Molecular fossil chronicles and geochemical probes of Paleogene coals from Meghalaya, India

Abstract

The Paleogene coals from the Jaintia, Khasi, and the Garo hills of Meghalaya, India, are studied for biomarker distributions, carbon isotope signatures (δ13Cbulk), and Rock-Eval pyrolysis (REP) to elucidate their organic matter (OM) source, depositional environment, thermal maturity, and hydrocarbon generation potential. Sulfur-rich OM (1.52 wt%–24.57 wt% S dry-basis) with Hydrogen Index (HI) values ranging from 172 mg HC/g TOC – 353 mg HC/g TOC suggest the presence of type IIS-IIIS admixed kerogen. Mixed gymnosperm – angiospermous OM input to the peatland is implied by δ13Cbulk values spanning from −26.7‰ to −25.5‰, −27.4‰ to −25.1‰, and −27.0‰ to −25.2‰ in the Jaintia, Khasi, and Garo coals, respectively. Further, the presence of pimarane, isopimarane, labdane, rimuane, phyllocladanes, and ent-beyerane, mark the conifer OM input, substantiated by the 1,7-dimethylphenanthrene (1,7-DMP), 1,6-dimethylnaphthalene (1,6-DMN), and 1,2,5- trimethylnaphthalene (1,2,5-TMN) compounds. The abundances of these gymnosperm markers decline from the Garo to the Jaintia coals implying plausible shifts in paleovegetation. Compound A from the oleanoid skeleton and subordinate amounts of oleanane indicate variable contributions from angiosperms. The oleanane index declines from the Garo to Jaintia coals suggesting a lower contribution of angiospermous OM from Garo to the Jaintia samples, which may also indicate a shift in the OM depositional environment towards the Jaintia Hills. Dibenzothiophene isomers increase while dibenzofuran isomers decrease from the Garo to Jaintia coals suggesting shifts from terrestrial to marginal marine depositional environment. Additionally, the gammacerane index increases from the Garo to Jaintia coals implying a transition from a stratified brackish water column or marine transgression to the peatland of the Jaintia Hills. Moreover, the studied coal samples exhibit variable thermal maturity. The mean random vitrinite reflectance of the Garo coals (R¯r=0.48%–0.61%) indicates subbituminous to bituminous rank, while the Khasi (R¯r=0.44%–0.60%) and Jaintia coals (R¯r=0.26%–0.75%) belong to subbituminous to bituminous D, and lignite to bituminous C rank, respectively. Tmax of these coals ranges from 414 °C to 446 °C, indicating the samples span from pre-catagenetic to catagenetic stages of maturation. Saturated C32-bishomohopane epimer and C30 βα-moretane/αβ-hopane ratios, along with aromatic MPI-3 and 3-MP/retene ratios, exhibit an increasing maturity trend from the Garo Hills to the Jaintia Hills samples.