Paleoclimate and paleodepositional condition of Ekizköy (Muğla-Milas) Middle Miocene coals: Implications from biomarker data

Abstract

In this study, saturated, aromatic biomarker distributions and organic geochemical characteristics and depositional environments of Middle Miocene aged coals in Ekizköy (Muğla-Milas) region were investigated. The average TOC, HI and OI values are 46.24 %, 153 mg HC/gTOC and 77 mg CO₂/gTOC, respectively. Pr/Ph ratio is 0.9 and Pr/C₁₇ and Ph/C₁₈ are 0.2 and 0.3. Considering the paleoclimate data (TOC, CPI, ACL, P_wax, P_aq, Q_wood/grass, Q_grass/plant and Q_wood/plant), it is likely that the climate was moderately humid and hot for the period represented by sample M-4 and relatively more humid and temperate for the period represented by sample M-8. High C₂₉(C₂₈ + C₂₉) MA steroid ratio indicates that Ekizköy coals are composed of terrestrial OM (Organic Matter). 20S/(20R + 20S) (for C₂₉), 22S/(22R + 22S) homohopane (for C₃₁), ββ/(αα + ββ) sterane, moretane/hopane ratios that show the maturity of OM are indicative of immature OM. C₂₈TA/(C₂₉MA + C₂₈TA) steroid and MA(I)/MA(I + II), TA(I)/TA(I + II), MDR, MPI, MPR ratios obtained from aromatic biomarkers are low and show an immature character. Since coals contain Type III kerogen and immature organic matter, they can only generate gas under appropriate temperature and pressure. Saturated and aromatic biomarker data in Ekizköy coals show that they precipitate in a lacustrine environment dominated by suboxic conditions.