川中地区侏罗系沙溪庙组储层特征及控制因素

Reservoir characteristics and controlling factors of Jurassic Shaximiao Formation in central Sichuan Basin

四川盆地中部地区侏罗系沙溪庙组浅水三角洲砂体是致密气勘探的重要领域,有利储层的成因是制约高效勘探的关键问题。综合岩心和薄片资料,采用扫描电镜、阴极发光和地球化学等测试手段分析了沙溪庙组成岩演化序列,从沉积和成岩两方面探讨了储层的控制因素。沙溪庙组储层以长石砂岩为主,平均孔隙度、渗透率分别为10.34%、1.75×10^(-3)μm^(2),属于(中)低孔—低渗储集层。储层总体处于中成岩A期,依次经历了早期绿泥石形成、长石早期溶蚀→方解石、浊沸石沉淀→有机酸溶蚀、贴粒缝形成、钠长石形成→浊沸石溶蚀→石英溶蚀、晚期方解石、浊沸石、绿泥石充填的成岩过程。高能环境中沉积的水下分流河道砂体粒度粗、杂基少是优质储层形成的有利微相。压实作用、胶结作用是2种主要的减孔机制,浊沸石与自生绿泥石对储层物性的影响有着两面性。原生孔隙大量保存、溶蚀作用强烈,胶结作用弱等多方面制约着优质储层的发育,且原生孔隙的保存与绿泥石薄膜有关。浊沸石胶结物充填导致储层孔隙度较低,大量微裂缝的出现是提高砂体渗透率的关键。

The widely developed shallow-water delta sandbodies of Shaximiao Formation in central Sichuan Basin are an important area for tight gas exploration. The formation of favorable reservoirs is a key problem restricting efficient exploration. The diagenetic sequence of Shaximiao Formation is analyzed by means of SEM,CL,geochemistry,and the controlling factors of reservoir are discussed from the aspects of sedimentation and diagenesis. The reservoir in the study area is dominated by arkose sandstone,with an average porosity and permeability of 10.34% and 1.75×10^(-3)μm^(2),belonging to(middle)low porosity and low permeability reservoirs.The main diagenetic events of the reservoir are as follows:Early chlorite formation,early feldspar dissolution → calcite and laumontite precipitation → organic acid dissolution,microfracture formation,albite formation → zeolite dissolution → quartz dissolution,late calcite,laumontite and chlorite. The subaqueous distributary channel sandbodies with coarse grain size and less impurity base are favorable sedimentary microfacies for the formation of high-quality reservoirs in high-energy environment. Compaction and cementation are the most important pore reduction mechanisms,and the effects of laumonite and authentically chlorite on reservoir physical properties have two sides. The development of high quality reservoir is restricted by many factors such as large amount of preservation of primary pores,strong dissolution and weak cementation. The preservation of primary pores is related to chlorite films. The filling of laumonite cement results in low porosity,and the occurrence of a large number of micro-fractures is the key to improve the permeability of sand body.