interval ranges in thickness from a wedge edge to approximately 100 ft (30.5 m), the average being 15 to 20 ft (4.5 to 6.1 m). The Codell is well developed in the southern Denver basin, is absent in a broad northeast-trending area in the central Denver basin, and is sporadically developed in the northern Denver basin.
The variation in geographic distribution and thickness results from regional unconformities at the base and top of the Codell. The Carlile Shale (50 to 200 ft, 15.3 to 61 m), in the Denver basin and marginal outcrop area, has four members which, in ascending order, are the Fairport Chalk, Blue Hill Shale, Codell Sandstone, and Juana Lopez Limestone. The unconformity at the base of the Codell Sandstone member has a hiatus which increases in magnitude to the west across the basin. The sandstone is transitional with the underlying Blue Hill in central Kansas, but overlies Fairport equivalents in most of the Denver basin and the underlying Greenhorn Formation along the northwest flank of the basin. The unconformity at the top of the Codell places the Fort Hays Limestone Member of the Niob ara in erosional contact with either the thin (1 to 3 ft, 0.3 to 0.9 m) Juana Lopez or the Codell.
Outcrop and core studies clearly show three types of sandstones which developed during sea level changes in late Turonian and early Coniacian time. The Codell is related to processes in three different environmental settings:
1. The marine (or shoreline) bars have a transitional base with the underlying Blue Hill Shale Member. The sandstones have good porosity and permeability and a sheetlike distribution. These sandstones occur in Kansas and the southern Denver basin and are not currently productive of petroleum.
2. Tight bioturbated and reworked marine shelf sandstones generally are without a central-bar facies. These sands may also be associated with thin, irregular, relict, or palimpsest shelf deposits which locally are coarse grained and conglomeratic. Recent petroleum discoveries have been made in this sandstone facies in the west-central part of the Denver basin. Productive depths range from 4,000 to 8,000 ft (1,219 to 2,438 m). Net pay ranges from 3 to 25 ft (0.9 to 7.6 m). Porosities range from 8 to 24%. Permeabilities are generally less than 0.5 md. Trapping of petroleum appears to be stratigraphic.
3. Tight sandstones of marine (?) origin fill large scour depressions (valleys?) which were eroded into underlying Fairport or Greenhorn strata. Although these sandstones are the thickest found in the Codell, they are generally tight and occur mainly in the Wyoming portion of the basin. Only minor production has been found in this facies.
Variation in thickness and reservoir quality is related to original environmental control, paleostructure which locally influenced the unconformities, fracturing, and diagenesis. Where fracturing is important to reservoir quality, the Codell and overlying Fort Hays Limestone may be a co-mingled reservoir.
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