Geological Characteristics And Favorable Target Selection Of Longmaxi Formation Shale Gas In Dingshan Area, Southeast Sichuan

A group of organic shale is widely developed in the Longmaxi Formation of Lower Silurian in southeastern Sichuan,which has huge shale gas exploration potential.The Dingshan area and surrounding areas have made many breakthroughs in recent years,but there are still insufficient geological features and evaluations of shale gas in this area.Therefore,based on the regional geological background,This master's thesis studies the geological characteristics of shale gas in the Longmaxi Formation through field outcrops observation,logging observation,drilling core observation and experimental analysis.Combined with the evaluation methods of typical shale gas basin selection areas at home and abroad,this paper discusses the parameters and thresholds suitable for evaluating favorable areas,preferably in the Dingshan area.The paper strives to provide a theoretical basis for the next step of shale gas exploration and development.The main understanding of the paper is as follows:The Longmaxi Formation in the Dingshan area is basically intact.According to the characteristics of lithology,electricity and paleontology,the Longmaxi Formation is divided into two sections.From the S1L2 to the S1L1,the rock color changes from shallow to deep,the stone content increases gradually,and TOC content increases gradually.S1L1 Formation is the main gas producing Formation for shale gas exploration and development.The main research is on the S1L1 Formation,and the S1L1 Formation is divided into the S1L11,S1L12 and S1L13.Tne Longmaxi Formation is located in the sedimentary facies belt of the hydrostatic,anoxic and reducing environment of the offshore shed,which can be divided into shallow water shelf subfacies and deep water shelf subfacies.The TOC content ranged from 0.42%to 6.50%,average being 2.82%.The distribution of TOC was the largest in the range of 2%-4%.The TOC content increased gradually with the increase of the buried depth in the longitudinal direction.The main distribution of high quality shale(TOC?2%)at the bottom of the Longmaxi Formation.The thickness of high-quality shale in the whole study area is about 30m;The organic matter type is mainly type I through the analysis of the kerogen composition and kerogen carbon isotope(?13C);Both plastid reflectance and rock pyrolysis analysis showed that the maturity of organic matter had reached a high and mature stage,and the maturity(Ro)ranged from 1.84%to 2.63%with average being 2.19%.Through the X diffraction analysis,the mineral components include quartz,feldspar,calcite,dolomite,pyrite,and clay minerals,among which siliceous minerals are the most abundant,and clay minerals are second.The bottom of the Longmaxi Formation shale has a high content of siliceous and TOC,and has a "two high" coupling characteristics.The content of brittle minerals decreased gradually from bottom to top.The brittleness index values calculated by mineral component method and Rickman brittleness index method were 49.1%and 52.6%respectively.The pores were observed by argon ion polishing experiments,and the pores were divided into organic pores,inorganic pores and micro-cracks.The cracks were observed through core observation and imaging logging data,and the cracks were divided into structural joints,inter-layered joints,slip-off joints and pressure-dissolved joints.Porosity distribution is between 1.22%and 7.12%,average being 4.07%,mainly distributed in 2%?4%.From the S1L13 to the S1L11,the porosity increases gradually.S1L11 has the highest porosity,generally more than 4%;permeability is between 0.00014mD and 39.38mD,40%of the total sample permeability is less than 0.01mD,the longitudinal penetration is much smaller than the lateral permeability,which can be found by full-diameter permeability.The gas content of shale is between 1.03m3/t?12.53m3/t measured by analytical method.The total gas content increases with the increase of depth,which is more than 3.0m3/t in the largest at the bottom of Longmaxi Formation.11 key parameters,namely effective shale thickness,organic geochemical characteristics,shale brittleness index,porosity,gas content,buried depth,top and bottom plate conditions,distance from Qiyueshan fault,fault development degree,formation pressure coefficient,were selected to evaluate the geological characteristics of the shale in the Longmaxi Formation.And the multi-factor integrated superposition method is used to optimize the favorable area for shale gas exploration.