Organic petrology and geochemistry of the Upper Ordovician-Lower Silurian Renheqiao Formation shales of the Baoshan Block, western Yunnan, China

Abstract Lower Paleozoic black shales are important source rocks worldwide. The Upper Ordovician-Lower Silurian Renheqiao Formation of the Baoshan Block is a low-maturity equivalent of the Wufeng-Longmaxi (WF-LMX) Shale of the Sichuan Basin. However, organic matter (OM) characteristics in these low-maturity Lower Paleozoic shales are not well understood. In this study, 50 Renheqiao Formation shale samples collected from seven outcrop sections and one drill core were investigated with organic petrology, organic geochemistry, Rock-Eval pyrolysis, N 2 and CO 2 adsorption, and scanning electron microscope (SEM) analyses to study the OM content, type, thermal maturity, and the development of OM-hosted pores in these Lower Paleozoic shales. The total organic carbon (TOC) content of the Renheqiao Formation shales varies, with the maximum content of 10.07 wt%. Rock-Eval pyrolysis results show that present OM in the Renheqiao Formation shales is Type IV kerogen, a result of advanced thermal maturation. Graptolite reflectance ( GR o ) ranges from 1.26% to 1.85%, and equivalent vitrinite reflectance ( EqR o ) converted from GR o ranges from 1.08% to 1.51%, indicating that the studied Renheqiao Formation shales are dominantly within the late-mature stage. EqR o based on Rock-Eval T max shows large variations, which indicates that Rock-Eval T max is not a reliable thermal maturity indicator for the Lower Paleozoic Renheqiao Formation shales. Caution should be applied when assessing the thermal maturity of high-maturity black shales based on T max when the S2 values are too low. Organic petrographic observations show that OM in these shales is dominated by solid bitumen (> 70 vol% of total OM), with minor contributions by graptolites and chitinozoans. The specific surface area and pore volume of shales are controlled by TOC content. Organic pores are hosted by solid bitumen and were not observed in graptolites when examined under the SEM. Although the Renheqiao Formation has a lower thermal maturity than the over-mature WF-LMX Shale, it is mature enough that primary oil-prone macerals have been thermally transformed and could not be identified under the microscope.