泥质岩成岩系统过程及其对SiO_2赋存状态的制约——以渝东南地区龙马溪组为例

Process of Diagenetic System in Shale and Its Restrict on Occurrence of SiO_2:A Case Study of the Longmaxi Formation in the Southeast District of Chongqing

SiO_2的赋存状态对页岩气开发具有重要意义,其成因机制与成岩系统具有密切关系。综合利用野外露头、钻井岩心等资料,采用X-射线衍射分析、阴极发光、扫描电镜及能谱分析等分析手段,系统研究了渝东南龙马溪组页岩成岩系统主控因素、成岩过程模型及成岩系统过程对SiO_2赋存状态的影响。渝东南地区龙马溪组页岩成岩现象丰富,成岩系统过程也相当复杂;早期成岩主控因素受沉积机制影响较大,中后期受成岩机制制约,晚期主要受到构造机制的影响;后期构造运动使得页岩成岩系统具有明显的阶段性特征,喜山期页岩地层的大幅度隆升,不仅使页岩与外界系统连通性加强,溶蚀作用和微裂缝变得发育,还导致页岩储层物性、有机质生烃演化、油气保存条件等发生了明显的变化;在成岩演化不同阶段,不同的化学流体特征制约着页岩中硅质成因,页岩中SiO_2赋存状态在早期主要以陆源石英及蛋白石为主,中后期以微晶自生石英为主,晚期在石英脉体中发育中—粗晶石英。

The occurrence of SiO2 has important significance for the development of shale gas, and its genetic mechanism is closely associated with the diagenetic system. Based on the data from field outcrops and drilling cores, we systematically studied the main controlling factors of diagenetic system, the diagenetic process model and its influences on the SiO2 occurrence of shale in Longmaxi Formation, by means of X-ray diffraction analysis, cath- ode luminescence, scanning electron microscope, energy spectrum analysis and other research methods. The results show that the process of diagenetic system of shale in Longmaxi Formation is quite complicated and the diagenetic phenomenon is rich. The diagenetic control factors were greatly influenced by deposition mechanism in the early stage, and restricted by the diagenetic mechanism in the middle and late stage, while they were mainly affected by the tectonic mechanism in the late stage. The diagenesis system had apparent phase characteristics influence by tectonic movement, especially for Himalayan stage. During this period, shale formation were greatly uplifted, which not only promoted the connectivity of shale and the outside system, and made dissolution and micro cracks become more developed, but also changed reservoir physical property, hydrocarbon generation and preservation conditions of hydrocarbon obviously. Generally, the different chemical fluids restricted the origin of siliceous in different evolution stages of diagenetic. The occurrence of SiO2 was mainly composed of continental quartz and opal in the early stage and slowly began to increase typomorphic quartz, while the medium-coarse grained quartz began to develop in the quartz veins after tectonic movements.