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    渝东北地区龙马溪组页岩孔隙结构特征及其主控因素

    Pore structure characteristics and main controlling factors of the Longmaxi shales in northeastern Chongqing

    • 摘要: 为了探究渝东北地区龙马溪组变形页岩孔隙结构特征及其主控因素,本次研究采集12件渝东北地区龙马溪组页岩样品,利用场发射扫描电镜、液氮吸附和二氧化碳吸附等方法系统表征其孔隙结构特征,并与四川盆地不同构造带龙马溪组页岩孔隙结构参数进行对比。研究结果表明:渝东北地区龙马溪组页岩孔隙主要包括有机质孔、无机孔和微裂缝,与四川盆地川南地区成熟度相近的海相页岩相比,样品有机孔发育程度偏低,孔隙规模偏小。此外,无机孔、微裂缝在渝东北地区龙马溪组页岩中都较为发育。根据孔裂隙发育特征,可将无机孔划分为粒间孔、黏土矿物层间孔、粒内孔三类,将微裂缝划分为有机质相关微裂缝和无机矿物相关微裂缝。渝东北地区龙马溪组页岩TOC含量对微孔的影响作用呈阶段式变化,在TOC<5.93%时,TOC含量与微孔体积、比表面积呈现较强的正相关关系,在TOC>5.93%时,则呈现负相关关系,TOC含量与介孔参数并无相关性,而黏土矿物、脆性矿物含量与页岩孔隙发育均没有显著相关性。扫描电镜图像显示,构造挤压强度增加导致渝东北地区龙马溪组页岩中孔隙数量减少,且孔隙连通性在一定程度上被破坏,这使得页岩储层中游离态页岩气向吸附态页岩气转化,从而使气体发生强烈的吸附作用,并且主要以吸附态的形式封存起来,提高了页岩储气能力。该研究成果为复杂构造区页岩气勘探提供了理论依据。

       

      Abstract: To investigate the pore characteristics and controlling factors of the Longmaxi shales in northeastern Chongqing, this study samples 12 shales from the Longmaxi Formation and utilizes physical experiments, including field emission scanning electron microscopy (FE-SEM), liquid nitrogen adsorption and carbon dioxide adsorption, to characterize the pore structures. The pore structure parameters are then compared with those of shales from different tectonic zones in the Sichuan Basin. The results indicate that the pores of shales in the Longmaxi Formation in northeastern Chongqing are predominantly comprised of organic matter pores, inorganic pores and micro-fractures. Compared to the marine shales with similar maturity in southern Sichuan, these samples exhibit relatively fewer and smaller organic matter pores but more developed inorganic pores and micro-fractures. Based on the pore-fracture characteristics, inorganic pores can be classified into three types: intergranular pores, interlayer pores in clay minerals, and intragranular pores. Micro-fractures are categorized as organic matter-related micro-fractures and inorganic mineral-related micro-fractures. The influence of TOC content on pore structure occurs in two stages: when TOC content is less than 5.93%, it exhibits a strong positive correlation with micropore volume and specific surface area; when TOC content is more than 5.93%, it shows a negative correlation with these factors, and no correlation with mesoporous parameters. In addition, there is no significant correlation between the inorganic mineral (clay and brittle mineral) content and pore parameters. The SEM images also show that the tectonic compression reduces pore volume and pore connectivity, leading to the conversion of free gas into adsorbed gas in shale reservoirs, thus improving the gas storage capacity of the shales. The research results provide a theoretical basis for shale gas exploration in complex structural areas.

       

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