Environmental Evolution And Patterns Of Organic Matter Enrichment Of Late Ordovician-Early Silurian In The Southern Margin Of Sichuan Basin

Late Ordovician-Early Silurian transition period is an important geological history period.Global plate tectonics,paleoclimate conditions,paleo-ocean environment and biota types occurred during this relatively short period of time(453?440 Ma).With these great changes,the Yangtze area deposited two sets of organic-rich shale Wufeng Formation and Longmaxi Formation under this global background.It is a key exploration layer for shale gas exploration and has received extensive attention.In this thesis I conduct the research chronologically on the stratum of Wufeng Period(corresponding to Katian),Guanyingqiao Period(mainly corresponding to Hirnantian)and Longmaxi Period(corresponding to Rhuddanian).Systematic measuring and continuous sampling were conducted.The sedimentary facies and lithofacies paleogeography of Wufeng Formation,Guanyinqiao Formation and Longmaxizu Formation were restructed based on the field geological survey and the petrology and mineralogy data.Shale gas material basis was briefly analyzed in the study area based on the data of organic geochemistry,polishing-scanning electron microscope(SEM)and pore structure.Major,trace,and rare earth elements researches were conducted to make better sence of the vertical sedimentary environment evolution of Wufeng Period to Longmaxi Period and establish the patterns of organicmatter enrichment.Following achievements were obtained:? Affected by multiple geological events,the lithofacies and sedimentary facies changed significantly and rapidly of the Late Ordovician-Early Silurian in the southern margin of the Sichuan Basin.The main manifestation is that the weakening of the paleo-continental activity caused the decrease of the terrigenous flux and the weakening of the tectonic skew.In addition,the impact of the global glacial events led to the marine regression during Hirnantian period and formed a set of carbonates Guanyinqiao Formation which is between two sets of black shale.?The appearance of calcarenite at the top of the Wufeng Formation in the southern margin of the Sichuan Basin indicates the development of local small-scale underflow caused by the frequent occurrence of volcanic activities during Wufeng period.It also implies that the climate of entering the ice age did not continuous shift to be cold.It is the petrological evidence of the alternating volatility of cold and warm under the general trend of overall temperature reduction.It is also suggested that the cold wheather was not the only cause of the extinction and the recovery of the Hirnantian funa was in relatively warm condition.?The southern margin of Sichuan Basin which is characterized by large stratum thickness,high TOC content,moderate Ro and good development of the small-scale pores and fractures has great material basis of shale gas.The study indicates that in addition to the Changning area,Wufeng-Longmaxi Formation in the north of Xuyong-Gulin-Xishui area has significant potential for shale gas enrichment which deserves further exploration.?The average values of Al/(Al+Fe+Mn),?Ce and ?Eu are 0.65,0.77 and 0.86,which indicate that excess-Si(Sixs)is in biogenic origin.The contents of Si and Sixs which are significantly negative correlated with the contents of Ca and Ti are mainly affected by terrigenous flux and paleo-water depth.The lower values of Mo/TOC and Mo/U indicate that the extreme degree of watermass restriction of the early period of Wufeng Formation,the reason for which was that a lot of underwater paleouplifts were out of the sea surface caused by the decline of sea level which divided the Yangtze Sea into multiple restricted areas.Organic-rich intervals formed in moderately restricted environment.The intervals with dense bentonite layers have higher paleoproductivity.?Stronger positive correlation between paleoredox proxy coefficient and TOC indicates that paleoredox is the main controlling factor of organic matter enrichment.Due to the different biohabitat types and the different influences of terrigenous flux on paleoproductivity and paleoredox,the correlation between paleoproductivity and TOC is poor in the mass.A comprehensive analysis suggests that organic matter enrichment patterns in different sedimentary stages of Wufeng-Longmaxi Formations are different.Organic matter enrichment of Wufeng-Longmaxi Formation was mainly effected by anoxic environment which was caused by deep water environment with low terrigenous flux during non-extinction period.It was mainly effected by high sedimentation rate of bio-precursors which was caused by explosion of bio-precursors and "marine snow" during the first episode extinction period(corresponding to the terminal stage of Wufeng Period),and by high sedimentation rate of bio-precursors and extremely anoxic environment caused by rapid transgression and organics decomposition during the second extinction period(corresponding to the incipient stage of Longmaxi Period).