Quantitative Characterization Of Lithofacies And Reconstruction Of The Sedimentary Process For Upper Yangtze Wufeng And Longmaxi Organic Rich Shales

The Ordovician-Silurian transition is a time interval when the global plate convergence and a series of environmental perturbation took place and triggered the 1st Mass Extinction in Earth's history.During this crucial period,the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation,containing organic-rich shale,were widespread in South China and are the most important shale gas producers in China nowadays.The deposition of these organic-rich shales was associated with some contemporaneous global and regional geological events,such as the Hirnantian glaciation,global marine transgression,volcanic events,bottom current intrusion and etc.It is of great significance to conduct high-resolution stratigraphic construction,isochronous correlation,shale facies classification,paleoclimate and paleomarine environment reconstruction in order to better understand the genetic mechanism and interrelation between the geological events and the deposition of the Wufeng and Longmaxi formations.This thesis made use of data from drillcores,outcrops,geochemical analyses(sampling at 0.01-1m interval)to investigate the spatial and temporal distribution of the Wufeng and Longmaxi organic-rich shales within the isochronous stratigraphic framework in the Upper Yangtze area.The thesis also focusses on the paleomarine environment,paleoclimate and paleoproductivity during the deposition of the Wufeng and Longmaxi formations,hence successfully reveals the genetic mechanism and controlling factors of different lithofacies in order to guide advantageous lithofacies prediction in the future shale gas exploration.The major breakthrough of this study are as follows:1.Under the biostratigraphic constraints(Age of graptolite biozones),a high-resolution isochronous sequence stratigraphic framework of the Wufeng and Longmaxi formations was constructed based on the integration of well log data,mineralogical,geochemical and petrophysical analyses.Detailed stratigraphic correlation among the Upper Yangtze area was achieved.The Wufeng Formation was interpreted to be a condensed section with four cycles of sea-level fluctuation which was driven by the 1.2-myr Earth long-obliquity cycle.The average sedimentation rate of the Wufeng Formation was estimated to be 0.2cm/kyr.The Longmaxi Formation consists of three members.The Long-1 Member was interpreted to be a 3rd-order depositional sequence that can be subdivided into a transgressive systems tract(TST),an early highstand systems tract(EHST)and a late highstand systems tract(LHST),with an average sedimentation rate of 2.5cm/kyr.The paleogeography of the Upper Yangtze area was featured by containing 3 sags and 2 submarine highs in between the sags.and the depocenter of each sag was showing a northwestern migration trend throughout the Ordovician-Silurian transition.2.The Wufeng and Longmaxi organic-rich shales display varying total organic carbon content ranging from 0.2% to 6%.Four major types of shale lithofacies were identified including siliceous shale,carbonate shale,mixed shale and argillaceous shale.and they can be further divided into 13 subtypes.By constructing three large basin-scale cross sections,the spatial and temporal distributions of each lithofacies within the isochronous stratigraphic framework in the Upper Yangtze area were clearly revealed.Based on data collected from 258 drill cores and 120 measured profile of outcrops,the sedimentary thickness and the depocenter of each lithofacies within each system tract was clearly exhibited.The results show that the lithofacies distribution pattern in the Upper Yangtze area was primarily dependent on the landform structure of the basin,paleoclimate and paleomarine conditions,and the influences of dominant geological events.3.Systematic sampling was carried out in 3 sags.Using collected samples,this thesis constructed high-resolution geochemical profiles of the Wufeng and Longmaxi organic-rich shales after conducing detailed geochemical analyses.As the geochemical features reveal the fingerprint of global and regional geological events within the sedimentary archive.The proxies including the paleoproductivity,redox conditions,and terrigenous input were used to examined the deposition environment across the Ordovician-Silurian transition.The outer shelf of the Upper Yangtze area was weakly restricted and prone to the influence of upwelling and bottom current instruction.It was associated with high primary productivity and good preservation conditions during the deposition of the Wufeng Formation and Long-1 TST,and significant clastic dilution during the deposition of Long-1 EHST and LHST.The middle shelf was moderately restricted,subjected to seasonal upwelling during the deposition of the Wufeng Formation and Long-1 TST and intermittent bottom current instruction during the deposition of Long-1 EHST.The inner shelf was strongly restricted,showing good preservation conditions throughout the transition.On the other hand,the volcanic influx was conducive for the development of organic siliceous shale though favoring the boom of primary productivity on surface water and expansion of oxygen minimum zone in the water column.4.This thesis reconstructed the depositional history and formation mechanism of different lithofacies under the influence of the contemptuous global or regional geological events,hence established the genetic and prediction models of lithofacies within the Wufeng and Longmaxi formations based on the following parameters.The development of Wufeng and Long-1 TST siliceous shale was greatly influenced by volcanic and upwelling events through primary productivity elevation.It is characterized by low sedimentation rate(<1cm/ky),anoxic-euxinic environment(V/Cr>4),high paleoproductivity(Ba/Al >300ppm/%)and high TOC%>4.The development of mixed shale in the Long-1 EHST was under moderately restricted basin environment and disturbed by bottom current intrusion.It is characterized by low speed bottom current flow,moderate sedimentation rate(>1-2cm/ky),dysoixc environment(V/Cr>2-4),moderate productivity(Ba/Al >200-300ppm/%)and medium TOC%>2-4.The gray-argillaceous shale facies in Long-1 EHST is the product of enhanced terrigenous input due to regional regression.It is characterized by low flow velocity and high sedimentation rate(>4cm/ky),dysoxic-oxic environment(V/Cr<2),low paleoproductivity(Ba/Al <200ppm/%)and low TOC%<2.The Wufeng and Long-1 TST siliceous shale was recognized to be the most advantageous lithofacies for shale gas exploration and production.