Developmental Controlling Factors of Upper OrdovicianLower Silurian  High Quality Source Rocks in Marine Sequence, South China

Key words: 
• South China /
• marine /
• source rock /
• Upper OrdovicianLower Silurian /
• geochemistry /
• upper welling

Abstract: From the angel of paleoclimate and paleooceangraphy, combined with Paleozoic paleogeography characters, the developmental controlling factors of Upper Ordovician Lower Silurian high quality source rocks in marine sequence, South China were discussed in this paper, with analysis of geochemical data in typical section from upper Yangtze. The researches indicate geochemical indicators such as V/（V+Ni）, Ni/Co, Ceanom, δEu and δ13Corg have good relationship with the content of organic carbon, and the abnormal of these indicators illustrate that the high quality source rocks of Upper Ordovician Lower Silurian were developed in anoxic environment. However, from the researches of paleoclimate and paleooceangraphy, the developmental controlling factors of Wufeng Formation in Upper Ordovician are different with those of Longmaxi Formation in Lower Silurian. As the ice age reached its crest in Wufeng period, the intense upper welling formed in Yangtze area, which improved the organic production. The thin argillite with high abundance of organic matter covered in Yangtze area are the best indicators of upper welling. The global paleoclimate came into a warm period as Longmaxi Formation deposit, which were disadvantage for lager scale upper welling. In this period, the effects of upper welling were limited in deep slope around Yangtze craton. High quality source rocks of Longmaxi Formation were developed in early period of transgression. Because during initial transgression, coarsergrained siliciclastics are constrained to fill, and the anoxic environment were preserved. During latter transgression, as the deep water and surface water have abundance time to mix, the anoxic environment were destroyed in deep sea, as a result of which, the preservation condition of organic matter become bad.