Organic Geochemistry and Depositional Environment of the Oltu Gemstone (Coal) in the Erzurum Area, NE Anatolia, Turkey

The Oltu Gemstone is located in the north of Oltu town (Erzurum–NE Turkey) city as a low rank coal. The Oltu Gemstone occurs as lenticular forms with thickness not exceeding centimeter size and lateral continuity of a few meters within the Liassic–Lower Malm Olurdere Formation consisting chiefly of claystone, sandstone, and volcanics. Coals that are operated as Oltu Gemstone are represented by very high TOC (67.39–78.56% wt), high hydrogen index (HI) values (314–379 mg HC/g TOC) and very low oxygen index (OI) values (1–3 mg CO2/g TOC). Low Pr/Ph ratios indicate that coals were prevented from oxidation and deposited under anoxic conditions. In Oltu Gemstone samples, C29 dominates over C27 and C28 steranes. In general, high (C19 + C20)/C23 tricyclic terpane, low Ts/(Ts + Tm), diasterane/sterane, and C31R/C30 hopane ratios were recorded. C29 MA steroids dominate with respect to others, and the C29/(C28 + C29) MA ratio is mostly high. DBT/P ratio of Oltu Gemstone samples has low values. Tmax values of Oltu Gemstone samples (between 416 and 436 °C) reflect immature–early mature character. 22S/(22R + 22S) homohopane, 20S/(20R + 20S), and ββ/(αα + ββ) sterane ratios and low moretane/hopane ratios, relatively high C28-TA/(C29-MA + C28-TA), MA(I)/MA(I + II), TA(I)/TA(I + II), MPI-3 (β/α MP), and MDR ratios indicate early mature character for the Oltu Gemstone samples.