鄂尔多斯盆地延长组陆相页岩孔隙演化模式与影响因素

Pore Evolution Model and Influencing Factors of Continental Shale in Yanchang Formation, Ordos Basin

储层孔隙对页岩气赋存状态和流动机制具有关键控制作用。页岩孔隙演化模式研究能够深化页岩成储机理的认识,但目前相关研究仍然薄弱。以鄂尔多斯延长组陆相低熟页岩(镜质体反射率R_o=0.73%)为研究对象,开展热模拟实验,结合X射线衍射(X-ray diffraction, XRD)分析、低压氮气吸附、高压压汞(mercury intrusion capillary pressure, MICP)测试,分析不同成熟度页岩的孔隙特征,深入探讨陆相页岩孔隙演化模式及其影响因素。结果表明,随热演化程度升高,页岩孔隙经历先增加后减小的趋势,孔隙演化受控于有机质生烃、矿物转化和压实作用。有机质生烃能大幅促进总比表面积和总孔体积的增加,矿物转化能促进溶蚀孔和微裂隙的形成,压实作用对中孔和宏孔比表面积和孔隙体积的增长有明显的抑制作用。建立了鄂尔多斯盆地延长组陆相页岩孔隙演化模式:0.73%3.48%,有机质的芳构化堵塞微孔,孔隙之间的合并使得中孔和大孔体积增加。

Reservoir porosity plays a key role in controlling the occurrence and flow mechanism of shale gas. Study of shale pore evolution can deepen the understanding of shale reservoir formation mechanism;however, relevant studies remain poor. Shales in low-maturity(R_o=0.73%) from the Yanchang Formation in Ordos Basin was studied in anhydrous pyrolysis, X-ray diffraction(XRD), low-pressure nitrogen adsorption, and mercury intrusion capillary pressure(MICP) experiments. Pores characteristics of the shale of different maturity were analyzed and the pore evolution mode and influencing factors of shale were discussed in depth. Results show that as maturity increases, the porosity of shale experience a trend of first increasing and then decreasing, and the pore evolution is controlled by hydrocarbon generation of organic matter, mineral transformation, and compaction. Hydrocarbon generation of organic matter can significantly increase the total specific surface area and pore volume, and clay mineral transformation can promote the formation of dissolved pores and microfractures. Compaction has a significant inhibiting effect on the growth of specific surface area and pore volume of mesopores and macropores. Pore evolution model of the continental shale in the Ordos Basin was established. When R_o is in the range of 0.73%≤R_o≤1.37%, hydrocarbons were generated from a large amount of organic matter, and the specific surface area and volume of pores increased rapidly. When R_o was in the range of 1.37 %≤R_o≤3.84%, the pore specific surface area and volume were reduced due to compaction and calcite filling. When R_o>3.48%, the aromatization of organic matter blocked micropores, and the combination of pores increased the volume of mesopores and macropores.