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

Among the different Rock-Eval parameters, the hydrocarbons released under the S-2 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= (S-2/TOC)*100]. The present study documents the role of sample weight/amount used for analysis on the Rock-Eval S-2 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 mm-500 mu m and 212-75 mu m) and different sample weights (5-15 mg for organic matter rich rocks and 30-60 mg for shale) using a Rock-Eval basic cycle (heating rate at 25 degrees C/min). Although S-2 is reported as mg hydrocarbon (HC)/g rock, with increase in sample weight, an increase in hydrocarbons released under the S-2 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 degrees C/min. The impact of sample weight on S-2 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 S-2 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 wt %, elemental analysis should be used to derive atomic H/C and O/C ratios for Van Krevelen diagram-based kerogen typing.