Characteristics of biomarker compounds and their implications for Lower Cambrian black shale on the northern margin of Sichuan Basin

The formation of the sag in the central Sichuan Basin may have influenced the development of major reservoir and source rock formations, such as the Lower Cambrian Qiongzhusi (Niutitang) Formation. This study analyzed the organic geochemistry, including n-alkanes and isoprenoids, steranes, terpanes (hopanes series), of organic matter in the Qiongzhusi Formation on the northern margin of the Yangtze Plate to investigate the depositional environment and formation mechanism of the intracratonic sag. The results indicate that the organic matter in the study area was sourced from eukaryotic phytoplankton and bacteria. The presence of gammacerane and low pristane/phytane ratios indicates that the top of the water column contained enough oxygen to provide favorable conditions for eukaryotic phytoplankton to thrive. The presence of 4-methyl steranes, a typical molecular indicator of hydrothermal vent microbes, in the Qiongzhusi Formation suggests anaerobic organisms thrived at the bottom of the water column as a result of extension during the formation of the intracratonic sag. A paleoceanographic model of the study area was constructed. Both methane oxidizing and sulfate reducing bacteria along with other hydrothermal vent microbes may not only serve as sources of organic matter, but also promote the propagation of algae in shallow water to improve organic productivity in this ancient environment.