Abstract
The major oil and gas fields of the Denver basin produce from the D and J zones of the Lower Cretaceous Dakota sandstone. Lateral and vertical variations in grain size and permeability of these members localize oil accumulations both in conjunction with, and independent of, structural anomalies. Although only about half the fields may be classed as stratigraphic traps, some of the most prolific producers, Adena, Little Beaver, Harrisburg, and Bobcat, are stratigraphically controlled. In the producing area, the D zone is a continuous siltstone body containing local lenticular and shoestring sandstones. The J zone is a more continuous sandstone containing numerous silt and shale barriers near the top of the zone. Stratigraphic traps in the D and J zones are divided into five classes based on the form of the reservoir: (1) isolated sand lenses, (2) shoestring sand bodies, (3) updip barriers in sand bodies, (4) updip drop in roof of reservoir sand, (5) erratic cementation of sand.
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