An in-depth study was carried out on the settings and sedimentary geochemical characteristics of source rocks in the lower Cambrian Maidiping-Qiongzhusi Formations of southern Sichuan Basin.The analysis of the rock compositions

organic carbon content

hydrocarbon generation biological combinations

and redox indexes as well as their correlations

on source rock samples from the Maidiping and Qiongzhusi Formations in Well Jinye 1 of Jingyan-Qianwei area

shows that the two formations are quite different in terms of mineral composition and hydrocarbon generation environment.The Maidiping Formation is dominated by siliceous dolomite and has a moderate organic carbon content.Hydrocarbon was mostly generated by microbial mats composed of small shallow-water shell creatures.A strong reducing environment can be inferred from its high Uranium index (greater than 1.7).It is suggested that

instead of being caused by deep water

the strong reducing environment is due to the thriving and rapid consumption of oxygen of the shell creatures in the sea bed.A combination of these understandings with a correlation analysis of sedimentary geochemical parameters indicates a tidal flat sedimentary setting for the deposition of the formation.The Qiongzhusi Formation is dominated by siliceous and clayey shale and varies in organic carbon abundance from the lower to upper members.Different from the Maidiping Formation

its hydrocarbon-generation organisms are mainly amorphous flocculent aggregates of organic matter.The Uranium index varying between 0.8 and 1.7 and a positive correlation between the organic carbon content and the redox properties of the samples all indicate a shallow-to-deep shelf setting of oxidation to strong reduction for the formation.In summary

various factors

including the tectonic-sedimentary-lithofacies paleogeographical environment

primitive biological type

and rock mineral composition during the Early Cambrian in southern Sichuan Basin

serve to jointly determine the quality of the source rocks and their sedimentary and geochemical characteristics.