Influence of Mineralogy, Organic Matter and Thermal Maturity on the Diagenetic Evolution of Pore Systems in the Shales of Cambay and Raniganj Basin, India

Abstract

The mineral composition, organic matter, thermal maturity, and diagenesis control the shale reservoir properties, including the pore distribution, variable fluid distribution, microstructure, heterogeneity, etc. The shale layers of the Cambay Formation and the Raniganj Formation are considered prospective shale gas resources in India. This paper focuses on the detailed analysis of shale from the Cambay Formation (Eocene age) of the Cambay Basin and the Raniganj Formation (Permian age) of the Raniganj Basin, India. The whole rock X-ray diffraction data indicates that quartz is the dominant mineral in both the shales (Raniganj and Cambay shale) of the Raniganj and the Cambay Basin. The main clay minerals are clinochlore, smectite-kaolinite, illite, and kaolinite in the Raniganj shale, while kaolinite, montmorillonite, and illitesmectite are the dominant clay minerals in the Cambay shale. The total organic content (TOC) ranges from 4.97 to 17.88 wt.% and 1.16-7.6% in the Raniganj and Cambay shales respectively. For the studied shale section, the TOC shows a gradual decrease with respect to depth in both shale types. The Raniganj shale has 3.8 to 8.3 percent total porosity, and the Cambay shale contains 1.16 to 6.5% total porosity. The increasing thermal maturity with the higher TOC corresponds to the more developed organic pores in the Raniganj shale. In contrast, the secondary pores are developing in the Cambay shale due to dissolution activities and enhancing the storage capacity. The study infers that the mineral composition and diagenesis are playing a crucial role in the pore system evolution of the shale in both the basins.