This article is concerned with the identification of hydrocarbon zones in unconventional plays through lithofacies classification using wireline logs. Identification of hydrocarbon zones in unconventional formations is often different than in conventional formations. For example, in conventional reservoirs, the fractional volume of clay (Vclay) is generally correlated strongly and positively with gamma ray (GR); high GR values commonly indicate low hydrocarbon potential. In unconventional reservoirs, however, Vclay is often correlated weakly with GR, and sometime even negatively. There are several possible explanations for this change of correlation. In source-rock reservoirs, high GR often indicates high total organic carbon (TOC), kerogen, and gas or oil concentrations. In some tight-gas sandstone reservoirs, hydrocarbon-bearing sandstones have abnormally high GR.
The change of the correlation between commonly used wireline logs in conventional formations to a different correlation between those same logs in unconventional formations is, in fact, a manifestation of the Simpson's paradox—a counterintuitive phenomenon in probability and statistical data analysis. Here we show how to classify lithofacies and identify high-potential hydrocarbon zones in unconventional resources by discerning the Simpson's paradox when analyzing relationships between wireline logs. We present examples from a tight-gas sandstone reservoir and a shale-gas reservoir.