Abstract
In this study, n-alkane and isoprenoid, saturated and aromatic biomarker distributions of Miocene coals in the Çan (Çanakkale) region, were studied to investigate organic geochemical characteristics, depositional environment, and hydrocarbon potential of these coals.Çan formation (Middle Miocene) unconformably overlies the basement Çan volcanites (Upper Oligocene-Lower Miocene). This formation is composed of lignite, clayey lignite, and ligniferous clayey levels of various thicknesses interlayering with clay and tuffite. The formation represents sediments of small lacustrine basins that are isolated by faults that are formed contemporaneously with Early-Middle Eocene volcanism. Coaliferous units are overlain by lower agglomerate levels.The Rock Eval analyses show that the average TOC (Total Organic Carbon) content of the coal is 53.71% and the average HI (Hydrogen Index) value is 180.5 mgHC/gTOC. OI (Oxygen Index) values (avg. 22.33 mgCO2/gTOC) show that the depositional environment of peat deposits is suboxic. According to HI-Tmax, S2-TOC classification diagrams and Pr/n-C17vs. Ph/n-C18 diagram, the organic matter is composed of Type II and Type III kerogen mixing and represents a transitional environment. In gas chromatograms, n-alkanes with high, mostly odd-numbered carbons are dominant and they are associated with a slight algal contribution. CPI22-30 (Carbon Preference Index) and TARHC (Terrigenous/Aquatic Ratio) values indicate a dominance of high-carbon numbered n-alkanes, indicating the presence of terrestrial organic matter.Paq (aquatic plant n-alkane proxy) values (0.2; 0.3) indicate a climate with relatively wet conditions or low precipitation, the presence of emergent macrophyteses, and the scarcity of hydrophilic plants. Pwax (vascular plant n-alkane proxy) ratio (0.8; 0.7) shows moderately wet conditions and a relatively low water level.The TOC, ACL (n-alkane average chain length), Qwood/grass, Qwood/plant, and Qgrass/plant values indicate the presence of woody plants in a peat bog and relatively wet and moderately warm climate conditions.Based on DBT/P (dibenzothiophene/phenanthrene) ratio, sample Ç-2 points to the character of marine shale and other lacustrines whereas sample Ç-6 indicates fluvial/deltaic depositional environment.Overall, 20S/(20R + 20S), ββ/(αα+ββ) sterane, 22S/(22R + 22S) homohopane, and moretane/hopane ratios that reflect the maturity of organic matter are indicative of immature organic matter. MA(I)/MA(I + II), TA(I)/TA(I + II) and C28TA/(C29MA + C28TA) steroid, MPI, MPR, and MDR ratios calculated from aromatic biomarkers are quite low, which indicate an immature nature.The n-alkane, saturated and aromatic biomarker data on the Çan coals reveal that deposition should have taken place in a terrestrial environment changing from lacustrine to fluvial under predominately suboxic-oxic and acidic conditions.
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