Reservoirs Characteristics And Accumulation Mechanism Of Shale Gas In The Upper Paleozoic Taiyuan Formation, Lin-Hong Area, Shanxi

The organic-rich shale with good development in the Carboniferous-Permian in the Linfen-Hongtong area of Shanxi Province has good shale gas accumulation conditions.The paper takes the Taiyuan Formation shale strata in Linfen-Hongtong area as the research object,based on the collection of regional literature,regional geological survey,drilling core cataloging and experimental analysis and testing,from regional geological features and reservoir sedimentation.The characteristics of the shale reservoirs and the reservoir-forming mechanism of the Taiyuan Formation in Linfen-Hongtong area were studied in terms of fabric characteristics,source rock and reservoir characteristics,reservoir control factors and reservoir formation mechanism.The results show that:There are many folds and faults in the Linfen-Hongtong area.The mainly folds are anticlines,presented in NE and NNE.The faults,which composed of normal faults,show NNE?NWW and NE directions,developing secondary or hidden barrier structure.The northeastern part of the study area characteristic with complex structure,folds and faults,and the bedrock tends to change greatly,while the southern structure is relatively simple,the dip angle of the stratum is gentle,and the fault distribution is relatively sparse.The sedimentary environment of the Taiyuan Formation is mainly composed of delta,barrier-lagoon and shallow sea platform,whose lithology consists of mudstone,carbonaceous mudstone,siltstone,sandstone,coal seam and limestone.Among them,the thickness of mud shale is relatively large,ranging from 39 to 83m,with an average thickness of 52m.It is relatively stable in lateral longitudinal space,with good ductility and continuity,and relatively high organic matter content;Shale depth is between 1500and 2500m,which is conducive to the integration of shale gas.The organic matter types of the shale of the Taiyuan Formation are all type III kerogen;The abundance of organic matter is high,and the TOC content is generally distributed between 1.32%and 18.96%;the maturity of organic matter is between 1.03%and 1.75%,and the main body is scattered at the mature stage.A large amount of methane is generated,which provides a rich material basis for shale gas accumulation.The mainly mineral composition of mud shale is clay minerals and quartz,with average values of 56.39%and 35.31%respectively,and its brittleness coefficient exceeds 30%.The nano-micron pores of the Taiyuan Formation shale reservoirs are mainly composed of organic pores,mold pores,dissolution pores,intercrystalline pores and intergranular pores.The pores of mud shale are mainly micropores and small pores,which are the main pores of shale gas occurrence,desorption and seepage in Taiyuan Formation.The porosity of the shale is mostly distributed between 0.45%and 2.41%;the total specific surface area is between 2.10 and 9.41m~2/g;the pore volume is between 0.0049 and 0.021cm~3/g;From the specific surface area and pores,the volume angle,the Taiyuan Formation Xishan section and the Mountain scale section reservoir are more conducive to the adsorption and occurrence of shale gas.The results of isothermal adsorption experiments show that the maximum adsorption capacity of shale in Taiyuan Formation varies greatly,ranging from 0.41-1.51m~3/t,with an average of 1.04m~3/t.The overall adsorption capacity of shale gas is lower than that of North America and South.The shale gas accumulation conditions of the Taiyuan Formation are analyzed from the perspective of organic matter thermal evolution.The Linfen-Hongtong area has a high degree of thermal evolution and has reached the stage of gasification,providing a reliable material basis for the formation of shale gas in the study area.Dark muddy-shale develops more organic nanopores,which makes the reservoir have strong adsorption capacity and provides sufficient space for shale gas accumulation.From the point of the tectonic evolution,the Crust-Permian over lying strata were denuded during the late Yanshanian movement,but the Taiyuan Formation was never exposed to the surface and began to settle in the late Himalayan movement,forming a good formation.The oil and gas reservoir caprock,and the Taiyuan Formation mud shale itself is relatively thick,with low permeability,strong gas preservation capacity,and good sealing effect on shale gas;comprehensive organic matter thermal evolution and tectonic evolution,it is considered that Linfen and The organic matter in Hongtong area has a high degree of thermal evolution and good shale gas preservation conditions.The mechanism of shale gas accumulation is complicated,and it has multiple characteristics of coalbed methane,root-edge gas and conventional natural gas.The initial stage of accumulation is the adsorption mechanism,the middle stage is the filling and piston transport mechanism,and the later stage is the displacement and accumulation mechanism.According to the characteristics of shale gas and coalbed methane longitudinal accumulation model,it can be divided into two major categories:One is consist of shale gas reservoir,coalbed methane continuous co-generation reservoir and the other includs shale gas reservoir,coalbed methane reservoir.This paper has 56 pictures,14 tables,and 159 references.