Distribution and geodynamic setting of the Late Neoproterozoic- Early Cambrian hydrocarbon source rocks in the South China and Tarim Blocks

The Late Proterozoic-Early Cambrian, one of the most critical intervals in Earth's history, is characterized by dramatic tectonic, climatic, oceanic and biological changes. In China, thick black mudstones and shales formed in intracontinental rifts during this period, associated with interglacial and postglacial phases of the Neoproterozoic global-scale glaciations. Previous studies have shown these black mudstones and shales were potential hydrocarbon source rocks that contributed to the Proterzoic-Early Paleozoic oil and gas discoveries. However, less work has been done to systematically characterize these important source rocks. In the South China Block, these excellent source rocks (similar to 30-200 m) were discovered in the Nanhua (Cryogenian), Sinian (Ediacaran), and Lower Cambrian Systems, with total organic carbon (TOC) contents of 2-10%. In contrast, black mudstones in the Nanhua and Sinian Systems in the Tarim Basin had low TOC contents (0.3-1.5%) perhaps affected by intense weathering, whereas excellent source rocks (TOC similar to 6-18%) existed in the Cambrian Yuertusi Formation (12-80 m). Our study indicates the formation and distribution of excellent source rocks were mainly controlled by the Late Proterozoic-Early Cambrian rifts, which was supported by a decrease in thickness towards marginal areas especially for the Nanhua and Sinian Systems. The Early Cambrian source rocks occurred per-vasively due to a global marine transgression and those formed under deep-water anoxic conditions at slope areas were of the best quality with TOC up to 15%. These source rocks could be focuses for future exploration in the deep or ultra-deep depths in the studied blocks as well as their peripheral areas.