SOURCE ROCK POTENTIAL AND ORGANIC GEOCHEMISTRY OF CENOMANIAN‐TURONIAN BLACK SHALES, WESTERN TAURUS, SW TURKEY

Abstract

The depositional environment and hydrocarbon source rock potential of Cenomanian‐Turonian black shales of the Dereköy and Ballik Formations in SW Turkey were investigated by organic geochemical methods. In detail, 33 samples from three section of the Dereköy Formation, and 15 samples from one section of the Ballik Formation were analysed for elemental (TOC, Rock ‐Eval pyrolysis), C15+‐lipid and biomarker compositions.Based on maximum pyrolysis degradation temperatures of not more than 420°C, all the shale samples are classified as immature, corresponding to a vitrinite reflectance of less than 0.45% Rr and a lignite to sub‐bituminous coal stage. This is confirmed by relatively high isoprenoid to n‐alkane ratios as well as by high biomarker contents. According to this maturity stage, and both total organic carbon contents of 6–41% and hydrogen indices of 255–708 mg HC/g TOC, the Cenomanian‐Turonian black shales exhibit fair to excellent source rock potential with mixed Type II and Type I kerogen.Relatively high isoprenoid to n‐alkane ratios may indicate at least partial (bio‐) degradation/evaporation/waterwashing and selective modification of the lipid composition due to the nature of the outcrop. However, very similar unimodal n‐alkane distributions in the gas chromatograms of four selected shale samples, with a predominance in the C16 to C17 region, clearly point to a significant contribution of algal and/or bacterial type organic matter with low terrigenous organic input.C27, C28 and C29 steranes in shales from both formations have similar distributions (C29>C27>C28). High C31 R homohopane / C30 hopane ratios indicate a marine depositional environment. This is confirmed by the presence of gammacerane in all the black shales investigated which in general indicates salinity. Pregnanes in one sample (BA‐6) may point to hypersaline conditions.