黔西六盘水煤田大河边向斜龙潭组煤系页岩微观孔隙特征及其影响因素

Pore characteristic and its influencing factors of Longtan shale in Dahebian syncline,Liupanshui coalfield,western Guizhou Province

黔西地区龙潭组煤系页岩厚度大、有机质丰度高、含气性好,是中国南方重要的页岩气勘探目标层位。以区内大河边向斜龙潭组煤系页岩全岩样品和干酪根样品为例,开展场发射扫描电镜、气体吸附、XRD矿物成分分析、TOC含量测定及显微组分分析等实验,讨论了中低成熟度煤系页岩孔隙发育特征及其主控因素。结果表明:该页岩为低成熟—中成熟页岩(镜质体反射率为0.68%~0.95%),孔隙类型有无机孔、有机孔和微裂缝,其中无机孔主要包括粒间孔、晶间孔、粒内孔和溶蚀孔,有机孔包括气孔和残余胞腔孔,微裂缝主要包括构造缝、有机质演化异常压力缝、成岩收缩缝和贴粒缝。全岩样品孔体积介于0.0046~0.0397 cm^(3)/g之间,干酪根样品孔体积介于0.0165~0.1020 cm^(3)/g之间,页岩中干酪根对孔隙的贡献率大于矿物。黏土矿物、方解石、有机质丰度和成熟度等对页岩总孔隙体积影响较大,黏土矿物含量、有机质成熟度与页岩孔隙体积呈显著正相关;方解石含量、TOC含量与页岩孔隙体积呈显著负相关。有机质的存在会充填微裂隙以及中孔、大孔,低成熟—中成熟阶段是干酪根大量裂解生成油气的阶段,同时产生大量有机孔。黏土矿物和有机质成熟度导致页岩孔隙分形维数增大,孔隙结构复杂程度增加,TOC含量导致页岩孔隙分形维数减小,孔隙结构复杂性降低。

The Longtan coal-bearing shale in western Guizhou is characterized by its large thickness,high TOC content and gas content,and it is a significant target for shale gas exploration in southern China.This is a case study of the shale and kerogen from the shale samples of the Longtan Formation in the Dahebian syncline,west⁃ern Guizhou Province.Scanning electron microscopy observation,gas adsorption measurement,XRD analysis were conducted to research the pore characteristic and its main controlling factors of these selected samples.The results indicate that the shale is a low-medium mature shale(with a vitrinite reflectance of 0.68%−0.95%),and its pore types are inorganic pores,organic pores,and microfractures.Inorganic pores include intergranular,intercrystalline,intragranular,and dissolution pores.Organic pores include bubble and cellular pores.Microfrac⁃tures mainly include structural fractures,abnormal pressure fractures of organic matter evolution,diagenetic contraction fractures,and adherent fractures.The pore volumes of the shale and kerogen samples range in 0.0046−0.0397 cm^(3)/g,and 0.0165−0.1020 cm^(3)/g,respectively,and the contribution rate of kerogen to pores in shale is greater than that of minerals.Clay minerals,calcite,organic matter abundance and maturity have a significant effect on the pore volume in shale.There is a significant positive correlation between clay min⁃erals content and pore volume,and between thermal maturity and pore volume.However,both calcite content and TOC content are negatively correlated with pore volume.Organic matter will fill micro cracks,as well as mesopores and macropores.During the low-medium maturity stage,kerogen is extensively cracked to generate oil and gas,and generate a large number of organic pores.Clay minerals and thermal maturity lead to an in⁃crease in the fractal dimension of shale pores and an increase in the complexity of pore structure.TOC content leads to a decrease in the fractal dimension of shale pores and a decrease in the complexity of pore structure.