Pennsylvanian and Permian sediments in the sparsely drilled western panhandle of Nebraska reflect a transition through time from shallow-marine to restricted environments. Middle and Upper Pennsylvanian and Lower Permian beds are cyclic; a typical cycle sequence includes black shale and carbonate mudstones, wackestones, packstones, and grainstones.

The cyclic sequences, particularly those in the Desmoinesian series, can be compared to productive sequences in the Midland basin, the Four Corners, and the Dodge City embayment. The black shales are excellent source beds. Diagenetic processes both enhance and inhibit porosity. Dense, intratidal dolomites are the norm but the porous, supratidal dolomites associated with a shoal and strandline assemblage of carbonate grainstones in the Continental 1-35 Duncan (NW¼ SW¼, Sec. 35, T25N, R57W) strongly imply that a few feet of added elevation is the difference between tight rocks and those with effective porosity.

Lack of structural leads and scant deep drilling put strong emphasis on the stratigraphic interpretation of seismic data and modeling. A synthetic trace derived from a sonic log can be systematically altered by replacing high-velocity tight rock with porous-rock low velocities. Ideally, the modeled porous synthetic trace will compare favorably to the actual seismic traces.

Other factors must be integrated. Low-velocity rocks can be other than porous. Assuming porosity, where is it effective? At what position is the hydrocarbon trap along the updip edge of the effective porosity?

In addition to the Permian-Pennsylvanian cycles, the Niobrara chalks and the Cretaceous "D," "J," and Codell sandstones are potentially productive in the Nebraska panhandle.

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