Organic geochemical characteristics of highly mature Late Neoproterozoic black shales from South China: reappraisal of syngeneity and indigeneity of hydrocarbon biomarkers

•Cryogenian Datangpo Formation and Ediacaran Doushantuo Formation (South China).•These Neoproterozoic formations exposed to high thermal maturity (EqVRo = ~2.5%).•Rigorous experimental procedure used to analyse indigenous geochemical composition.•All the black shales lack indigenous steranes, and one contains a trace of hopanes.•Previously published biomarkers from these formations may be due to contamination.