LOW ACCOMMODATION DEPOSITIONAL SYSTEMS AND RESERVOIR BODIES IN THE LOWER CRETACEOUS DAKOTA FORMATION, SUBSURFACE OF NORTHWESTERN NEBRASKA

The “J” sandstone represents up to 30m of the Lower Cretaceous Dakota Group in the subsurface of northwestern Nebraska. The unit lies within a mudrock-dominated section and is hydrocarbon prospective in structural and stratigraphic traps. Fuenning, 1942, AAPG Bull., 26, 1517-1536 and Harms, 1966, AAPG Bull., 50, 2119-2149 concluded that the unit fills linear valleys. However, understanding of its external geometry and internal architecture is limited. An improved understanding of depositional history was achieved by describing and correlating cores, drill cuttings, and wireline logs from 8 hydrocarbon exploratory wells within Sioux County (25-30N, 50-57W), Nebraska. The “J” sandstone is composed of fine grained, amalgamated, sharp based sandstones, alternating with siltstone and shale, with abundant carbonaceous material. Composite layers of wavy, hummocky, and flat laminated sandstones occur repeatedly through the unit and are associated with soft sediment deformation. Ripple cross-lamination, lenticular bedding and millimeter scale rhythmic, pinstripe laminations of fine grained sandstone and siltstone also occur. Mudstone rip-up clasts, siderite, pyrite, and few mollusk shells occur throughout. Ichnofossils such as Diplocraterion habichi, Planolites, Rosselia, Cylindrichnus and equilibrium adjustment structures (EAS) are common in most of the unit, except in black shales. Bioturbation indices vary from 0-6. A rooted horizon is present near the top of the unit. The assemblage represents low diversity, size- reduced variants of the Skolithos and Cruziana ichnofacies. The near absence of marine fossils along with ichnofossil characteristics suggests stressed depositional environments. Physical structures and EAS indicate significant sediment flux to the area. Rhythmic, pinstripe laminations and abundant carbonaceous material imply near shore environments influenced by tidal activity. Together, this dataset suggests that the “J” sand in the study area was formed in estuarine depositional environments within drowned, incised valleys.