Use of Pickett Plots for Evaluation of Shale Gas Formations

Abstract

AbstractA practical method, based on the pattern recognition approach used in Pickett plots, is presented for preliminary yet accurate quantitative evaluation of shale gas formations. The method is inspired by a quick-evaluation, time-tested methodology developed by Passey et al. (1990) for shales, which utilizes primarily sonic and resistivity logs. In Passey et al. method the sonic and resistivity logs are overlain in such a way that the curves track each other in fine-grained non-source rocks. Separation of the curves indicates the presence of organic-rich intervals.The Pickett method presented in this study reproduces data published by Passey et al. with coefficients of determination (R2) greater than 0.99 for cases related to sandstones, limestones, dolomites and shales. As Pickett plots have been used thousands of times in the past for evaluation of the first 3 mentioned lithologies, this paper concentrates primarily on the evaluation of shale gas reservoirs. The advantage of the proposed Pickett plot for shale gas formations is that it allows quick estimates of water saturation, total organic carbon, and under favorable conditions, estimates of fracture intensity and diffusion. The objective of the proposed approximate approach, however, is not to replace detailed petrophysical and thermal maturity studies but to provide quick and accurate evaluations of shale gas formations.It is concluded that Pickett plots provide a powerful practical tool for quick evaluation of shale gas formations. Examples of applications and comparisons with previously published interpretations are presented in detail.