R.  Kara-Gülbay1, Ö. Sen2, S. Korkmaz*1 and I. H. Demirel3

1 Karadeniz Technical University, Department of Geological Engineering, Trabzon, Turkey.

2 Barite Mine Turkish Comp.,  Istanbul, Turkey.

3 Hacettepe University Department of Geological Engineering,  Ankara,  Turkey.

* Corresponding author, email: korkmaz@ktu.edu.tr

Lower Carboniferous (Tournaisian-Visean) shales, sandstones and limestones are exposed at the surface in autochthonous units in the Eastern Taurides, southern Turkey.  This study investigates the organic geochemical characteristics, thermal maturity and depositional environments of shale samples from two outcrop locations in this area (Belen and Naltas). The total organic carbon (TOC) contents range from 0.11 to 5.61 wt % for the Belen samples and 0.04 to 1.74 wt % for the Naltas samples. Tmax values ranging from 432–467 oC indicate that the samples are in the oil generation window and are thermally mature. Rock-Eval pyrolysis data indicate that the organic matter in the shales is composed mainly of Type II and III kerogen.

Solvent extract analyses of the samples show a unimodal n-alkane distribution with a predominance of low carbon number (C13-C20) n-alkanes. Pr/Ph ratios and CPI values range from 1.57-1.66 and 1.08-1.11, respectively. Pr/n-C17 and Ph/n-C18 ratios also indicate that the shales consist of mixed Type II/III organic matter.

Sterane distributions are C27> C29> C28 as determined by the sum of normal and isosteranes, suggesting marine depositional conditions. 20S/(20S+20R) and bb (bb+aa) C29 sterane ratios range from 0.51-0.54 and 0.53-0.57, respectively. These values are high and 20S/(20S+20R) sterane isomerisation has reached equilibrium values.

Tricyclic terpanes are abundant on m/z 191 mass chromatograms and C23 tricyclic terpanes are the dominant peak, which indicates a marine depositional setting.  C29 norhopane has a higher concentration than C30 hopane, and C30 diahopane and C29Ts are present in all the samples. Ts and Tm were recorded in similar abundances. Moretane/hopane ratios are very low.  22S homohopanes are dominant over 22R homohopanes, and the C32 22S/(22R + 22S) C32 homohopane ratios are between 0.58 and 0.59, indicating that homohopane isomerisation has reached equilibrium. C31 homohopanes are dominant and the abundance of homohopanes decreases towards higher numbers.

Although regional variations in the level of thermal maturity of Upper Palaeozoic sediments throughout the Taurus Belt region largely depend on burial depth, organic geochemical data indicate that the Lower Carboniferous shales in the eastern Taurus region (Naltas and Belen locations) have potential to generate hydrocarbons. These shales are thermally mature and have entered the oil generation window.

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