Methane sorption dynamics and hydrocarbon generation of shale samples from West Bokaro and Raniganj basins, India

Abstract

For the purpose of assessing methane sorption capacity, degree of conversion of organic matter and hydrocarbon prospectivity, twenty one borehole shale samples of the Lower Permian Barakar Formation were studied from the West Bokaro and Raniganj basins in India. Positive correlation between total organic carbon (TOC) content and sorption amount [methane sorption capacity (MSC) and Langmuir volume (VL)] was observed and indicates that organic matter abundance is a significant factor in controlling methane sorption in the studied shale samples. TOC-normalized MSC and VL shows polynomial relationship with ash yield and clay and hence point towards the importance of mineral matter on methane sorption capacity. Shales from the West Bokaro and Raniganj basin are marked by 'good' to 'excellent' TOC content, input of type III- II/III admixed organic matter and are capable of generating oil and thermogenic gas upon thermal cracking. Tmax (thermal maturity) value of shales from West Bokaro basin varies between 444 and 454 °C, while those from Raniganj basin vary between 438 and 464 °C. The sample showing highest MSC, VL and Tmax value, had a forty centimeter thick lamprophyre intrusion occurring immediately above it which might have resulted in increasing its maturity (Tmax value), cooking and thus increasing its MSC and VL. This sample also shows maximum fraction of conversion (f = 0.70; i.e. 70% of the hydrocarbon generation process has been completed).