Abstract

Garfield field, an old (1948) stratigraphic trap in central Kansas, produced 10 million bbl of oil from Cherokee (Desmoinesian, Pennsylvanian) conglomerate reservoirs and from Mississippian carbonate and sandstone reservoirs. The primary reservoir rock at Garfield conglomerate pool is the Pennsylvanian basal conglomerate (PBC), unique in that it accumulated as an unstratified alluvial-fan deposit, in a subtle structural depression that may be coincident with a microplate boundary. Each of these conditions is uncommon in the northern Mid-continent.

The PBC at Garfield pool is composed of pebbles, cobbles, and boulders of reworked Mississippian chert and limestone in a matrix of porous, fine-grained sandstone and sandy shale. Microporosity in the chert cobbles has access to the borehole through the porous sandstone matrix. The sandstone matrix was the product of reworking of the original shaly sand matrix and older sandstones from the substrate during short-term Desmoinesian marine transgressions of the alluvial-fan surface. Ultimately, Marmaton transgressive onlap smoothed and buried the fan surface.

A near-modern alluvial fan at Rocky Flats in central Colorado provides an ideal analog for the PBC deposit at Garfield pool. The Rocky Flats fan is nearly identical in scale, areal distribution, and thickness, and its constituents are remarkably similar to conglomerate zones, which produce hydrocarbons at Garfield pool. Future exploration in north-central Kansas and southern Nebraska should focus on the occurrence of conglomerate deposits in similar depositional settings where coincident with structural depressions.

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