Structural Control Of Shale Gas Accumulation In Shanxi Formation In Linfen-Hongtong Area

Linfen-Hongtong Area is located in the northern part of Linfen basin.The structure of this area is complex,and the research on shale gas is less.The research on shale gas geology and shale accumulation under structural control in this area can not only enrich the geological theory of shale gas,but also have theoretical guidance value for future shale gas exploration and development in this area.Based on detailed field investigation and experimental test,combined with drilling and seismic data,this paper uses structural geology theory,reservoir analysis technology and simulation technology to study the organic-rich shale reservoir structure and tectonic thermal evolution characteristics of Shanxi Formation in Linfen-Hongtong Area,and discusses the influence and control of tectonic action on shale gas accumulation.The following main knowledge was obtained:The study area is located in the northern part of Linfen Basin,which is a giant graben structure with NNE trend.Fold structures are mainly developed in the northern region,and faults in the whole region are mainly normal faults.Except for the eastern and northern regions of the study area,most of them are covered by quaternary system.Most of the faults are concealed,and the strike is mainly NNE,followed by NE.In the bedrock exposed area,shear joints are very well developed,while tensile joints are relatively rare,and the dominant strike of joints is NEE,NNE and NWW.There are magmatic rocks of Yansha epoch(Early-Middle Cretaceous)in the southwest.The tectonic stress field in the study area has experienced multiple evolution stages.The NE-SW compression and NW-SE tensile stress in the Himalayan period and the accompanying right-handed shear couple have the most obvious influence on the current tectonic pattern in the study area.Under the control of structure,the buried depth of the bottom boundary of Shanxi Formation in about two thirds of the study area is more than 1500m.In the north-south direction,the overall performance is shallow in the south and north,and increases to the middle.In the east-west direction,the performance is gradually increasing from east to west.The cumulative development thickness of organic-rich shale in Shanxi Formation is between 20 m and 50 m,which shows an increasing trend from northwest and east to southwest.Shanxi Formation mud shale is mainly composed of clay minerals,quartz,plagioclase and other minerals.The average content of organic matter is 2.82%,and the type is mainly type III.The organic matter maturity(R_o)of mud shale is between 1.26%and 1.58%,showing a the burial depth.Organic pores,intragranular pores,intragranular corrosion pores,pyrite intergranular pores and intergranular pores are developed in shale.Interlaminar fractures,intergranular fractures,intragranular fractures,grain edge fractures,cleavage fractures and bedding fractures are mainly developed.The porosity is 0.4524%?2.8091%.Mesoporous contributed the most pore volume,followed by micropores and macropores.Faults and magmatic heat have a strong effect on the formation transformation near the distance.The organic-rich shale of Shanxi Formation experienced four stages of tectonic-burial process,and the shale gas experienced a complex accumulation process:From the late Carboniferous to the end of Triassic,the organic matter entered the low mature stage,mainly generating biogenic gas and a small amount of thermogenic gas.The organic matter pores were less developed,and the shale gas accumulation was weak.Early Jurassic,hydrocarbon generation ended.In the Middle Jurassic,the heating temperature of organic matter increased,but the hydrocarbon generation was weak,and the pore content of organic matter increased.From Late Jurassic to the end of Early Cretaceous,the thermal evolution of organic matter increased,the pore content of organic matter increased,the porosity increased,and a wide range of hydrocarbon generation occurred,and the shale gas content showed an increasing trend.After the Middle Cretaceous,the organic matter stopped evolution,and the shale gas reservoirs of Shanxi Formation were adjusted.Tectonic action has destructive and adjustment effect on the shale gas that has been accumulated.Due to the tectonic-lifting effect and regional magmatic thermal effect,the differentiation of shale gas reservoirs in the region is caused.Therefore,the tectonic-thermal effect and the coverage of the overlying Quaternary have an important control effect on the preservation of shale gas in the study area.With the adjustment of gas reservoir,the shale gas reservoir with adsorption state is formed in Linfen-Hongtong Area.Under the control of structure,the total gas content of the organic shale in Shanxi formation is between 2.14m~3/t and2.25m~3/t.Six favorable shale gas enrichment areas are selected,including one primary enrichment area with an area of 143.12km~2 and five secondary enrichment areas with an area of 51.42km~2.There are 73 pictures,31 tables and 151 references in this paper.