Dakota "D" sandstone reservoirs represent a prograding, delta-front environment of deposition. The unit is 40 ft (12 m) thick and displays rapid lateral change in rock types. Examination of texture, composition, and sedimentary structures in five cores shows that the section is composed of three major facies. The facies are, in ascending order, (1) prodelta, bioturbated mudstone; (2) delta-front, burrowed, fine-grained sandstone; (3) fluvial, cross-laminated, medium-grained sandstone. These facies form either a single, thick prograding sequence, or two thinner sequences within the Dakota D section. The section thins abruptly northwestward and grades to a dominantly bioturbated sandstone which suggests that all facies were deposited in a location marginal to the main axis f the delta. The uppermost part of the section consists of thin beds of sandstone, mudstone, and shale that represent bay, marsh, and levee deposits.

The sandstones have a mean grain size of 0.18 mm (fine grained), and grain size increases upward from prodelta mudstone, through delta-front sandstone, to fluvial-channel deposits. Average detrital composition is 69% quartz, 10% feldspar, 13% rock fragments, and 8% clay matrix. Quartz increases and clay matrix decreases from prodelta through delta-front, to the fluvial-channel facies. Cement is largely calcite and averages 10% of bulk composition.

Permeability and porosity are variable. Fluvial-channel sandstones have the highest average values of 600 md and 22.8%, and delta-front sandstones decrease to about 100 md and 19%. Average values in the prodelta facies are less than 1 md and 19%.

Oil and gas are trapped by lateral facies change within the deltaic sequence. The total gas and oil column is 65 ft (20 m), and calculations suggest that about 40 ft (12 m) is trapped by capillary pressures within the delta-front facies. The oil-water contact shows a pronounced eastward tilt, and it is estimated that about 25 ft of additional oil is present by hydrodynamic flow.