蛋白石硅质页岩成岩过程中的孔隙结构变化特征

Pore network changes in opaline siliceous shale during diagenesis

为研究硅质页岩成岩演化过程中的孔隙结构变化,选取松辽盆地嫩江组低演化蛋白石硅质页岩和渝东南高演化五峰—龙马溪组样品,在X射线衍射分析基础上,采用氮气等温吸附技术,开展了成岩作用过程中蛋白石硅质页岩的孔隙结构变化特征研究。结果显示较低演化的蛋白石硅质页岩比表面积和孔容均远高于较高演化的蛋白石硅质页岩,低演化页岩2 nm以上孔隙主要集中在2~3 nm区间,其次分布在20~30 nm区间,2 nm以下孔隙主要集中在0.6~0.8 nm区间,总孔容主要由10~30 nm区间孔隙贡献;高演化页岩2 nm以上孔隙主要集中在2~3 nm区间,2 nm以下孔隙主要集中在0.6~1 nm区间,总孔容主要由2~30 nm区间孔隙贡献。将五峰—龙马溪组硅质页岩与其相比,发现纳米孔隙在成岩过程中大幅丧失,孔容缩小至原始的1/3~1/10。五峰—龙马溪组硅质页岩中2~30 nm区间的孔隙对孔容的贡献最大,30~200 nm范围内孔隙有微小贡献,2 nm以下的孔隙在成岩演化过程中几乎消失殆尽,显示出成岩作用对不同孔径孔隙的改造程度存在差异性。

The siliceous shale of the Nenjiang Formation in the Songliao Basin and the Wufeng-Longmaxi formations in the Sichuan Basin were analyzed using X-ray diffraction and N2 isothermal adsorption to study pore network changes during diagenetic evolution. The specific surface area and pore volume of shale with a low evolution degree are much higher than those with a high evolution degree. Pores larger than 2 nm in lower evolved shale mainly concentrated in 2-3 nm and 20-30 nm, while those smaller than 2 nm mainly concentrated in 0.6-0.8 nm, and their pore volume is mainly contributed by 10-30 nm. Pores larger than 2 nm in higher evolved shale mainly concentrated in 2-3 nm, while those smaller than 2 nm mainly concentrated in 0.6-1 nm, and their pore volume is mainly contributed by 2-30 nm. Compared with siliceous shale in the Wufeng-Longmachi formations, nano pores were greatly reduced during diagenesis, and pore volume reduced to 1/3-1/10 of the original. Pores of 2-30 nm are the major contributor to pore volume in the Wufeng-Longmaxi formations, and the next is 30-200 nm, and pores smaller than 2 nm almost disappeared, which indicated that the influence of diagenesis on various size pores is different.