Reservoir Characteristics And Shale Gas Preservation Of The Longmaxi Formation In The Changning Area,South Sichuan

To clarify the shale gas reservoir characteristics and gas reservoir preservation conditions of the Longmaxi formation in the Changning area,South Sichuan,on the basis of finding out the stratigraphic and structural background of the study area,this paper takes literature research,field geological investigation and laboratory experimental analysis as the approaches,structural geology,mineral petrology and sequence stratigraphy as the guidance,and software programming and information visualization as the means to accurately characterize the source rock-reservoir characteristics of Longmaxi shale,and reconstructs the formation,adjustment and dissipation of gas reservoir,the hydrocarbon generating potential of source rock and the preservation conditions of gas reservoir are evaluated,and the favorable areas for shale gas exploration are delineated,and the following knowledge is obtained:The Longmaxi formation is a set of black rock series,rich in graptolite and brachiopod fossils,the thickness of the Longmaxi shale is mainly in 100-500 ranges;The mineral components are divided into brittle minerals and non brittle minerals,in which the former is dominant and the latter is mainly clay minerals;The organic matter types are mainly type I and type II₁,with high hydrogen and low oxygen characteristics and good hydrocarbon generation potential;The total organic carbon content?TOC?is in 1.69%-9.73%ranges,and the TOC value of the bottom sample is significantly higher;The thermal maturity?R_o?value is in 2.0%-2.84%ranges,the thermal evolution degree is high.Micro and nano scale organic pores,intergranular pores,intragranular pores and microcracks are widely developed,with large pore size range and various pore forms,among which organic pores and clay mineral pores are the most developed;The pore types are mainly parallel plate pores,narrow neck wide body pores and ink bottle pores;And the contribution rate is macropores>mesopores>micropores,the specific surface area is mainly contributed by micropores;TOC and clay minerals contribute well to pore volume and specific surface area,which is positively correlated;Brittle minerals are generally not conducive to the development of pore volume and specific surface area;Macropore fractal dimension?D₃?>mesopore?D₂?>micropore?D₁?,indicates that the larger the pore size is,the higher the complexity of the pore surface is,and TOC is negatively correlated with D₂,positively correlated with D₃,brittle mineral content is positively correlated with all three,while clay mineral content is the oppsite.The total erosion thickness of the overlying sediments of the Longmaxi is more than 3000 m,including less than 500 m in the Silurian,Devonian and Carboniferous,and more than 2500 m in the overlying sediments of Triassic;The sedimentary burial history of the Longmaxi formation is mainly includes six stages,of which the maximum burial depth is reached in the Early Triassic to the end of Jurassic;The thermal evolution history is divided into three stages,of which the highest temperature is reached in the early Triassic to the early Cretaceous,about 210?;The evolution of hydrocarbon generation is divided into four stages,in which the Longmaxi formation entered the peak of oil generation from the end of Carboniferous to the end of Triassic,and reached the peak of gas generation from the middle Jurassic to the early Cretaceous.Shale has the characteristics of"low porosity and low permeability",with excellent sealing performance,the sealing mechanism includes physical property sealing,pressure sealing and hydrocarbon concentration sealing;The thickness of high-quality shale in the study area gradually increases from south to north;the lithology of underlying limestone of Baota formation and silty shale in the upper Longmaxi formation is dense,with good sealing ability,which can effectively prevent the vertical loss of gas;The Shiniulan formation-Hanjiadian formation and the Jialingjiang formation are two sets of high-quality regional caprock;In structure,the Jianwu syncline and the Luochang syncline have gas enrichment conditions,the large and medium-sized faults are mainly developed in the core of the Changning anticline and the erosion area and shallow burial area of the Longmaxi formation in the east of the study area.In the core area of the Jianwu syncline,the pressure coefficient is the highest,which is more than 2.0%,gradually decreasing to both sides,and the core area of the Changning anticline is mostly erosion area;In general,the gas content of the Longmaxi formation is higher,which is generally greater than 1.8 cm³/g;The calculation results show that the loss ratio is between 19.00%and 22.40%;The core area of the Changning anticline is mainly the erosion area and shallow burial area,which are non economic areas,and gradually transition to the two wings into class?and class?reservoirs;The preservation conditions of the Luochang syncline and the Jianwu syncline are the best;In addition,the burial depth of the north east corner is more than 4500 m,due to the limitation of mining technology,it is temporarily divided into class?reservoirs;Generally speaking,class?and class?reservoirs have great exploration and development value.The Changning area is one of the key areas of marine shale gas development in South China,the research results have certain reference significance for shale gas development in the study area,and certain theoretical significance for the study of shale gas reservoir selection and evaluation parameters.There are 87 pictures,18 tables and 293 references in this paper.