Insights from Rock-Eval analysis on the influence of sample weight on hydrocarbon generation from Lower Permian organic-matter rich rocks, West Bokaro basin, India

Abstract

Abstract Among the different Rock-Eval parameters, the hydrocarbons released under the S2 peak of Rock-Eval is of significance as it indicates the residual hydrocarbon content of the rock. Further, through its relationship with total organic carbon (TOC) content, it helps in calculating hydrogen indices (HI) which helps in understanding the type of organic matter present in a rock [HI= (S2/TOC)*100]. The present study documents the role of sample weight/amount used for analysis on the Rock-Eval S2 parameter for unconventional source-rock characterization. For the purpose of this study, a vitrain band sample type (manually isolated from a coal), a high-TOC shale sample type, and a carbonaceous shale sample type were analyzed at two different particle sizes (viz. 1\u202fmm-500 μm and 212-75 μm) and different sample weights (5–15\u202fmg for organic matter rich rocks and 30–60\u202fmg for shale) using a Rock-Eval basic cycle (heating rate at 25\u202f°C/min). Although S2 is reported as mg hydrocarbon (HC)/g rock, with increase in sample weight, an increase in hydrocarbons released under the S2 peak of Rock-Eval was observed for all three sample types for both the particle sizes. The observations were further validated using a Norwegian geochemical standard (JR-1). Further, all samples were reanalyzed by conducting pyrolysis experiments at a lower heating rate of 5\u202f°C/min. The impact of sample weight on S2 and HI was observed to be more pronounced for the JR-1 standard (higher hydrocarbon yield) than the Van Krevelen Types III-IV organic matter-bearing rocks. It thus calls for interpreters to be aware of the influence of mass of organic matter on hydrocarbon generation, and to monitor the maximum S2 values of organic matter-bearing rocks, within the flame ionization detector (FID) detection limits. Further, it is recommended that for Type III organic matter-bearing rocks with TOC content>20\u202fwt %, elemental analysis should be used to derive atomic H/C and O/C ratios for Van Krevelen diagram-based kerogen typing.