Organic Matter Enrichment And Evolution Of Sedimentary Environment Of The Wufeng-Longmaxi Shale In Southeastern Margins Of The Sichuan Basin

In Earth history,the Late Ordovician–Early Silurian transition was a particular epoch,during which the Earth surface was extremly abnormal active and lots geological events occurred,including the climate cool down and global glaciation,biological extinction crisis,frequent sea level changed,marine anoxic event,widespread development of black shale and the reginal volcanism activity,all event were concentrated in this shot time,make the palaeoenvironment of the Ordovician–Silurian transition become much more complicated.The Late Ordovician Wufeng Formation–Early Silurian Longmaxi Formation marine black shale is a tipical strata to reseach the evolution of palaeoenvironment in the Sichuan Basin.The strata not only across the Ordovician–Silurian transition,but it contain abundant biological fossil zone,which can exactly reveal the relationship between the marine,biological and atmosphere climate.In addition,the Late Ordovician Wufeng Formation–Early Silurian Longmaxi Formation is present the main shale gas?oil?exploration and development target in Sichuan Basin.and currently drawing much attention among scholars all over the world.The study on the characteristic of palaeoenvironment of Late Ordovician–Early Silurian,is conducive to understand the mechanism of organic matter enrichment,and further provide some important advisement for the shale gas?oil?exploration and development in Sichuan Basin..In this article,three continuous and well exposed section of Wufeng Formation and Longmaxi Formation from southeast Sichuan Basin were chosen to drill and sample to test.We try to combine the organic geochemistry?carbon,nitrogen,sulfur?,element geochemistry?major and trace element?and stable isotope geochemistry?carbon isotope,nitrogen isotope?,to reconstruct the palaeoenvironment conditions of Wufeng Formation and Longmaxi Formation,including the redox condition,ocean primary productivity,eustatic sea level change,terrigenous influx and water colmn restrict condition.Base on this study,we construct the development model of Wufeng Formation and Longmaxi Formation,then reveal the distinction of mechanism of organic matter accumulation between the two series black shale,also investigate the relationship between paleoclimate and the carbon and nitrogen isotope perturbations during Ordovician–Silurian transition.The major conclusions are summarized below:?1?Redox sensitive proxies?V/Cr,V/?V+Ni?,U/Th?reveal the water mass was mainly anoxic and reducing condition during the deposition of Wufeng-Longamxi Formation in Sichuan Basin,but also accompanied by intermittent oxic and dysoxic condition in the lower part of Wufeng Formation,upside of Longmaxi Formation and Guanyinqqiao Bed.?2?High value of P/Al,Ba_bio indicate that,during the late Ordovician-early Silurian,Yangtze sea had a moderate to high primary productivity water columns.High productivity ensure there were sufficient organic matter to accumulate in the Wufeng-Longmaxi Formation.?3?Redox condition is the most critical factor controlling the accumulation of organic matters,and the mechanism of organic matter enrichment conform to the“preserved model”but not“productivity model”.?4?The Wufeng Formation was deposited in a strong restrict anoxic water column according to the U-Mo covariation.Meanwhile,a relatively moderate restrict water mass prevail during the depositional of Longmaxi Formation..?5?The late Ordovician-early Silurian in Sichuan Basin record a positive shift both in the carbon isotope and nitrogen isotope.The caused of carbon isotope shifts was the increased organic carbon burial fraction and development of black shale,which conform to the“burial michenism”.The increase in?¹⁵N values recorded during the Hirnantian glaciation may have been influenced by the long term anoxic environment and enhance of denitrification in sediment porewaters.