Willard J. Guy¹

(1) Kansas Geological Survey, Lawrence, KS

ABSTRACT: The Recognition and Characterization of the Morrow Sandstone Reservoirs as a Low Resistivity/Low Contrast Pay

In 1990, Cliff Nolte published a paper in the RMAG publication Morrow Sandstones of Southeast Colorado and Adjacent Areas entitled "Anomalous Mechanical Well Log Resistivities in the Middle Morrow Sandstones of Southeast Colorado". Nolte documented the problems that arise in the evaluation of Morrow sandstones with both high and low resistivities due to varying water salinities, interstitial clays, pyrite, cementation, and/or residual oil saturation.

Much has been learned over the past ten years regarding the wireline analyses of the Morrow sandstone reservoirs. It is not enough to just calculate the water saturation and porosity. Additional data is required such as the bulk volume water, the resistivity-porosity patterns, the reservoir lithology, the spontaneous potential and a broad geological and engineering knowledge of the Morrow reservoirs..

The variability of the resistivity values in sandstones is usually the result of varying amounts of clays, which can typically hold a significant amount of bound water. This bound water can effectively reduce the resistivity of the Morrow reservoir. This reduction in resistivity results in an increase in the calculated water saturation and the bulk volume water. A log-log crossplot of resistivity-porosity (Pickett, 1973) is used for analysis rather than a shaly sand technique, which requires additional unknowns such as the resistivity of the interstitial shale or clay. The plot is a graphic portrayal of the Archie equations with a very significant added feature being the pattern of the crossplots. This added feature allows for the approximate determination of the irrreducible water saturation, the transition zone thickness, the pore geometry, and the effective porosity.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado