Origin of organic matter and hydrocarbon potential of Permian Gondwana coaly shales intercalated in coals/sands of the Barapukuria basin, Bangladesh

Abstract

This paper presents the characteristics of source organic matter (OM), hydrocarbon potential (HC) and the depositional system of Gondwana coaly shales in the Carboniferous-Permian succession in drill hole GDH-40, Barapukuria basin, Bangladesh. Twelve layers intercalated in coals and sands were examined using Hydrogen Index (HI) of Rock Eval analysis, biomarkers in saturated HC fractions, polycyclic aromatic hydrocarbons (PAHs), and stable carbon (δ13Corg) and nitrogen (δ15Ntot) isotope ratios. The coaly shales showed a wide range of total organic carbon (TOC) content (2.3–30.3 wt%), intermediate to high TOC/TN ratios (14–39) and low total sulfur (TS) contents (<0.3 wt%), suggesting deposition in a freshwater environment occasionally influenced by brackish water. OMs with low to moderate HI (20–204 mg HC/g TOC) were deposited in a shallow terrestrial basin. The HC potentials are high in the middle and lower parts of the succession, with increasing TOC content dominated by terrestrial plants. The higher total HC generation potential up to 60.1 mg HC/g is attributed to coaction of high TOC and HI values. The HI values show good positive correlations with Pr/Ph ratios (r = 0.90), n-C>25/n-Ctotal alkanes (r = 0.86), phenanthrene (r = 0.75), methylphenanthrene (MP, r = 0.75), Tm/Ts (r = 0.74), sterane C29/C(27+28+29) (r = 0.69), Pr/n-C17 ratio (r = 0.69) and TS (r = 0.68). The coaly shales also contain abundant retene. Higher δ15N ratios coupled with high HI values suggest that the climate had a dry season. Very small amounts of oleanane and low ratios of oleanane/C30-hopane show negative correlations with HI values (r = −0.68). A significant input of algae and/or planktonic organisms occurred in the upper part of the core, estimated based on regular sterane compositions, but this did not increase the HI values. Relatively abundant hopanes (steranes/hopanes = 0.11–0.40) implied greater contribution of methanogen in the upper part of the succession and increased activity of aerobic bacteria in the middle to lower parts, respectively. Organic maturity is considered to be in the early to medium stage of oil generation, according to C29 sterane 20S/(20S + 20R) ratios of 0.28–0.49. Based on these results, most OMs originated from ferns, pteridospermic gymnosperms, peat mosses and/or submerged/floating plants, occasionally supplemented by a specific gymnosperm OM with epicuticular wax. Potential of HC generation of these coaly shales probably increased with increasing deposition of liptinite materials. Fluoranthene (Fla), pyrene (Py), benzo[e]pyrene (BePy) and benzo[bjk]fluoranthene (Bflas) are abundant PAHs in the coaly shales, and the ratios of low Fla./Py, benzo[a]anthracene (BaAn)/228 and indeno[1,2,3-cd]pyrene/(indeno[1,2,3-cd]pyrene + benzo[ghi]perylene) (InPy/(InPy + BghiP)) suggest that these 4- to 6-ring PAHs were mainly formed by small- to medium-scale wildfires during the dry season.