Organic Geochemical Properties of the Miocene Coal Seam in the Ömerler Field, Tunçbilek Basin, Western Türkiye: Implications for Depositional Environment

In this comprehensive study, which was conducted based on data obtained from the Ömerler Field İğdekuzu underground mine located in the Tunçbilek Basin (Kütahya/W. Turkey), the aim was to reveal the organic matter content, origin, thermal maturation level, depositional environment, and conditions of Miocene-aged coals. Coal samples collected from the underground coal mine were subjected to proximate analysis, Rock-Eval pyrolysis/Total organic carbon (TOC), gas chromatography (GC), gas chromatography–mass spectrometry analysis (GC–MS) of aromatic and saturated biomarkers and extraction analysis. On an air-dry basis, the average moisture (Mad), volatile matter (Vad), ash (Aad), fixed carbon (FCad) and total sulphur contents of coals are 2.79%, 33.93%, 18.71%, 44.31% and 1.40%, respectively. TOC content is so high; the highest mean TOC content is 86.75% calculated in the upper coal seam, followed by the middle seam with 85.14% and the lower seam with 79.01%. In the GC–MS of the coal samples, the n-alkane distribution varies between C12 and C36. The lowest homohopane index (HHI) value was calculated as 0.02 for OR-5, and the highest HHI value was 0.05 for TB-8. The upper coal seam sample exhibits the order of C29>C28>C27 sterane (%), and the lower and middle coal seam samples exhibit the order of C29>C27>C28 sterane (%). The coal samples under study exhibit early mature oil window behaviour according to all geochemical maturity parameters. These coals indicate a mixed organic matter of Type II/III and III kerogens, as demonstrated by medium and long-chain n-alkanes, terrestrial/aquatic organic matter ratios (TAR), aquatic plants to terrestrial plants ratios (Paq), C24 Tet/C26 Tri and C19/C23 Tri ratios, and distributions of C27–29 ααα20R, likely deposited in a paralic, oxic-dyoxic transitional environment. Although the TOC value of the Miocene-aged coal samples is so high, they have not yet reached the level of producing hydrocarbons due to low and medium hydrogen index (HI), low quality index (QI), production index (PI), and bitumen index (BI) values, but it has been evaluated that they have the potential to produce gas.