Petrographic characteristics and methane sorption dynamics of coal and shaly-coal samples from Ib Valley Basin, Odisha, India

• Primary role of organic matter in methane sorption
• Relation between TOC-normalized Langmuir volume and ash yield
• High ↑Vt60 indicating thermogenic gas generation
• Disparity between Tmax and vitrinite reflectance

Hydrocarbon generation potential, methane sorption capacity, maturity and petrographic composition of ten borehole (2 shaly-coals and 8 coals) samples from Ib Valley, India were determined. Rock eval pyrolysis and total organic carbon (TOC) analysis indicate that all the samples have excellent TOC content and hydrocarbon generation potential. Though Tmax values indicate the samples are mostly immature in nature, vitrinite reflectance analysis shows that the samples are within the vitrinite reflectance oil window. Moreover ↑Vt60 (vitrinite grains having reflectance greater than 0.60%), indicates that thermogenic gas generation is occurring to some degree in almost all sample intervals. TOC content of the samples (25.35–58.15 wt.%) shows good positive relation with Langmuir volume (VL; 0.481–0.551 mmol/g rock) and methane sorption capacity (MSC; 0.317–0.379 mmol/g rock), indicating that organic matter abundance is the significant factor in gas sorption capacity of these samples. While Fourier Transform Infrared Spectrometry (FTIR) spectrum of coal samples shows the presence of aromatic H, the same was not observed in the case of shaly-coal samples which may be attributed to their lesser organic matter and greater content of clay.