Paleoenvironmental Settings of the Soma Coal Basin (Turkey): Insights from Maceral Data, Biomarker, and Carbon Isotopic Composition

Abstract

Organic geochemical (TOC, pyrolysis, biomarker) and petrographic (maceral analysis) investigations together with organic carbon isotope studies were carried out to characterize in detail the depositional environment, determine the organic matter type, and assess the hydrocarbon production potential of three coal seams (KP1-upper, KM3-middle, and KM2-lower) in the Soma (Manisa, Western Anatolia) coal field in Turkey. The total organic carbon value of the upper coal seam ranged between 11.7 and 55.75%, the middle coal seam between 20.12 and 62.86%, and the lower coal seam between 50.03 and 65.71%. Coals in all three seams are characterized by low hydrogen index (HI) values (<151 mg HC/g TOC), low bitumen index (BI) (<19 g HC/g TOC), and quality index (QI) between 23 and 156 mg HC/g TOC. According to Rock-Eval pyrolysis data, the organic matter type of the coals is type III kerogen. Huminite reflectance, Tmax, and biomarker data (22 S /22S+22R (C 32 ) sterane, ββ/(αα + ββ) (C 29 ) sterane, and MPI-1) indicate that the organic matter is not thermally mature and that the Soma-Manisa coal has reached the sub-bituminous rank. Rock-Eval data shows that coal is gas-prone and has not reached the maturity threshold required for initial gas production. The dominant maceral group is huminite while liptinite and inertinite macerals have been found in minor amounts. Groundwater index (GWI), vegetation index (VI), tissue preservation index (TPI), and gelification index (GI) parameters indicate a transition from limnic-limno-telmatic to limno-telmatic-telmatic environment from the upper seam to the lower seam. N -Alkane distributions show that paleoclimatic conditions have changed from KP1 to KM2. The higher abundance of pristane compared to phytane and low C 35 /C 31 –C 35 homohopane index values demonstrate that the coals were deposited in a suboxic–oxic environment. The predominance of n -alkanes with generally high carbon number, relative variable abundances of C 27 –C 28 –C 29 steranes, δ 13 C values, C/N ratios, and very low gammacerane index indicate a terrestrial ecosystem with nonmarine influence, although algae and microorganisms also contributed to the biomass.