Sedimentary Characteristics And Basic Geological Conditions Of Shale Gas In Lower Carboniferous Jiusi Formation In Weining Area,Southwest Guizhou

Shale gas is a kind of natural gas storing in black shale or carbonaceous shale,which has huge resource potential and has been paid more and more attention as a new field of oil and gas exploration.The Lower Carboniferous Jiusi formation is an important shale gas target formation in Guizhou province,and its sedimentary center is Weining county.The fold and fault structures are well developed in the study area,and a typical thrust nappe structure system is formed.The lower Carboniferous Jiusi formation in the study area is influenced by the main fault ziyun-yadu fault.Based on the theory of sedimentary petrology,lithofacies paleogeography,well logging geology and oil and gas geochemistry,the basic geological survey of shale gas is further carried out on the basis of previous work.Four geological sections have been measured in the Carboniferous exposed area of Weining area:Heituhe section of Weining;Majiayakou section of Yulong town of Weining;Mulaqing section of Yina town of Weining and Hengdi section of Longjie town of Yiliang County.In addition,the core of GWD1 well and cuttings of ZK2 well in Weining area are observed,and the changes in the logging curve were analyzed,and samples were taken for analysis.It is recognized that in the study area,The Jiusi formation in the study area is in integrated contact with the bottom Yanguan formation and the overlying Shangsi formation.The lithology of the lower member of Jiusi formation?C₁j¹?is mainly black carbonaceous shale,fine sandstone,siltstone and argillaceous siltstone.Coal line is developed in shale,and parallel bedding is developed in argillaceous siltstone.The lithology of the upper member of Jiusi formation?C₁j²?is mainly dark gray medium thick limestone,marl and black shale interbedded.The strata of Jiusi formation are distributed in Weining area.The thickness of the strata varies greatly,ranging from 200m to 1000m.The thickest is located in Caohai-heituhe of Weining,which thins rapidly to the East and West until it pinches out.According to the analysis of the sedimentary marks of the Jiusi formation of Lower Carboniferous in the study area,it is considered that the sedimentary environment of the Jiusi formation is coastal shallow sea.According to the analysis results,the area can be divided into two types of sedimentary facies:shore facies and shelf facies.Under the influence of transgression,the sedimentary facies of the lower member of Jiusi formation is shoreline to clastic shelf from land to sea.With the rising of sea level,the scope of the shelf has been expanded.In the upper member of the Jiusi formation,the sedimentary facies are:shore,clastic shelf and mixed shelf from land to sea.According to the analysis of sampling test data and collected data in the study area,the TOC of black shale of Jiusi formation is ranges from 0.80%to 2.40%,and the distribution of main body is ranges from 1.00%to 2.00%,the highest is in Caohai town-Xueshan town-Heituhe town.The shale with higher TOC is concentrated in the lower section of Jiusi formation.The types of kerogen are?₁ and?₂.The Ro of the dark shale of Jiusi formation in the study area are ranging from 1.44% to 2.68%,with the main body are ranging from 1.50%to 2.50%.It is in the mature or over mature,and wet gas or dry gas,which is conducive to shale gas accumulation.The highest Ro is located in Heitu River,which is more than 2.5%.It is dry gas,and it is positively correlated with burial depth in longitudinal direction.The content of clay minerals and brittle minerals are very high in the black shale of Jiusi formation in the study area,which indicates that the organic rich shale is easy to form natural fractures and fissures.The content of carbonate in the Jiusi formation is very small,which is only visible in GWD1 well.The high content of illite and illite/smectite clay minerals indicates that the black shale of Jiusi formation has entered the stage of late diagenesis.The high content of illite and illite/smectite indicating that the black shale of Jiusi formation has entered the stage of late diagenesis.The roof of Jiusi formation in the study area is Shangsi formation?C₁s?,and the floor is Yanguan formation?C₁y?.The formation thickness is large,the distribution is wide,the lithology is mainly limestone and relatively dense,which can form a good trap condition.The thickness of organic rich shale?TOC>1%?is closely related to the distribution of sedimentary facies.The sedimentary center is be range from Caohai town to Xueshan town,which decreases rapidly to the East and west sides until it pinches out.Through the analysis of TOC value,organic matter type,Ro value,buried depth,reservoir physical property and shale gas preservation conditions,it is concluded that the area with shale gas accumulation potential is located in the southeast wing of haiku syncline.