Abstract

The Upper Pennsylvanian (Missourian) Drum Limestone in Montgomery County, Kansas, is characterized by an oolite bed that thickens into a paleobathymetric low. The ooids formed on a shallow subtidal shelf in association with bryozoan-algal banks. The oolite contains abundant, well-preserved, seemingly delicate fossils, which were protected from breakage and abrasion by thick algal coatings acquired before introduction into the high-energy oolitic environment. The diversity and lack of dwarfism in the fauna suggest the organisms did not live in the principal ooid-forming environment. Instead, they were probably mixed with the ooids during storms.

Diagensis created moldic and oomoldic porosity, but isolated the molds, resulting in low permeabilities. Wholesale leaching and cementation began when deltaic distributaries eroded into the oolite soon after its deposition. These rocks would have been well-suited to accommodate hydrocarbons had they escaped freshwater invasion, or had fracturing connected the secondary pores formed during freshwater diagenesis.

Because many modern and ancient thick oolites have formed by upward shoaling and accentuation of preexisting topography, potential oolitic petroleum reservoirs have been sought on paleobathymetric highs. Oolite deposits in paleobathymetric lows may also be significant and should not be neglected in the search for petroleum.