The Shale Reservoir Characteristics Of Chang 7 Member In The Yanchang Formation And Its Controlling Factors On Gas-bearing,the Southeastern Part,Ordos Basin

The first exploration breakthrough of continental shale gas was obtained in the Triassic-deep lacustrine shale in the Yanchang formation in southeastern region of Ordos basin,belong to the Yanchang oil company.The re-examination results of numbers of exploration wells and old wells show that there is a great potential of shale gas resources in this area.However,the study of Continental shale reservoir and gas-bearing characteristics is not good enough,which restricts the further exploration and development of shale gases in this area.The gas-rich shale reservoir in the 7^th member of Yanchang formation?Chang 7?in southeastern Ordos Basin,was chosen as research object because of its most abundant data available and was studied based on the investigation of its sedimentary background and source direction in the research area by using outcrop,core,logging,and other data,combined with the thin-sections observation,scanning electron microscope observation,X-ray diffraction analysis and other methods and techniques.Firstly,we identifies the sedimentary facies of the Chang 7 member,divides the sedimentary facies based on the their depositional characteristics,and determine the distribution of sedimentary facies,including the shale buried depth and horizontal distribution in the area.Secondly,this paper studies the lithological combination zoning characteristics and mineral composition of Chang 7,as well as the geochemical characteristics such as kerogen type,organic matter abundance and thermal maturity using parameters from rock pyrolysis,organic carbon content,vitrinite reflectance,kerogen maceral composition analysis.Thirdly,the characteristics of pore and fracture development of shale reservoir are summarized.The characteristics of shale pore and specific surface area are analyzed,and the properties of shale reservoir are analyzed by comprehensive physical property analysis and isothermal adsorption,etc.In addition,based on the understanding of the basic characteristics of shale reservoir,the main factors affecting porosity and fracture development of shale are summarized,and the influence of tectonic action on porosity and fracture development of shale reservoir is analyzed,too.Finally,according to the on-site gas desorption and the isothermal adsorption experiment,combined with the gas logging data,the gas-bearing characteristics of the 7 shale in the research area was studied qualitatively and quantitatively and the characteristics of the different occurrences of the shale gases are studied.The main control factors of the different occurrence state shale gas are further analyzed,and the model of controlling the gas content in the shale is established.On the basis of the above research,together with the characteristics of shale reservoirs and geochemical characteristics in this area,the appropriate assessment parameters are selected to evaluate and forecast the favorable area of shale gas in the study region.The study of Chang 7 shale in the southeast of the Ordos Basin shows that:?1?The overall depth of the Chang 7 has the tendency of the deepening from east to west,and thickness of shale in the 3^rd and 2^nd sections of Chang 7?Chang 7³ and 7³?are mostly distributed in the southwestern part of the research area.?2?The organic carbon content of Chang 7 shale is4.59 wt%,the organic matter type is mainly?₁ type and?₂ type,and the thermal maturity level of organic matter is mainly in mature stage.?3?The pore types in the Chang7 shale reservoirs are mainly composed of primary intergranular pores,dissolution pores and organic matter-hosted pores due to hydrocarbon generation,followed by intergranular pores,cracks and inner-granular pores with medium-good porous structure,average porosity at 2.36%and permeability at 3.18 mD.?4?The fracture development of 7 shale pores is mainly controlled by the degree of organic matter abundance and maturity,mineral composition,laminar development and tectonic movement.?5?The Chang 7 shale has a tendency in gas-bearing content of decreasing from west to east and from south to north generally.Gas-rich layers are mainly concentrated in the bottom of Chang 7² and Chang 7³.The adsorption gas has the biggest contribution to the shale total gas content,and is mainly controlled by the organic matter abundance,the organic matter maturity,the clay mineral content and the development of micropore.?6?The Chang 7 shale is divided into three grade promising regions,such as the prospect,favorable area and the target area.The shale gas favorable zone in the Chang 7² and 7³ is concentrated in the southwestern part of the research block,the southwestern part is the most favorable region for shale gas exploration.