Paleoenvironmental significance of biomarker and stable isotope (2H, 13C, 15N, 18O, and 34S) investigations on a Pliocene coal-bearing sequence (Denizli Basin, southwestern Turkey)

Abstract

Pliocene coal, clayey coal, and coaly claystone sediments from the Yatağan Formation (Denizli Basin, southwestern Turkey) have been investigated with respect to maceral association, mineralogical composition, n-alkane, isoprenoid and saturated-aromatic biomarker characteristics, and stable isotopes (2H, 13C, 15N, 18O, and 34S). The main purpose of the present study was to determine the hydrocarbon potential and paleodepositional conditions of coals in the Denizli Basin. The abundances of huminite, liptinite, and inertinite macerals were 46.14%, 8.06%, and 1.57%, respectively. The huminite/vitrinite reflectances (%Ro) of the coal, clayey coal, and coaly claystone samples ranged between 0.26 and 0.37 %Ro, implying sub-bituminous B and C coal ranks. The total organic C (TOC, wt.%) content and the Tmax values of the investigated samples ranged between 0.74 and 43.80 wt% and 390–440 °C, respectively. In addition, the coaly horizons were characterized by very low H index (HI), O index (OI), production index (PI), and Tmax values. Moreover, a high waxiness degree, C preference index (CPI), odd-to-even predominance (OEP), terrigenous/aquatic ratio (TAR), and high C19 tricyclic terpane/C23 tricyclic terpane (C19/C23 TT) and C24 tetracyclic terpane/C26 tricyclic terpane (C24*/C26 TT) biomarker ratios indicated a terrestrial plant input. These results were supported by stable isotope (2H, 13C, and 15N) investigations. Although the Pliocene coals have source rock potential according to the TOC content and pyrolysis analyses, they have been determined to be at the diagenesis and immature stages as a result of organic petrographic, and saturated-aromatic biomarker investigations.