鄂尔多斯盆地延长组长8油层组砂岩中绿泥石膜的生长模式

Growth pattern of chlorite film in Chang 8 sandstone of Yanchang Formation in Ordos Basin

为了研究鄂尔多斯盆地延长组砂岩中绿泥石膜与物性的关系,利用铸体薄片和带有能谱仪的扫描电镜技术对绿泥石膜的赋存特征进行细致观察和分析,研究其成因和生长模式对物性的影响。结果表明,绿泥石膜由里层膜和外层膜组成,外层膜依据晶体大小细分为小晶体亚层膜和大晶体亚层膜。小晶体亚层膜生长于早成岩阶段B亚期,此时绿泥石结晶速度快,产生的晶体较小,晶形较差,集合体密集排列,对流体具有阻滞作用。里层膜生长在颗粒表面溶蚀产生的小孔腔中,在小晶体亚层膜的双向阻滞作用下,里层膜具有贫铁、镁而富硅质的化学组成,因生长空间狭窄致使晶体小、晶形差、集合体杂乱堆积。进入中成岩阶段,随着孔隙连通性变差和油气进入,流体流速减小,物质供应变慢,晶体结晶速度放缓,可形成稀疏的大晶体亚层膜。通过占据结晶底质,石英质碎屑颗粒表面的小晶体亚层膜阻止石英次生加大,而长石质碎屑颗粒表面的小晶体亚层膜和骨架颗粒表面的里层膜,通过阻滞颗粒溶解或消耗颗粒溶解出来的硅质流体,致使原生孔隙流体中的硅质浓度不足以生成大量的自生石英颗粒,它们共同作用抑制石英胶结,从而保护了原生粒间孔隙。

The sandstone property is closely related to the chlorite film of Yanchang Formation in Ordos Basin. The data of cast thin slice and scanning electron microscope with energy disperse spectroscopy were used to analyze the chlorite film of Chang 8 sandstone, so as to study the growth pattern of the chlorite film and its influence on physical properties. The result shows that the chlorite film consists of inner layer film and outer layer film, and the outer layer film can be separated into small crystal sub-layer film and big crystal sub-layer film according to the grain size of crystal chlorite. The small crystal sub-layer film developed in the early diagenetic stage B, and the crystal has small size, poor shape and dense arrangement after rapid crystallization, and the chlorite film had the capacity to retard fluid exchange between the primary pore and the small cavity formed by the dissolution of the framework grain. The inner layer film developed in the small cavity and had low Fe and Mg content with high Si content under the bidirectional block of the small crystal sub-layer film, whose crystal is small in size and poor in shape and disorderly packed because of too narrow space for the growth. As the connectivity of the prima- ry pore changed for the worse and the oil emplaced in the middle diagenetic stage, the velocity of the fluid de- creased and the supply of the substance slowed down, which reduced the growth rate of chlorite. In such a pro- cess, some crystal chlorites increased in size and sparsely arranged and formed the big crystal sub-layer above the small crystal sub-layer film. The small crystal sub-layer film on the surface of quartziferous clastic particle inhibited quartz overgrowth by occupying the crystalline substrate. The small crystal sub-layer film on the surface of feldspathic clastic particle retarded the dissolution of the feldspathic clastic particle, and the inner layer film in the surface of the framework grain consumed the silicic fluid, which both led to insufficient silicic fluid to generate large amounts of authigenic quartz particles in the primary pore. The chlorite film protected the primary pore from quartz cement.