Abstract: Log Response of Authigenic Illite in a Dolomite Reservoir, Upper Copper Ridge Dolomite, Richland County, Ohio

Eric Daugstrup, Annabelle Foos

Petrophysical properties of the Cambrian upper Copper Ridge Dolomite, Richland County, Ohio were investigated by thin section petrography and log analysis. This unit is composed of nonplanar dolomite with planar-e and planar-s dolomite void fills. Intercrystalline and vuggy porosity are the most abundant porosity types. As the percentage of planar-e and planar-s dolomite increases there is a corresponding increase in porosity. Authigenic illite and baroque dolomite are the main porosity destroyers.

Authigenic illite was observed infilling up to 95% of the pores. The illite has a honeycomb and fibrous morphology and occurs as pore fillings and pore bridges. Its presence results in a large loss of effective porosity. The presence of illite caused the neutron log porosity and water saturation values to be anomalously high. Illite did not significantly effect the bulk density porosity determination. There is a positive correlation between percent illite in pores and the ratio neutron porosity/bulk density porosity.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994