Investigation Of Pore Characteristics Of Taiyuan Formation Coal-series Shale In South Central Qinshui Basin

The shale gas of the Taiyuan Formation in the Qinshui coalfield is a new object explored in recent years.Formed in the marine-continental transitional environment,its reservoir characteristics and controling factors are different from marine and continental shale.For the purpose of understanding the pore characteristics for coal-series shale and its controlling factors,the samples are conducted by a series of experiments to study the pore structure of coal series shale of Taiyuan formation,including:total organic carbon?TOC?analysis,polarizing microscope,Rock-Eval,X-ray diffractometer?XRD?,field emission scanning electron microscope?FE-SEM?,X-ray computed tomography?X-CT?etc..Through these tests,the organic geochemical characteristics,petrology,pore structure and fractal characteristics,methane gas adsorption capacity for coal-series shale of Taiyuan Formation are comprehensively studied,and the influencing factors of pore structure and fractal characteristics are discussed as well.The results show that the TOC varies from 0.6 to 11.7%with a mean value of 3.0%.Although organic matters are in the high-over mature stage?the average value of R_o is 2.37%?with a Humic-type kerogen,they still have potential for hydrocarbone generation.The shale samples are rich in Al₂O₃and SiO₂,containing a small amount of other oxides,such as K₂O,Fe₂O₃,MgO,etc.The mineral consists mainly of clay minerals?average content is57.2%?and quartz?average content is 36.7%?with a small amount of pyrite,siderite and carbonate.Clay minerals are mainly kaolinite and illite with a small amount of chlorite.The nitrogen adsorption curves of shale samples are type II,indicating that the pores are mainly micropores and mesopores with a small number of macropores.The average pore size for shale samples is 9.3 nm and pores less than 20 nm account for more than 50%of the total pore volume;the type of hysteresis loop on desorption curve is between H2 and H3,and it is mainly H3,which indicates that morphology of the majority pores in shale samples is slit type and the minority is ink bottle shape.There are three types of pores in SEM images for shale samples:organic pores,non-organic pores and micro-cracks.The organic pores include three types:hydrocarbon-generating pore,structural pore and intergranular pore.Non-organic pores mainly include intergranular pores,clay layered pores,intercrystallineporesanddissolvedpores.Thenano-scale hydrocarbon-generating pores of organic matter,micro-nano mineral intergranular pores and interbedded pores of clay minerals are developed and micron-scale cracks have limited development in coal-series shale.The shale in the study area has undergone various diagenesis such as compaction,cementation,dissolution,authigenic mineral precipitation and rupture during diagenesis,and it has important effects on pore development.Among them,compaction is the main controlling factor for pore morphology and pore size.The organic hydrocabon has an important significance for secondary pore development.Based on FE-SEM analysis and 3D pore reconstruction with X-CT images,porosity in coal-series shale is a complex system which is characterized by random distribution of pores in space,diversified morphology,wide pore size distribution,strong heterogeneity and obvious fractal characteristics.There are two different fractal dimensions D₁ and D₂ in the nitrogen adsorption curves?0-0.5?and?0.5-1?with the average values of2.496 and 2.720,respectively.After the binarization of the X-CT image of the shale samples,the pore fractal dimensions are calculated by the box method,which shows that the pore fractal dimension also increases with the increase of TOC.Organic matter has obvious effects on the development of micropores and mesopores?<10 nm?.The distribution of organic matter in shale is heterogeneous.The pore fractal dimensions increase as TOC increases.The smaller the TOC is,the more unevenly organic matters distributes,the less pores'number is,and the smaller pore fractal dimensions are.With the increase of TOC,the distribution of organic matter tends to be uniform,the number of pores and the fractal dimensions of pore increase,while the average pore size decreases.FTIR results show that coal shale has similar macromolecular structure to coal.By analyzing the relationships between FTIR structure parameters and pore structure parameters,it is shown that the I₁ has a reducted function relationship with the T_max value,while the DOC₁ has an increasing function relationship with the T_max value.Organic matter hydrocarbon generation and aromatic structure condensation are controlling factors for nano-scale organic materrs'pores development.Brittle minerals have a positive effect on pore development and preservation,and clay minerals have an adverse effect on pore development and evolution.The higher the TOC is,the more developed micropores and the mesopores below 10 nm in the shale are,the larger the adsorption energy and adsorption space the shale has,the stronger the methane adsorption capacity of shale is.Brittle minerals are more favorable for the adsorption of methane than clay minerals.The coarser pore surface is,the larger the pore fractal dimension D₁ is,which is more conducive to the adsorption of methane gas molecules.