The Upper Pennsylvanian (Missourian) Drum Limestone, cropping out in Montgomery County, Kansas, is characterized by a thick body of cross-bedded oolite formed by filling a paleobathymetric depression. This oolite contains abundant, well-preserved, seemingly delicate fossils, which were protected from breakage and

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abrasion by thick algal coatings acquired before introduction into the high-energy oolitic environment. The fauna is robust (not dwarfed, as is common in oolites) and is taxonomically diverse. The ooliths formed in association with small bryozoan-algal banks on a shallow subtidal shelf. Mountainous regions to the south, and lowlands to the northeast and southeast, provided terrigenous sediments to the study area through deltaic processes before, during, and after Drum deposition.

Diagenetic alteration of the oolite has created much moldic and oomoldic porosity, but isolation of the molds has resulted in extremely low permeabilities. Wholesale leaching and cementation began (1) when deltaic distributaries eroded into the oolite soon after its deposition, allowing fresh water into the pore system of the oolite; and (2) perhaps when the oolite shoaled and formed islands which may have served as conduits for fresh rainwater. Similar rocks with higher permeabilities, resulting from less cementation or late fracturing, would be well-suited for hydrocarbon accumulation.

Because many modern and ancient oolites have thickened by upward shoaling and accentuation of preexisting topography, potential oolitic petroleum reservoirs in the subsurface have been sought on paleobathymetric highs. Oolitic accumulations in paleobathymetric lows should not be neglected in the search for petroleum.

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