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Keywords:

  • Coal;
  • carbargillite;
  • mudstone;
  • Barapukuria Basin;
  • Dighipara Basin;
  • source rocks;
  • Bengal Basin;
  • Bangladesh;
  • Permian;
  • Gondwana Group

In the Barapukuria and Dighipara coal basins, NW Bangladesh, the Basement Complex is overlain by the coal-bearing Permian Gondwana Group. In the present study, 36 core samples collected from five boreholes in these two basins were analysed using organic geochemical and organic petrological methods. Based on the results of biomarker analyses (TIC, m/z 191 and m/z 217 fragmentograms) and maceral composition (proportions of vitrinite, liptinite, inertinite), three organic facies were identified: coals, carbargillites and mudstones. Together with other evidence, cross-plots of HI versus Tmax and Pr/nC17 versus Ph/nC18 indicate that the coals, as expected, were dominated by terrestrial organic matter (OM). The carbargillites contained a mixture of terrestrial and probable Type II aquatic OM, and the mudstones contained mostly terrestrial OM. Accordingly the coals, carbargillites and mudstones are interpreted to have been deposited in swamp-dominated environments in a delta-plain setting which was subject, in the case of carbargillites, to periodic flooding. Suboxic conditions were indicated by very high Pr/Ph ratios and a high content of inertinite macerals.

All the samples analysed were immature or early mature for hydrocarbon generation, as indicated by mean vitrinite reflectance (%Ro) of 0.60–0.81%, Rock-Eval Tmax of 430–439°C, and biomarker ratios (hopane C32 22S/(22S+22R)) of 0.57–0.60. Carbargillites showed potential for both liquid and gaseous hydrocarbon generation; coals were mainly gas-prone with minor liquid hydrocarbon potential; and mudstones were dominantly gas-prone. The oil-prone nature of the samples was attributed to the presence of resinite, cutinite, bituminite and fluorescent vitrinite. The presence of exsudatinite within crack networks, solid bitumen and oil droplets as well as bituminite at early oil-window maturities suggests that the organic matter may have expelled some hydrocarbons.