Accumulation Characteristics Between Coalbed Methane And Shale Gas In Taiyuan Formation In Southern Qinshui Basin

Southern Qinshui Basin is one of the CBM development focus regions in China,where sediment coal shale under the transitional facies.Taiyuan Formatiom is the major shale gas producing reservoir section,which well-developed set of dark shale and has good geological conditions.This paper selects the Taiyuan Formation coal and shale as the research object in Southern Q inshui Basin.Through data collection,field geological investigation,core observation,experimental test and theory analysis to expand system research includes to analy the Taiyuan Formation coal and shale gas accumulation conditions and accumulation mechanism and other core scientific issues.The results showed that:Southern Q inshui Basin is the main part of Q inshui synclinorium,the inside structure is simple,few faults.The target layer is coal bearing strata,which is formed in the clear water and muddy water alternating shoreline and delta sedimentary facies environment,develops 7¹6 layers coal,and 15^{t h} coal's thickness is largest,and the thickness value is between 1.0⁷.0m,15^{t h} coal is widely distributed in the whole region,showing the characteristics of north and south thickness and the tendency of the middle and the West.And Taiyuan Formation is also the main layer of the development of dark shale,which develops a total of 5 sets of shale systems with larger sedimentary thickness and more stable distribution,and the Q inxian-centered area of the syncline axis,because the buried depth exceed 1500m,is beneficial to the formation of shale gas.The type of organic matter in shale in study area is mainly of?type,with an average TOC being 2.01%,an average Ro being 2.30%,and has little change basically which means that the organic matter of clay shale of Taiyuan Formation has already entered into dry transom,and have a highest content of clay and a higher content of quartz,and the average brittleness coefficient is 35.7%,which is generally low.Coal seam is dominated by organic matter,15^{t h} coal Ro mainly beyond 3.0,individual less than 2.50;and the mineral composition content is mainly in 3.1¹5.4%,most of them are clay minerals,containing a small amount of carbonate minerals,basically does not contain other mineralsThrough mercury,nitrogen,CO₂ adsorption,isotherm and SEM experiments testing methods combined with many other scholars on the basis of coalbed literature data and holded fissure characterization and comparison on the coal with the pore of shale reservoirs in Taiyuan Formation,southern Q inshui basin.Fissures in 15^{t h} coal in study area are more developed,and the filling of some fractures is very serious,nd shale is mainly developing natural micro-fractures,which is advantageous to the storage and migration of shale gas and the activation of artificial fissure network.The coal seam in the area15^{t h} coal are more developed,and the filling of some fractures is very serious,and shale is mainly developed by natural micro-fractures,which is advantageous to the storage and migration of shale gas and the activation of artificial fissure network.The coal mainly develops primary pore,metamorphic pore,epigenetic pore,mineral pore four types,and shale mainly develops organic pore,intragranular pore,dissolved pores,pyrite intergranular pore,intergranular pore and micro cracks six types.Overall,the total pore volume of coal is about 5.5 times than that of shale,specific surface area is about 6 times times than that of shale,because the proportion of micro-pore and small hole is large.The average porosity of coal is 2.82%,and shale is 1.57%,at a slightly lower level.The permeability of coal is dominated by low permeability,and there is a strong heterogeneity on the plane,and the permeability of sha le has wide varation and average is 0.00615mD.The Taiyuan Formation has experienced four stages of hydrocarbon generation and evolution,including rapid burial in the first stage and regional thermal metamorphism in the third stage,which has an important influence on gas production and gathering.The target strata has formed two generation peak,Late Carboniferous to late Triassic based on bio-gas and late Jurassic based on pyrolysis gas.Comprehensivly analyzed reservoir forming conditions of shale gas and CBM,and there are many similarities and opposite factors in hydrocarbon generatio n,reservoir condition and preservation condition.CBM and shale gas are different in reservoir forming mechanism,CBM is a typical mechanism of adsorption and accumulation,and shale gas has both adsorption and piston type double reservoir forming mechanism.Besides,shale gas and coalbed methane reservoirs are generated in the vertical interval type and adjacent exhibit two kinds of features,and hydrocarbon generation of thin coal improve the entire content of shale gas reservoirs.