The biomarker geochemistry of Göynük (Kemer-Antalya) coals

This study focuses on the biomarker geochemistry of the Göynük (Kemer- Antalya) coaly units located in the southwestern part of Antalya. These coals are found within the Carboniferous-Permian-aged Pamucakyayla Formation. The Pamucakyayla Formation is predominantly composed of quartz-rich sandstones in shades of gray and brown, with occasional thin layers of shale in grayish or yellowish tones and coal bands in certain regions. The formation has been identified as dating from the Carboniferous to the Lower Permian and resting conformably on the Devonian. The limestone bands within the formation, along with symmetrical ripple marks and small-angle cross-bedding, indicate a shallow marine environment. The formation, observed to be 380 meters thick in the study area, shows no lateral variation. Organic geochemical and petrographic analyses were conducted on coal and coal- bearing samples collected from the formation. These analyses aimed to determine the organic matter content, type, maturation level, and potential source rock characteristics of the coals. The Total Organic Carbon (TOC) content of the samples ranges from 0.13% to 30.45%, with an average TOC value of 15.29%. These values indicate a rich source rock potential for the formation. Hydrogen Index (HI) values range from 34 to 572 mg HC/g TOC, with an average of 303 mg HC/g TOC, suggesting that the samples predominantly contain humic organic matter and exhibit Type III and Type II kerogen characteristics. Oxygen Index (OI) values vary between 1 and 223 mg CO2/g TOC. Tmax values range from 444 to 476 °C, indicating that the samples are in mature to late-mature stages and contain kerogens with the potential to generate oil and gas. Vitrinite reflectance values range from 0.713 to 0.977, corresponding to bituminous coal ranks. The bitumen/TOC ratio ranges from 0.33 to 0.57, reflecting a mature to late-mature level of organic matter. Additionally, CPI and OEP ratios were calculated to be between 1.26-1.59 and 0.75-0.92, respectively, which also indicates mature to late-mature stages. The Pr/nC17-Ph/nC18 diagram provides consistent results, offering further insights into the maturation process of the samples. GC-MS analyses revealed the presence of short, medium, and long-chain n- alkanes in the samples, offering insights into their biological sources and coalification processes. In one sample, the dominance of long-chain n-alkanes indicates the presence of organic matter derived from terrestrial plants and a high maturation level. In contrast, the dominance of short and medium-chain n-alkanes in the other two samples reflects the diversity of biological sources and varying maturation stages. Pr/Ph ratios were determined to be 0.88, 6.26, and 5.44, respectively. High Pr/Ph ratios (>3) indicate deposition and preservation under oxic conditions, whereas a low Pr/Ph ratio (0.88) suggests deposition and preservation under anoxic conditions. These findings indicate that the coals were formed under varying depositional environments. In the analysis of the m/z 191 mass chromatograms of saturated hydrocarbon fractions, prominent peaks corresponding to tricyclic and pentacyclic terpanes were observed. Pentacyclic terpanes, particularly C30, C27, and C29-hopane compounds, were found to be dominant, while tricyclic terpanes were concentrated in the C19, C20, and C21 compounds. The high abundance of pentacyclic terpanes indicates a terrestrial organic matter origin and suggests deposition under oxic conditions. Tricyclic terpanes, on the other hand, provide insights into the thermal maturity levels of the samples. Similar sterane/hopane ratios were observed across all three samples, typically indicating anoxic or suboxic conditions. Such environments are commonly found in restricted marine basins, swamps, or coastal lagoons, where bacterial activity is high. A diasterane/sterane ratio above 0.5 reflects a high clay content and deposition in oxic environments. The C29/C30 hopane ratios in the samples reflect a mixture of marine and terrestrial organic matter, with the low C35/C34 ratio in sample NS-2 representing marine organic matter accumulation. In contrast, the higher ratios in NS-11 and NS-17 indicate the presence of terrestrial organic matter. The gamaseran index values suggest that the organic matter has undergone moderate maturation and was deposited in an anoxic environment. Consequently, NS-2 represents a deep marine setting, NS-11 reflects a shallow marine or coastal environment with mixed terrestrial and marine inputs, and NS-17 represents a coastal-deltaic environment with higher terrestrial organic matter input. The low moretane/hopane ratios (0.0432-0.194) across the samples indicate that terrestrial plant material was the predominant component during coal formation, with minimal marine or lacustrine contributions. The similarity in these ratios between the samples suggests that they were formed under similar environmental and geochemical conditions. The C35/C34 ratio was low in NS-2 but higher in NS-11 and NS-17. A low C35/C34 ratio typically indicates marine organic matter accumulation or anoxic depositional conditions, while higher ratios point to terrestrial organic matter or oxic environments. The low C25/C26 tricyclic terpane ratio in NS-2 suggests a predominance of terrestrial plant material with little marine organic input, whereas the moderate ratio in NS-11 reflects a mixture of terrestrial and marine sources. In NS-17, the high ratio indicates a marine environment with dominant marine organic matter. The observed gammacerane index values in the samples represent different depositional environments, in sample NS-2, it indicates a deep, anoxic marine environment; in NS-11, it reflects a shallow marine environment where terrestrial and marine sources mix; and in NS-17, it represents a more oxygenated coastal or deltaic environment with a higher contribution of terrestrial organic matter. XRD analyses of samples from the Pamucakyayla Formation revealed that quartz, clay, calcite, pyrite, and feldspar are the most common minerals. In the clay mineral analyses, illite and kaolinite were identified as the dominant minerals.