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The present study aims to illustrate the paradigms of saturated and aromatic biomarker distributions in the lithotypes of lower gondwana bituminous coals (r̄r = 0.50–0.68%), perhaps for the first time. The coal samples were collected from the early permian barakar formation of the rajmahal basin, india. These coals mostly comprise three lithotypes, i.e., Vitrain, durain, and fusain. The durain shows a considerable abundance than the other two bands and represents the bulk coal characteristics. The total organic carbon (toc), s1, and s2 parameters from the rock-eval pyrolysis depict insignificant variations among the lithotypes, unlike the hydrogen index (hi), which peaks in the durain samples. The micropetrography furnishes a significant amount of liptinite macerals (30.6–51.3 vol% on the mineral matter free basis) present in the samples, dominated by the sporinite and cutinite. These hydrogen-rich macerals, being one of the crucial micropetrographic constituents, would have enhanced the hi values of the durain lithotype. The waxy nature of durain is also substantiated by the long chain-length n-alkane (lclas) distributions, while the plausible scarcity of wax in the lignocellulosic plant materials would have led to lower abundances of the lclas in vitrain (detected only from the 2d-gas chromatography coupled with time-of-flight mass spectrometry (gc × gc-tofms)). Further, although the saturated diterpenoids are abundant in the durain and vitrain, their extremely low abundances (identified only through the gc × gc-tofms) along with the degraded lcla signature in fusain may hint toward its source from the biomass combustion. Besides, the distributions and interrelations of the 4-, 5-, and 6-ringed polycyclic aromatic hydrocarbons (pahs; m/z 202, 206, 228, 252, 276, and 278), like fluoranthene, pyrene, chrysene, benzo[g,h,i]perylene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[e]pyrene, and the isomers of the dimethylphenanthrene, dibenzanthracene, and benzochrysene further support the pyrogenic origin of the fusain lithotype. Meanwhile, these pahs would have derived from diagenetic/catagenetic origin to mixed sources (diagenetic/catagenetic maturation of organic matter and combustion origin) in vitrain. The durain shows a mixed to pyrogenic origin of these pahs and represents the pah distributions of the bulk coal samples. Besides, the thermal maturity assessment of these lithotypes may suggest that the molecular parameters respond more sensitively to the thermal maturation compared to the bulk organic geochemical proxies. © 2022 elsevier b.v. |
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