Organic Geochemistry of the Lower Permian Tide/Wave Influenced Carbonaceous Shale of Barakar Formation, Raniganj Basin, India

ABSTRACT The coal bearing lithostratigraphic units of the upper Barakar Formation of the Raniganj basin, India, were reinterpreted as mixed fluvio-tide/wave settings. The RockEval study from the shales in connection with the coal units of Ramnagar, Laikdih opencast-underground and Salanpur was performed to evaluate the source rock potential. The total organic carbon (TOC), S1, S2, Tmax, HI, and OI values indicate that the rocks are good to excellent source rocks. The majority of the organic materials are of Type III or IV, but some mixed sources are also evident. The palaeo-burial temperature calculated from the vitrinite reflectance gives a consistent result of 135°C. The hydrocarbons are indigenous, and the organic matter (OM) is mature enough to produce mainly gas with small potential for oil. But the palaeo-redox condition was fluctuating between oxic to anoxic. By SEM-EDX analysis, it was observed that kaolinite and illite/smectite are the primary clay minerals. Different types of porosities were developed; some of them are partially filled with secondary minerals. Hydrocarbon generation and escape have produced fracture porosity in these rocks. The escaping CH4-bearing fluid carried Ba, derived from dissolution of K-feldspars and/or organic matters, and moved to the interlamellar pores, where mixed with oxic sulfate-rich water to form barite. In some shales, anhydrites are also precipitated as a late diagenetic mineral. This oxic water was responsible for the destruction of some organic matter and reduced the hydrocarbon generating capacity of the rocks. The relative hydrocarbon potential ratio also gives low oxic values and high anoxic values, indicating the fluctuation of redox conditions in the system.