Simplified petrophysical reconnaissance tool for evaluation of unconventional shales

Abstract

Petrophysical evaluation of unconventional shale reservoirs is complex due to the formation's heterogeneity, requiring advanced methods to estimate parameters such as effective porosity (${\varnothing }_e$), effective water saturation ($S_{we}$), and permeability ($k$). While various techniques are available to characterize these reservoirs, this study presents a simplified workflow for reconnaissance evaluations aimed at identifying sweet spots. The proposed method utilizes standard resistivity and porosity logs (e.g., triple or quad combo), making it accessible and reproducible for use in commercial software or spreadsheets, even by non-experts. By integrating modified Archie's equation, a log-based total organic carbon (TOC) estimate, and Lewis's cutoffs into a single composite curve, this approach streamlines the process of sweet spot identification. The combined methods were tested on the Wolfcamp Shale in the Permian Basin, as well as the Woodford and Bakken shales, consistently identifying similar sweet spot intervals. The results demonstrated a strong alignment between the intervals flagged by Archie's method and those identified using Lewis's cutoffs, with the upper section of Wolfcamp B meeting sweet spot criteria, while the lower section showed less consistency. Similar patterns were observed in the Woodford and Bakken shales, where primary sweet spots were determined based on consistent cutoff satisfaction. The methodology was further validated by comparing the results with landing depths from DrillingInfo reports, confirming its effectiveness across different formations. Thus, this cost-effective method presented in this study reduces the reliance on expensive tools like nuclear magnetic resonance (NMR) or core analysis for sweet spot delineation, making it especially valuable for operators with budget constraints. Furthermore, the methodology provides an accessible approach for professionals of varying expertise levels, facilitating more informed decisions regarding lateral landing depths and net interval definitions in unconventional reservoirs.