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ABSTRACT: Aux Vases Sandstone: Zeigler Field, Frank County, Illinois

Berverly Seyler

The Aux Vases Sandstone (Middle Mississippian) in the Illinois basin is a prolific but problematic oil reservoir. Problems in exploration, production, and development of Aux Vases reservoirs are primarily due to reservoir heterogeneities of both depositional and diagenetic origin.

Examination of five closely spaced cores at Zeigler oil field in Franklin County, Illinois, demonstrates the Aux Vases is composed of subangular to rounded, very fine to medium-grained sandstone. High-angle crossbeds form the primary sedimentary feature observed in reservoir sandstones. Conventional core analysis porosity measurements range from 20 to 27% and permeability measurements range from 10 to 700 md. X-ray diffraction, scanning electron microscopy with energy dispersive analysis, and thin section analyses show authigenic chlorite and illire clays coat sand grains and line pore throats.

Presence of clay minerals and their distribution on a microscopic scale is interpreted to contribute significantly to reservoir problems commonly experienced in the Aux Vases. Clay-related problems may be: (1) low electric log resistivity, which results in erroneous high water saturation calculations for actual water-free oil wells; (2) geochemical reactions with treatment and injection fluids; and (3) potential for compartmentalized reservoirs owing to distribution of clays.

Examination of the depositional and diagenetic factors related to clay minerals and porosity development in the Aux Vases at Zeigler field may allow comparison to other similar Aux Vases reservoirs in the Illinois basin and may suggest tailored methods of exploration, drilling, treatment, and development to optimize hydrocarbon production.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990