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The Delaware Mountain Group consists of a 1,000-m thick section of Permian siltstone and sandstone that was deposited in a euxinic, deep-water, intracratonic basin. More than 100 oil and gas fields produce from the upper part of the Delaware Mountain Group (Bell Canyon Formation) in western Texas and southeastern New Mexico. Stratigraphic traps occur where sandstone-filled channels are incised into less-permeable, interchannel siltstone. Subparallel, erosional channels are relatively broad, shallow features (0.5 to >8 km wide, 1 to >35 m deep) which trend at high angles to the basin margin and extend at least 70 km basinward. Channels are filled with siltstone and thick-bedded, moderately well-sorted, very fine sandstone. The sandstone contains abundant large- and small-scale tr ctive-produced stratification, generally lacks texturally graded sedimentation units, and shows no regular vertical sequence of stratification types. Channel erosion and sediment transport are interpreted to have resulted from long-lived, clay-free, density underflows of fluctuating flow strength. The flows may have originated by storm-ebb flushing of hypersaline shelf lagoons.

Reservoirs are subarkosic, poorly cemented sandstones with high intergranular porosity (15 to 25%) and relatively low permeability (<200 md). The presence of authigenic, pore-lining clay (principally chlorite) greatly affects reservoir properties in these sandstones. Source-rock analyses of the interbedded siltstones show large amounts of unstructured kerogen (TAI~2) and extractable organic matter (1,120 to 2,550 ppm), with high concentrations of hydrocarbons in the extractable organic matter (515 to 1,560 ppm). Delaware Mountain Group siltstones are good to very good source facies and are the most likely source for oil in Bell Canyon reservoirs.

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