Study On Tuff Reservoir Characteristics And Formation Mechanism Of The Chang 7 Member In The Southern Ordos Basin

The tuff which is closely related to volcanic activity,has become one of the research hotspots in the field of earth sciences.However,there are still few studies on tuff in the Jinghe area of the southern Ordos Basin,especially for the study of the sedimentary genesis,material sources and the characteristics of the tuff reservoirs in the study area,and there is no typical and systematic description method of tuff reservoirs.The study focuses on the tuff reservoirs in the Chang7₃ section at the bottom of the Chang 7 member,the purpose of this thesis is to clarify the lithofacies types and sedimentary genesis,the source of volcanic material,the characteristics and its controlling factors of the tuff reservoirs in the study area,by observing core and field profiles,and using experimental methods such as optical microscopy,SEM,XRD,major and trace element analysis,nitrogen adsorption,and high-pressure mercury intrusion.Studies have shown that tuff reservoirs can be subdivided into vitric tuff,crystalline vitric tuff,tuffite,tuffaceous sandstone and crystal tuff.The content of tuffite is the highest.The lamellar tuff in“Zhangjiatan Shale”is formed by sedimentation of volcanic clastic materials,and tuffs at the bottom are deposited by gravity flow such as debris flow and turbidity.The major and trace element analysis shows that the tuff reservoir is characterized by medium acidity,low sodium and high potassium.The tuff at the bottom is more acidic and trace elements are rich,but the element U is rich in the lamellar tuff.The parent magma of the tuff belongs to the feldspar series magma,the source magma is formed in a volcanic arc environment related to the subduction,and mixed contamination of continental crust material.Macropore is considered as the main storage space of tuff reservoir.The pore types of vitric tuff and crystalline vitric tuff are mainly intergranular pores,with coarser pore-throats,poor pore-throat sortability;crystal tuff is mostly developed with crystal surface cracks,the content of intergranular pores in clay minerals of tuffite and tuffaceous sandstone is high and the pore-throat sorting is better.The study shows that the higher the content of glassy debris and crystal debris,the better the reservoir quality.Porosity,permeability,and macropores have a positive correlation with the content of quartz and feldspar,and a negative correlation with the content of clay;the coarser the particle size of the tuff reservoir,the more developed the sedimentary structure,the stronger the devitrification and dissolution,and the better the reservoir quality;cementation is an unfavorable factor.Through the analysis of logging characteristics,the multiple regression fitting method is used to predict the thickness,porosity and brittleness index of the tuff reservoir.On the whole,from the south,southwest to north and northeast,the thickness of the tuff reservoirs increased,and the porosity and brittleness index increased.Based on these three parameters,a comprehensive evaluation method for reservoir classification was established,and tuff reservoirs were divided into three types:type I reservoir is finger-shaped and distributed in the north and northeast of the study area,and type II reservoir is mainly distributed in the north of the study area.Type III reservoir is the most widely distributed,mainly in the middle and south of the study area.