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Abstract: The Effects of Authigenic Clay Coating on Saturation Exponents in the Mississippian Basal Chester Sandstones in Western Kansas

ASQUITH, G. B., Dept. of Geosciences and the Center for Applied Petrophysical Studies, TEXAS TECH UNIVERSITY

A Mississippian well drilled in western Kansas encountered two basal Chester sandstones. Both of these sands appear from the SP, gamma ray and neutron-density logs to have very low clay content. Using the Archie Equation (m=n=2) the water saturations of the upper sand (Rt 20-40 ohm-m) ranged from 25-29% with porosities from 12-14%. The water saturations of the lower sand (Rt 2.5-3 ohm-m) ranged from 58-62% with porosities from 14-16%. Thus indicating that the lower sand should produce predominantly water. However, dielectric (EPT) water saturations were similar (Sxo=43-46%) in both sands. When both these sands were perforated the well IPP for 157 BOPD. SEM analysis revealed that both sands contained well rounded quartz grains. However, in the lower sand the grains have a thin coating of illite-smectite mix-layer clay. In the upper sand the grains have no clay coatings.

Work of Diederix (1982) indicates that sands with clay coatings have a lower saturation exponent (n) when compared to sands without clay coatings. Diederix (1982) believes that brine trapped in the clay micropores ensures better conduction at low water saturations (i.e. lower n) than in clean sands with no clay micropores (i.e. higher n). Saturation exponents based on dielectric water saturations are 2.48 (upper sand) and 1.42 (lower sand). Archie water saturations based on these saturation exponents ranged from 27.1-35.4% (upper sand) and 28.9-41.3% (lower sand). Thus indicating that both sands have similar water saturations.

AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas