Trace and rare earth elements in the Permian shales: geochemical paradigms

The present study investigates the geochemistry of the shale samples from the Barren Measures Formation of the Jharia Basin through major and trace element distributions as well as through bulk mineralogical attributes. The rare earth element distribution shows the dominance of light rare earth elements (LREEs) over heavy rare earth elements (HREEs). The chondrite-normalized REE pattern shows fractionation of LREEs but the almost flat distribution of the HREEs. These distributions, along with a negative europium anomaly (Eu/Eu* ranges from 0.58 to 0.66) and negligible cerium (Ce/Ce*) anomaly, may indicate the terrigenous source of these REEs. The SiO2 content (53.02 to 60.22 wt%), the ratio of Al2O3/TiO2 (21.09 to 26.80) as well as the ratio of the TiO2/Zr content (63.14 to 88.31) may indicate felsic to intermediate source rocks, which is supported by the La/Sc (2.86 to 4.11), La/Co (2.70 to 6.77), Th/Co (1.00 to 3.87) and Th/Cr (1.11 to 2.73) ratios along with the Eu anomaly. These shale samples consist of substantial amounts of kaolinite and muscovite, as reflected by K2O/Al2O3, SiO2/Al2O3 and index of compositional variation values. Such enrichment of kaolinite and muscovite in the samples would be the consequence of moderate to heavy precipitation under sub-humid to humid palaeoclimatic condition reflected by the C-value [(Fe + Mn + Cr + Ni + V + Co)/(Ca + Mg + Sr + Ba + K + Na)], Rb/Sr range (0.56–1.65) and large TiO2 content. This source rock weathering coupled with oxidation would have led to the loss of uranium and consequent enhancement of the Th/U ratio (2.99 to 6.59). The V, Cr and Ni concentrations further suggest sub-oxic and anoxic redox conditions. Besides, in the mineralogical aspects, the X-ray diffraction reflects that the quartz is the most dominant mineral present in the samples, followed by muscovite and kaolinite along with some chlorite, carbonate and feldspar. The large amounts of brittle quartz grains and low amounts of ductile clay minerals in the samples may suggest the brittle nature of these shales. Finally, from the ranges of the brittleness indices (BI1 = 48.88 to 71.99 and BIM = 49.24 to 68.77) the shales from the Barren Measures Formation of the Jharia Basin were found to be very brittle and, hence, are apposite for carrying out hydraulic fracturing for the shale gas exploration.