渝东南盆缘转换带五峰组——龙马溪组页岩气储层孔隙特征与演化

目前关于页岩气储层孔隙演化及其机理的研究仍处于探索阶段,研究成果尚未达成共识。为此,基于岩心分析和样品热模拟实验,采用氩离子抛光扫描电镜、原子力显微镜等实验分析技术,对四川盆地东南部及其转换带(以下简称渝东南盆缘转换带)上奥陶统五峰组—下志留统龙马溪组页岩气储层的孔隙类型、孔隙结构、孔径变化规律等进行研究,探讨有机孔隙演化特征,分析伴生矿物对孔隙演化的影响,建立该区页岩孔隙演化模式。研究结果表明:(1)该区五峰组—龙马溪组优质页岩储层主要发育裂缝、无机孔隙、有机孔隙等3种储集空间类型,其中后者是页岩气赋存的主要储集空间,其又可细分为无定形干酪根孔、结构型干酪根孔、沥青质孔及生物化石孔等4类;(2)有机质收缩缝先在有机质颗粒一侧出现,随温度升高缝宽变大,与有机质颗粒的收缩有关;(3)有机孔隙以密集分布于有机质内部的"海绵状"孔为主,当R_o介于1.56%～3.50%时以大孔—介孔为主,R_o 3.50%时大孔减少,介孔和微孔增加;(4)有机质类型及黏土、硅质颗粒、黄铁矿等伴生矿物的含量对孔隙发育有着重要的影响。结论认为,该区五峰组—龙马溪组页岩的孔隙演化模式为:无机孔隙度随埋深增加大幅度减小、有机孔隙度先增大后减小,总孔隙度呈现出先降低、再增加、再持续减小的变化趋势。

Pore characteristics and evolution of Wufeng–Longmaxi Fms shale gas reservoirs in the basin-margin transition zone of SE Chongqing

At present, researches on the pore evolution of shale reservoir and its evolution mechanism are still at such a groping stage that a consensus has not yet reached. Based on core analysis and thermal simulation experiments, the pore types, pore structures and pore-size change rules of shale gas reservoirs of Upper Ordovician Wufeng–Lower Silurian Longmaxi Fms in the southeastern (SE) Sichuan Basin and its basin-margin transition zone (hereinafter referred to as the basin-margin transition zone of SE Chongqing) were studied by means of argon ion polishing-scanning electron microscopy and atomic force microscopy. Then, the evolution characteristics of organic pores were discussed, and the influence of associated minerals on pore evolution was analyzed. Finally, a pore evolution model for the shale gas reservoirs in this area was established. And the following research results were obtained. First, three types of reservoir spaces are mainly developed in the high-quality shale reservoirs of Wufeng–Longmaxi Fms in this area, including fracture, inorganic pore and organic pore. And the organic pores provide the primary reservoir space of shale gas, which can be divided into four categories, i.e., amorphous kerogen pores, structured kerogen pores, asphaltene pores and paleontology fossil pores. Second, organic contracted fissures are related to the contraction of organic matters, first appearing on one side of the organic matters and then becomes wider and wider with the increase of temperatures. Third, organic pores are mostly the "spongy" pores distributed densely inside the organic matters. When Ro is in the range of 1.56–3.50%, macropores and mesopores are dominant. And when Ro exceeds 3.50%, macropores decrease while mesopores and micropores increase. Fourth, the types of organic matters and the content of associated minerals (e.g. clay minerals, siliceous particles and pyrite) play an important role in the development of pores. In conclusion, the pore evolution law of Wufeng–Longmaxi shale in the basin-margin transition zone of SE Chongqing is that with the increase of burial depth, inorganic porosity decreases significantly, organic porosity increases first and then decreases, and the total porosity shows a change trend of decreasing first, then increasing and finally decreasing continuously.