Development Environment and Formation Mechanism of Lower Cambrian Source Rocks in the Chengkou Area, Northeast Chongqing, South China.

The Lower Cambrian black shale at the northern margin of the Upper Yangtze Platform is an important source rock with large sedimentary thickness and high organic carbon content, which gives it important oil and gas exploration value. Restoring its developmental environment and clastic provenance is of great significance for the prediction of hydrocarbon source rock distribution. By systematic field outcrop sampling in the Chengkou area, South China, and geochemical testing, this study investigated the formation environment and source supply of the black shale and discusses the effect of climate change and terrigenous input on the formation of the black shale. It also reconstructs the process by which these excellent-quality hydrocarbon source rocks were formed. The results showed that the black shale has high Al₂O₃ content (7.38-13.90%), K₂O content (1.01-2.93%), Na₂O content (1.98-2.88%) and total Fe₂O₃ content (2.35-5.36%), and is enriched by trace elements such as V (average 242 µg/g), Cr (average 83 µg/g) and Ni (average 58 µg/g). CIA, K/Al and Rb/Al ratios indicate that the climate was warm and humid during the period of black shale deposition. V/Cr, U/Th, Ce/Ce* and MoEF-UEF indicate that the sedimentary waterbody underwent an anoxic → suboxic → anoxic → suboxic cycle. A positive Eu/Eu* anomaly also suggests the occurrence of early submarine hydrothermal activity. The Th/Sc - Zr/Sc chart indicates that the sedimentary components of the black shale did not undergo a recycling process, and it is therefore possible to effectively identify the location of the source. The Co/Th-La/Sc and La/Th-Hf diagrams confirm the presence of both felsic and mafic source rocks in the black shale. The Hannan paleo-uplift may have been an effective source area. [ABSTRACT FROM AUTHOR]