Source potential and reservoir characterization of the Cambay Shale, Cambay Basin, India: Implications for tight gas and tight oil resource development

Abstract

New organic–geochemical investigations of the Eocene Cambay Shale (Cambay Basin, India) from five wells and an open-cast lignite mine reveal that the total organic carbon ranges from 0.37 to 10.68 wt. %, with an average of 2.43 wt. %. The pseudo–Van Krevelen diagram, hydrogen index versus the maximum pyrolysis yield temperature (Tmax) crossplot, and the visual kerogen assessment of the Cambay Shale indicate the dominance of type III kerogen, with some well samples showing mixed type III and II kerogen. The vitrinite reflectance values range between 0.46% and 0.7%, with Tmax values ranging from 387°C to 441°C and are consistent with an immature to early oil generation stage. The dominance of vitrinite macerals and high pristane–phytane (Pr/Ph) ratios (>6) of the well samples indicate an oxic to dysoxic depositional environment, whereas the presence of Botryococcus braunii and low Pr/Ph ratios (<1) in the lignite mine samples suggesting a more reducing brackish water environment. The estimation of hydrocarbons generation, expulsion, and retention data suggest a low retention of 10%–12% of generated hydrocarbons within the Cambay Shale. The mineralogical data show an abundance of clay minerals (average 62.9%), implying poor to moderate mineral brittleness index. The kerogen type and maturity level along with the high clay content and poor to moderate brittleness indicate low fracability, thereby restricting the production potential of the Cambay Shale for tight gas or tight oil exploration.