Gamma-ray log shape used as a facies indicator: critical analysis of an oversimplified methodology

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Abstract
Gamma-ray log shapes are often used by geologists to determine sandstone grain size trends and hence depositional facies. However, for the simple methodology of relating shapes to facies to be valid, there must be consistent relationships between gamma-ray log values and clay content and between clay content and grain size. Log to core comparisons and sandstone textural analyses show that these relationships are naturally variable. Consequently, correlation of log shape with grain size trend is tenable only under very limited (and defineable) conditions. A universal application of gamma-ray log shape to grain size trend and depositional facies is wrong.