生物蛋白石早期成岩相变特征及对硅质页岩孔隙发育与孔径分布的影响

Early diagenesis characteristics of biogenic opal and its influence on porosity and pore network evolution of siliceous shale

卢龙飞 刘伟新 俞凌杰 张文涛 申宝剑 腾格尔

LU Longfei LIU Weixin YU Lingjie ZHANG Wentao SHEN Baojian BORJIGIN Tenger

页岩油气富集机理与有效开发国家重点实验室, 无锡 214126 中国石化 石油勘探开发研究院 无锡石油地质研究所, 无锡 214126

State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Wuxi, Jiangsu 214126, China Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi, Jiangsu 214126, China

为研究生物成因硅质页岩成岩演化以及在该过程中页岩物性和孔隙结构变化特征，选取松辽盆地嫩江组蛋白石硅质页岩和四川盆地东部上奥陶统五峰组-下志留统龙马溪组硅质页岩，利用X射线衍射、氦气孔隙度、氮气吸附和高压压汞等手段，开展了页岩矿物相变化、孔隙发育和孔隙结构特征等综合分析。结果显示，生物蛋白石发生脱水和重结晶作用较早，在早成岩阶段即完成了向准晶态蛋白石-CT和晶态石英的转变过程。在蛋白石-A向蛋白石-CT转化过程中，页岩总孔隙度从75%以上快速降低至30%附近，在继续向石英转化过程中孔隙损失速率迅速降低，降幅减小，仅降低了约5%，呈快速和缓慢两段式变化特征。同时，不同类型孔隙的孔体积分布也发生较明显变化，大孔损失较多，微孔损失较小，孔隙组成从初始以大孔和介孔为主逐渐向以介孔和微孔为主转变。生物成因硅质页岩早期成岩阶段机械压实和化学压实（压溶）作用近乎同步进行，对页岩改造作用强，造成页岩孔隙减小的同时，又使页岩的硬度增大，支撑和抗压实能力增强，从而使早期成岩中后期及后续成岩作用的改造和破坏减弱。生物成因硅质页岩早期快速成岩定型是其在成岩中后期与晚期仍然能够保持高孔隙特征的重要原因。

Opaline siliceous shale from the Nenjiang Formation in the Songliao Basin and siliceous shale from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the eastern Sichuan Basin were selected to study the diagenetic evolution of biogenic siliceous shale and the characteristics of shale physical properties and pore structure changes during this process. X-ray diffraction, helium porosity, nitrogen adsorption, and high-pressure mercury intrusion were used to analyze mineral composition, total porosity, and pore structure characteristics. The dehydration and recrystallization of the biogenic opal occurred early, and the transition to quasi-crystalline opal CT and crystalline quartz was completed at the early diagenetic stage. During the conversion of opal-A to opal-CT, the total shale porosity decreased rapidly from more than 75% to around 30%. During the conversion to quartz, the rate of pore loss decreased rapidly, and the decrease was only about 5%, showing two stages. At the same time, the pore volume distribution of different types of pores also changed significantly. The loss of macropores was larger than the loss of micropores. The composition of pores gradually changed from macropores and mesopores to mesopores and micropores. In the early diagenetic stage of siliceous shale, the mechanical compaction and pressure solution occurred synchronously and had a strong effect on shale transformation, which reduced shale porosity, increased hardness, and enhanced the support and resistance to compaction, reducing the transformation and destruction of the early to middle and subsequent diagenesis. The characteristics of rapid diagenesis of siliceous shale in the early stage of biogenesis are the important controls for maintaining high porosity in the middle and late stages of diagenesis.

国家自然科学基金 U1663202;国家自然科学基金 41972164;国家自然科学基金 U19B6003-02;国家油气重大专项 2016ZX05036002;中国石化科技部项目 P15097

https://doi.org/10.11781/sysydz202003363