Depositional environment and geomechanical properties of Cambay Shale: potential reservoir for shale oil and gas in India

Abstract

This study aims to investigate mineralogy, depositional environment and rock mechanical properties of Cambay Shale Formation in Jambusar-Broach block of South Cambay Basin, India, by an integrated approach of combining wireline logs and laboratory studies. The study involves the analysis of spectral gamma ray (SGR) logs and laboratory studies of core samples using X-ray diffraction (XRD), thin-section analysis and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS). The results show that in the Cambay Shale, mineral components are dominated by clay minerals, quartz and siderite with minor amounts of feldspar, mica and pyrite. Both thorium (Th)–potassium (K) cross-plots derived from SGR logs and XRD analysis have revealed that kaolinite and chlorite are the dominant clay minerals. The dominant clay mineralogy and the analysis of thorium to uranium ratio plots indicate the deposition of the shale in marginal marine to marine environment. Pyrite framboid size distribution, used as a redox proxy, indicates deposition under anoxic conditions. The geomechanical property of Cambay Shale has been analysed based on the overall mineral composition of the core samples. Mineralogy-based brittleness index (BI) has been evaluated and found to vary between 0.21 and 0.66, suggesting low to moderately high brittleness. Kaolinite and chlorite, the dominant clay minerals, are the less swelling clays and exhibit a lower plasticity. It may be concluded that these factors might facilitate the intrinsic fracking properties of Cambay Shale.