Fractal Characteristics and Interfering Factors of Microscopic Pores in Marine-Continental Transitional Coal Shale:A case study of the Taiyuan Formation in the northern Qinshui Basin
[Objective]To analyze the pore fractal characteristics and interference factors from the coal measure shale of the Taiyuan Formation in the northern Qinshui Basin.[Methods]Total organic carbon(TOC)content,matu-rity,X-ray diffraction,and low-temperature nitrogen adsorption experiments were conducted on samples of the Taiyuan Formation in the Yangquan block.The fractal dimension of a sample pore was calculated based on the(Frenkel Halsey Hill,FHH)theoretical model,which analyzes the influence of mineral content,organic geochemi-cal characteristics,and pore structure parameters on the pore fractal dimension.[Results]The results show that the TOC content of coal measure shale in the Taiyuan Formation is between 0.57%and 6.40%,and the average is 3.18%;the vitrinite reflectance of organic matter(Ro)is between 1.96%and 3.24%,and the average is 2.49%.The coal-measure shale micropores have double fractal characteristics,of which fractal(D1)is between 2.507 9 and 2.663 9,dimension(D2)is between 2.527 1 and 2.809 4,and the organic matter content and maturity are positively correlated with D1 and D2.The pore structure parameters have a positive correlation with D1 and D2,but the correla-tion coefficient with D2 is higher than that of D1,indicating that micropores have stronger influence on pore structure parameters.The input of terrigenous clastic quartz with high sorting and roundness leads to regular pore morphology,resulting in a negative correlation between quartz content and D1 and D2.Carbonate minerals and feldspar primarily provide macropores,and their contents are related to shale.Both D1 and D2 are negatively correlated,and the pore size of the clay minerals decreases under long-term compaction,whereas the number of micropores increases,and the pore shape is complex,which is positively correlated with fractal dimensions D1 and D2.[Conclusions]The micropores of the coal measure shale in marine continental transitional facies have dual fractal characteristics.The increase of organic matter content,maturity,pore structure parameters,and clay mineral content can lead to the increase of micropore fractal dimension,and an increase in terrigenous clastic quartz,feldspar,and carbonate mineral content can lead to a decrease in micropore fractal dimension.
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