Relationship Between The Origin Of Organic-rich Shale And Geological Events Of The Upper Ordovician-lower Silurian In The Upper Yangtze Area

Organic-rich shales have been deposited throughout the geological history of Earth in specific geological periods and geological environments.Organic-rich shales are the products of changes in the interactions of the lithosphere,hydrosphere,atmosphere and biosphere and are also the symbol and embodiment of the evolution of open complex dynamical systems of the earth.Widely distributed organic-rich shales have been preserved in the Yangtze Platform of the South China Craton.The Upper Ordovician Wufeng-Lower Member of Lower Silurian Longmaxi shales are excellent source rocks with abundant siliceous minerals and organic matter.The sedimentary processes and development characteristics of these units were likely influenced by environmental changes on geological timescales.This study is aim to study the development characteristics and change of shale lithofacies based on establishing a high frequency(4^th order)sequence stratigraphic framework in the study area.Then identify volcanic activity,Hirnantian Glaciation,bottom-current activity and turbidity current of Late Ordovician-Early Silurian and their evolutionary sequence using sedimentological observations and geochemical analyses?e.g.,petrology,mineral composition,sedimentary structure,biology and geochemistry?.Finally deduce the influence of geological events on sedimentary environment and composition and source of shale,and establish a genetic depositional model of shale by the combination effects of four geological events.In addition,the significance of this genetic model for shale gas exploration and development can then be revealed.Based on the identification of the 3 sequence boundaries and 2 maximum flooding surfaces,the Late Ordovician Wufeng Formation to the Early Silurian Longmaxi Formation were interpreted as two sequences using well logs,well cores,outcrops,mineral composition and paleontology data.From the bottom to the top,the Ordovician Wufeng and Guanyinqiao Formations were interpreted as Sq1,which consists of a transgressive systems tract 1?TST1,Parasequence Set 1?and a highstand systems tract?HST,Parasequence Set 2?.TST1 is represented by the siliceous shale of the Wufeng Formation,and the lime mudstone and siliceous mudstone of the Guanyinqiao Formation corresponds to the HST.The Lower Member of the Silurian Longmaxi Formation was interpreted as Sq2,with a transgressive systems tract 2?TST2,Parasequence Sets 3 and 4?,an early highstand systems tract?EHST,Parasequence Sets5,6 and 7?,and a late highstand systems tract?LHST,Parasequence Sets 8,9 and 10?.TST2 is characterized by siliceous shale,and EHST and LHST are represented by mixed?siliceous/clay?shale and clay-rich shale,respectively.The overall changes of shale lithofacies within chronostratigraphic framework show that the carbonate mineral content increased slightly from TST1 to HST,and siliceous mineral content decreased while the clay mineral content increased from TST2 to LHST.On the basis of establishing a high-frequency sequence stratigraphic framework,volcanic activity,Hirnantian Glaciation,bottom current activity and turbidity current were identified using petrology,mineral composition,sedimentary structure and paleontological features.Bentonite layers in the study area may suggest that volcanism occurred during the TST1 and TST2.According to the stratigraphic characteristics of bentonite rich interval,TST1 is classified as dense interval for bentonite development:the frequency of bentonite development?bentonite layers/time?is more than 1.5,and the cumulative thickness of bentonite?thickness of bentonite layers/thickness of shale?is more than 1%,which suggests first stage of intense and high-frequency volcanic activity.TST2 is classified as the sparse bentonite interval?frequency<1.5;cumulative thickness<1%?,which represents second stage of moderate and low-frequency volcanic activity.The HST yields abundant shelly fauna,demonstrating intense glaciation during this stage.The sea-level fall mainly affected the Qianzhong old land and its adjacent areas,and the Fuling-Nanchuan areas were mainly affected by lower ocean temperature during the glacial expansion.The subsequent glacial retreat caused a rapid sea-level rise during the TST2.The drag structure,preferentially oriented arrangement of minerals and graptolites,along with the“sediment drift”featured by silty and shaley laminae may suggest that the EHST?Parasequence Sets 6 and 7?was influenced by bottom-current activity.The sedimentary characteristics of Bouma Sequence?deformation structures and erosion surface,and with increasing clay minerals and siltstone during the LHST reflect that turbidity current caused more turbulent environment and higher terrigenous inputs.Based on the study mentioned above,the sedimentary processes of the organic-rich shales affected by these four geological events were deduced using geological and geochemical data.The genetic depositional model is as follows.Intense and frequent volcanic activity has dual promoting effects on organic-rich siliceous shale during the TST1 period.Volcanic ash can provide a significant supply of nutrients,which triggers high marine productivity.The extremely anoxic environment caused by volcanic activity lead to rapid burial and high levels of preservation of deposited organic matter.The low ocean temperature and relatively sea-level fall caused by the Hirnantian Glaciation was responsible for the lime mudstone and siliceous mudstone with moderate TOC content in the HST.The second stage of volcanism,favorable preservation conditions,a biotic recovery and high organism productivity during the postglacial sea-level rise stage played significant roles in the formation of the organic-rich siliceous shales in the TST2.Bottom-current activity?mainly in the Parasequence sets 6 and 7?has reconstruction effects on the organic-rich shale,which debased preservation potential of organic matter and formed the mixed shale lithofacies with moderate TOC content in the EHST.Turbidity current increased the clay mineral and enhanced terrigenous detritus,which destroyed anoxic environment and further debased preservation potential of organic matter in the LHST.Volcanism and postglacial effects not only benefit to form excellent source rocks in the TST1 and TST2,but also conducive to forming diverse reservoir space,and are favorable for artificial hydraulic fracturing and gas-bearing property in shale gas exploration and production.