Refined sedimentologic concepts and favorable economics accord further exploration for subtle hydrocarbon traps in mature areas such as Kansas. Upper Pennsylvanian sediments of the Lansing and Kansas City Groups in central and western Kansas represent a succession of cyclic reservoir carbonates and sealing shales deposited on a shallow cratonic shelf. A subsurface study conducted by the senior author demonstrates using cores that subaerial exposure occurred in many cycles during late regression across much of the shelf. This resulted in the flushing of the regressive carbonates by fresh or undersaturated water and the formation or redistribution of porosity in these rocks.

Primary and diagenetic porosity that are observed in cores and keyed to well logs commonly are associated with paleohighs or breaks in slope across the shelf. Selected maps of structure, thickness, and porosity from logs integrated with diagenetic and depositional facies from cores together provide necessary criteria to define subtle reservoir trends with the regressive carbonate.

A study of a shallow gas reservoir in the Nolans Limestone in central Kansas found that porosity is best developed in a transgressive dolomitic mudstone to moldic bivalve packstone. During later regressive sedimentation of the Nolans cycle hypersaline waters apparently percolated through the sediment and precipitated anhydrite and dolomitized the carbonate mud particularly in the upper Nolans.

The gas reservoir is structurally high with flanking water production and exhibits improved porosity development over the structural crest. Gamma ray--neutron crossplots and facies-ratio mapping aid in recognition of this subtle carbonate facies of the lower Nolans.

Characterizing carbonate facies and early diagenesis of these reservoirs in the framework of cyclic sedimentation is critical to minimizing risks in locating drill sites. Similar concepts as described here can be applied to other Middle and Upper Pennsylvanian and Lower Permian cyclic sequences in Kansas.

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