Median-permeability contour maps of the J sandstone, Dakota Group, in the Denver Basin, Colorado, Nebraska, and Wyoming

The Lower Cretaceous J sandstone of the Dakota Group (MacKenzie, 1965) is present in the Denver basin in eastern Colorado, southeastern Wyoming, and southwestern Nebraska. This informally named unit deposited during a regression of the Cretaceous epi continental sea and is composed primarily of sandstone and shale of deItaic and near-shore marine origin. The J sandstone can be divided into an upper transgressive sand, a middle marginal-marine and deltaic facies, and a lower prodelta sequence (Clark, 1978). The depth from the surface to the top of the J sandstone increases from about 4,000 ft on the gently-dipping eastern flank of the bass n to more than 8,000 ft near the steeply-dipping flank at the western boundary.

Permeability values compiled in this J sandstone study were determined from cores from 134 widely scattered boreholes. Median permeabilities, rather than average permeabilities, were used in order to minimize the effect of anomalous samples. Thirty-five oil companies and independent operators supplied core data. Core Laboratories of Denver, Colorado analyzed the core, which was submitted over a period of 25 years.