Characteristics Of Fine-Grained Sedimentary Rocks And Prospects Of Shale Gas During The O-S Transitional Period In The Leibo Area Of The Southwestern Upper Yangtze

During the transitional period between the Ordovician and Silurian on the southwestern margin of the Upper Yangtze,a set of fine-grained sedimentary rocks represented by the Wufeng-Longmaxi Formation were deposited,and the carbonates were also accepted in the Upper-middle Yangtze Region since the Early-middle Cambrian Dragon King Temple.The first set of fine-grained clastic deposits since sedimentation contained not only profound structural-sedimentary basin evolutionary information,but also a set of fine-grained sedimentary rocks dominated by shale,with large thickness,stable distribution,and high organic matter abundance.Features such as brittle mineral development have the potential to form large-scale shale gas resources.Since it has undergone multi-phase structural transformation since the Hercynian period,it is on the southwestern edge of the present Sichuan Basin.The information on the tectonic-sedimentary basin and shale gas contained in this set of fine-grained sedimentary rocks is still far from the basin’s abdomen.The same hierarchy is comparable.This severely restricts people’s understanding of the tectonic-sedimentary evolution of the Upper Yangtze region during the O-S transitional period and the objective evaluation of shale gas resource potential.Based on this,this article carried out a 1:1000 profile survey in the study area,and systematically collected series of samples that revealed the sedimentary environment and shale gas geological characteristics of the Wufeng-Longmaxi Formation,through geochemistry,pyrolysis experiments,and specific surface area The parameters of thickness,organic matter type,abundance,and vitrinite reflectance of the fine-grained sedimentary rocks in the Wufeng-Longmaxi Formation were obtained by pore size analysis and scanning electron microscopy.A shale gas geology comprehensive section of the study area was established,and the upper The sedimentary environment of fine-grained sedimentary rocks during the transitional period of the O-S in the Leibo area of the southwestern margin of the Yangtze,the developmental intervals of the organic-rich shale,spatial distribution,effective thickness,burial depth,and organic geochemical parameters,etc.,according to which,Leibo was selected.Regional shale gas favorable target area.The O-S transitional fine-grained sedimentary rocks in Leibo area on the southwestern margin of Upper Yangtze are mainly shale,carbonaceous shale,carbonaceous mudstone,mudstone,calcium-bearing carbonaceous shale,and contain a small amount of mud-microcrystalline limestone.Based on the rock composition characteristics,mineral distribution characteristics,and biological fossils of fine-grained sedimentary rocks,the Wufeng-Longmaxi Formation in Leibo District was divided into two sedimentary subfacies:shallow water shelf and deep water shelf.The deep-water shelf deposition developed in the Wufeng Formation-In the lower section of the Longmaxi Formation,the lithology is dominated by black carbonaceous shale and carbonaceous mudstone;the minerals are characterized by high quartz content,low clay mineral content,and the appearance of pyrite;developmental horizontal bedding,and abundant pen fossils Mainly distributed in Leibo-Yongshan-north of Yanjin area.Shallow water shelf sediments developed in the upper part of the Longmaxi Formation,and the lithology is dominated by yellow ash and gray-dark gray silt mud（page）rocks.The thin layer of argillaceous siltstone,mudstone,and biofouling limestone is evenly clamped.Features of gypsum;clay mineral content,low quartz content;development level bedding,local development of a small amount of staggered bedding,visible pen fossil production;mainly distributed in the Meigu-Jinyang-Weixin area.Due to the impact of the central Guizhou uplift,the study area showed a transitional relationship between the late Ordovician and the early Silurian sedimentary facies from south to north,and formed the northwestern town of the south bank and the thunder within the foreland basin of the Central Guizhou Uplift.Two high-TOC fine-grained sedimentary sediment centers in the northern part of the county of Po County.In the Wufeng-Longmaxi Formation of the study area,the organic kerogen was dominated by saprophytic components,followed by the inertin component,a small amount of vitrinite components,and almost no chitin component.The kerogen type was mainly based on type II₁ indicating the study area.The Wufeng Formation-Longmaxi Formation organic matter is mainly supplied by algae and other aquatic organisms.Overall,the organic carbon content of the TOC in the lower part of the Longmaxi Formation was significantly higher than that in the upper part,and the upward organic carbon content gradually decreased.It was formed on the plane near Zhongdu Town and Nanan Town and in the east of Leibo County.Two high-value areas with an organic carbon content of more than 4%have gradually decreased in other areas.The asphalt reflectance（R_b）distribution is between 2.86%³.55%with an average of 3.13%.The vitrinite reflectance（R_o）is distributed between 2.34%².72%with an average of 2.47%.The T_max distribution was between 345℃⁵78℃,indicating that the thermal evolution of the study area reached a high maturity-over-mature stage;the TOC was positively correlated with the quartz content and negatively correlated with the clay mineral content.Argon ion polishing-SEM experiments show that shale reservoirs are of various reservoir types.The main types of pores developed are mineral dissolution pores,organic pores and irregular microcracks.The characteristics of isothermal adsorption curves are very steep and increase rapidly,indicating that the adsorption capacity is large.The shale has a larger specific surface area and strong adsorption capacity.The porosity is positively related to the permeability and the feldspar content;the TOC is positively correlated with the BET specific surface area,the total pore volume of the BJH and the average pore diameter of the BJH,and the total volume of the clay mineral and the BJH.Negative correlation.Combined with the characteristics of rock thickness,geochemical parameters,and reservoir characteristics in the Leibo area,the Qingshui shale gas prediction area of Qili Town,Qijiang County was obtained using the integrated information superposition law.