Diagenetic Evolution and Petrophysical Characteristics of Oomoldic Facies in United States and Middle East Reservoirs

Roy Nurmi, Daniel Neuberger

The diagenetic evolution of oomoldic pore rocks from United States and Middle East reservoirs were studied to determine their variation. The oomolds in all of the reservoir samples appear to have formed prior to any compaction, and were also subject to early cementation, which preserved the delicate structure of these rocks. The most common oomold-filling mineral is calcite, regardless of whether the remainder of the rock is dolostone or limestone. Anhydrite is commonly the mineral filling oomolds in formations depositionally associated with evaporites. Partial cementation of individual oomolds is rare. However, partial filling of oomoldic wackestones by lime mud was observed.

The petrophysical characteristics of oomoldic rocks in different stages of diagenetic evolution were analyzed using thin sections, pore casts, porosity and permeability measurements, and saturation. The rocks used in the study included core samples from the Lansing-Kansas City Group, San Andres, Smackover, Arab, and Khuff formations. Selected samples from the Lansing-Kansas City Groups were also measured for formation factor, density, and for dielectric and acoustic properties. The study quantified the relationship of pore type and volume with both formation factor and permeability of oomoldic rocks. The framework mineralogy (calcite or dolomite) and microstructure, and the presence and nature of interparticle pore-filling cements are critical factors in assessing the reservoir potent al of an oomoldic reservoir facies. Furthermore, the geologic analysis of the pore system can greatly improve the determination of the Archie "m" exponent, which provides more reasonable evaluations of the saturation and permeability of these complex rocks.

AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.