Isopach maps and diagenetic features may be used to predict the distribution of reservoir-quality rock in the D-zone cyclothem of the Lansing-Kansas City Groups in southwestern Nebraska. The D-zone cyclothem was deposited during one major oscillation of the epeiric sea in Late Pennsylvanian (Missourian). This cyclothem records a transgression of sea level followed by a major regression. During the regressive phase there was a brief sea level transgression.
The D-zone cyclothem consists of the four basic lithofacies common to most cyclic deposits of this age in northwestern Kansas and southwestern Nebraska: (1) a thin lower carbonate unit deposited in a shallow-marine environment; (2) a laterally extensive lower shale unit of marine origin resulting from a terrigenous influx from the north; (3) a complex upper carbonate unit deposited in shoaling water during waning terrigenous influx; and (4) an upper shale unit deposited in tidal flat to nonmarine environments.
Core data and an isopach map of the upper shale unit suggest that several shoal areas existed in Hitchcock County during part of the Missourian. Pellet, ooid grainstone deposition was localized on these bathymetric highs. The bathymetric highs may have been formed by (1) differential compaction of the upper shale unit of the underlying E-zone over erosional topography, or (2) movement on the ancestral Las Animas arch.
The presence of equant-calcite fringing cements in pores of the grain-supported rock indicate early diagenesis in a freshwater phreatic zone formed during initial subaerial exposure. Limpid dolomite rhombs intergrown with the early calcite cements and replacing the edges of some framework grains suggest cementation in a mixing zone. The highest stratigraphic occurrence of dolomite, if plotted on a cross section, forms a line which transects facies boundaries and may represent either the position of the mixing zone or an early paleowater table. The majority of the leached porosity in the grain-supported rock occurs above this line. Dolomitization of underlying carbonate facies probably occurred contemporaneously as the mixing zone migrated through the porous mud-supported sediments. Further enhancement of porosity may have occurred in a vadose zone above a later paleowater table. The position of this paleowater table is indicated by the distribution of skeletal fragments replaced by red silica, dissolution cracks infiltered with nonmarine clay, and authigenic gypsum. These features formed during a later stage of diagenesis which look place contemporaneously with soil formation and calichification in the upper shale in a semiarid or arid environment.
Conclusions: (1) paleobathymetery is reflected in an isopach map of the upper shale unit; (2) distribution of grain-supported rock is controlled in part by formation of bathymetric highs while underlying shales compacted around preexisting topographic highs; (3) enhancement of porosity by dissolution in the grain-supported rocks occurred in the freshwater phreatic and vadose zones; (4) recognition of diagenetic features associated with formation of paleowater tables may be used to predict the distribution of porosity in these grainstones.