Spectral Gamma Ray Log-Based Shale Volume Estimation of a Gas Well, Bengal Basin

Abstract

Background: Formation evaluation plays a pivotal role in identifying lithology and its depth of occurrence in gas fields. This study focuses on the Bengal Basin, aiming to achieve lithology identification and shale volume estimation in a gas well. Method: The gamma ray (GR) log is utilized to measure natural radioactivity, with spectral gamma ray (SGR) employed to capture concentrations of potassium, uranium, and thorium in clastic sedimentary formations. Lithology identification is conducted using resistivity, SGR, and GR logs, while shale volume estimation utilizes standard models including GR and true resistivity approaches. Results: The lithology of the studied well predominantly comprises clastic sedimentary rocks, specifically sand and shale. The reservoir rock type is primarily sandstone, with sand being the dominant fraction and shales appearing laminated. Concentrations of thorium, uranium, potassium, and gamma ray are approximately 12.46 ppm, 2.28 ppm, 1.73%, and 100 API, respectively. The shale volume of the gas reservoir ranges from 12% to 29%. Conclusion: The estimated shale volume provides valuable insights for assessing effective porosity and hydrocarbon saturation in shaly sand reservoirs, facilitating gas resource estimation and reservoir characterization in the sedimentary basin. This study underscores the significance of comprehensive formation evaluation techniques in optimizing reservoir management and resource extraction strategies.