Log Interpretation of Shaly Sandstones: a General Model

Joel Foster Baker

The determination of water saturation from electrical resistivity measurements to evaluate the potential of reservoirs is a fundamental tool of the oil industry. Shaly sandstones are difficult to evaluate because clays are conductive and they lower the resistivity of the rock. A review of shaly-sandstone research concerning "volume-of-shale" equations reveals three theoretical categories: (1) laminated clay equations, (2) dispersed clay equations, and (3) equations that assume that the effect of the clays on the conductivity measurement is directly related to water saturation.

A new model for predicting the relative amounts of laminated and dispersed shales and accounting for their effects according to their abundance can be used for any sandstone, clean or shaly.

Equations representing each of the three theoretical categories and the new equation were tested on cored Wilcox sandstones from two wells. Cores were analyzed to determine the volume and distribution of clays and to correlate porosity with the well logs.

The sandstones from one well contained dominantly dispersed clays with gamma-ray readings from 50 to 90 API units, measured water saturations from 60 to 90%, and resistivities from 2.0 to 3.6 ohm/m. The sandstones from the other well contained dominantly laminated clays with gamma-ray readings from 13 to 15 API units, measured water saturations from 35 to 85%, and resistivities from 2.4 to 6.1 ohm/m. The new equation yielded the most consistently accurate results, especially in the more permeable zones, often predicting water saturations within about 5% of measured values.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.