Low Resistivity in Aux Vases Sandstone Reservoirs in the Illinois Basin Linked to Clay Minerals Coating Sand Grains

SEYLER, BEVERLY, Illinois State Geological Survey, Champaign, IL

The Mississippian-age Aux Vases Sandstone, one of the most prolific producers in the Illinois Basin at approximately one half billion barrels of oil, is a problematic reservoir because low resistivities (2-6 ohm-meters) indicate salt water saturation rather than oil saturation in oil productive zones. The cross-bedded reservoir sandstone facies is friable, clean and fine to very fine grained. Porosity ranges between 20 percent and 30 percent, and permeability ranges between 50md and 1000md, but is typically greater than 200md. Petrographic analysis shows most samples are subarkoses with a feldspar content between 5 and 7 percent.
Petrographic analysis has established that erroneously high water saturation calculations are due to diagenetic clay minerals coating virtually every grain and lining all pores. X-ray diffraction and SEM/EDX analyses have shown that diagenetic clay minerals in the reservoir facies are composed of a closely intergrown mixture of mixed-layered illite/smectite, aluminum-rich chlorite, and illite. Irreducible water bound in these clay minerals greatly diminishes the resistivity response, leading to very high water saturation calculations in zones that produce large amounts of oil and negligible amounts of water. This has led to difficulty in estimating reserves and determining producible zones in Aux Vases Sandstone reservoirs. Formation damage due to interaction of clay minerals with fluids introduced during drilling and production is also a common problem. Solutions to these problems include use of fluids compatible with clay minerals and use of oil saturation calculated independently of the standard Archie Equation.

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