The Original Organism Assemblages and Kerogen Carbon Isotopic Compositions of the Early Paleozoic Source Rocks in the Tarim Basin, China

Abstract

Original organisms are the biological precursors of organic matter in source rocks.Original organisms in source rocks are informative for oil-source rock correlation and hydrocarbon potential evaluation,especially for source rocks which have high-over level of thermal maturity.Systematic identification of original organism assemblages of the Lower Paleozoic potential source rocks and detailed carbon isotopic composition of kerogen analyses were conducted for four outcrop sections in the Tarim basin.Results indicated that the original organism assemblages of the lower part of the Lower Cambrian were composed mainly of benthic algae,whereas those of the Upper Cambrian and the Ordovician were characterized by planktonic algae.Kerogen carbon isotopic data demonstrated that the δ13Ckerogen values of source rocks dominated by benthic algae are lower than-34‰,whereas the δ13Ckerogen values of source rocks dominated by planktonic algae are higher than-30‰ in general.We tentatively suggested that the carbon species those are utilized by algae and the carbon isotopic fractionation during photosynthesis are the major controls for the δ13Ckerogen values in the Lower Paleozoic source rocks in the Tarim basin.Correlating the δ13C values of oils exploited in the Tarim basin,the original organism assemblages,and δ13Ckerogen values of source rocks,it implied that the Lower Paleozoic oils exploited in the Tarim basin should be sourced from the source rocks with original organism assemblages dominated by planktonic algae,and the hydrocarbon sourced from the Cambrian benthic algae should be of great exploration potential in future.Original organism assemblages in source rocks can provide important clues for oil-source rocks correlation,especially for the source rocks with high thermal maturity.