Organic geochemistry and depositional environment of the Aptian bituminous limestone in the Kale Gumushane area (NE-Turkey): An example of lacustrine deposits on the platform carbonate sequence

The Jurassic-Lower Cretaceous aged carbonate sequence is widely exposed in the southern zone of Eastern Pontides. Aptian black bituminous limestone is found in the upper part of this sequence in the Kale area (Gumushane). This limestone contains faunal remains (e. g., gastropod, ostracod, characean stems and miliolid type benthic foraminifera) that indicate a freshwater, lacustrine depositional environment. The total organic carbon (TOC) values of the bituminous limestone samples range from 0.11-1.30% with an average TOC value of 0.54%. The hydrogen index (HI) varies from 119-448 mg HC/g TOC (average HI 298 mg HC/g TOC) indicating that the limestone contains gas prone as well as oil prone organic matter. Pyrolysis data prove that the organic matter content in the bituminous limestone consists of Type II kerogen. The average T-max value for bituminous limestone samples is 438 degrees C (434-448 degrees C). Bitumen/TOC ratios for bituminous limestone are 0.05 and 0.04. The T-max values and the ratios indicate that the bituminous limestone samples contain early mature to mature organic matter. Analysis of solvent extracts from the two richest bituminous limestones show a predominance of high carbon number (C-26-C-30) n-alkanes. The Pr/Ph ratio and CPI value are 1.34 and 0.96, respectively. C-29 is the dominant sterane, with C-29 > C-27 > C-28. The bituminous limestone samples have low C-22/C-21 ratios, high C-24/C-23 tricyclic terpane ratios and very low C31R/C-30 hopane ratios (<0.25). These data are consistent with the bituminous limestones being deposited in a lacustrine environment. (C) 2012 Elsevier Ltd. All rights reserved.