Petrographic insights of organic matter conversion of Raniganj basin shales, India

• Fraction of conversion depends upon kerogen type and stage of maturity.
• Organic matter typifying based on HI values alone may be deceptive.
• Importance of organic matter in methane sorption
• Secondary role of clay minerals in methane sorption capacity
• Partial removal of aliphatics and disordering of kaolinite due to igneous intrusion

Shale gas has strikingly altered the international energy landscape and consequently research on shale gas systems has increased many folds in recent years. For the purpose of assessing hydrocarbon generation potential, methane sorption capacity and degree of conversion of organic matter, thirty nine borehole shale samples from different parts of Raniganj basin, India were studied. All the samples of the Barakar and Barren Measures Formations have excellent (> 4 wt.%) total organic carbon (TOC) content, while the Raniganj Formation shales show good to excellent TOC (3.13–29.74 wt.%) content. Relationship between hydrogen index (HI) and Tmax values of rock eval pyrolysis indicates that depending upon kerogen type, conversion of kerogen to hydrocarbons takes place at different maturities and typifying kerogen type based on HI values alone can be misleading. Petrographically calculated fraction of conversion (f) of kerogen also substantiates the above observation. Good positive correlations were observed between TOC and sorption amount [methane sorption capacity (MSC) and Langmuir volume (VL)] and hence indicate that organic matter abundance is a significant factor in controlling methane sorption for the studied shale samples. However, the results also indicate the role of mineral matter on methane sorption capacity of the samples. Fourier Transform Infrared (FTIR) analysis indicates the presence of kaolinite and quartz within the studied samples. The sample showing the maximum MSC, VL, Tmax, mean vitrinite reflectance (Romv), f and fixed carbon (FC) content, had a forty centimeter thick lamprophyre intrusion occurring immediately above it which might have resulted in cooking and maturing the sample. The FTIR spectra of this sample also show the partial removal of aliphatics and disordering of kaolinite due to the impact of heat.

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