博兴洼陷沙四上亚段滩坝砂岩次生孔隙形成机制

Mechanism of Secondary Pore of the Beach Bar Sandstones in the Upper Es_4 of the Palaeogene, Boxing Sag

根据砂岩铸体薄片、扫描电镜、地层压力分析,研究了博兴洼陷沙四上亚段滩坝砂岩次生孔隙的形成机制。研究表明博兴洼陷地层中的上升流为滩坝砂岩次生孔隙的形成创造了条件,硅酸盐和碳酸盐矿物在CO2和有机酸作用下的溶蚀是次生孔隙形成的主要因素,黏土矿物的转化也为次生孔隙的发育做出了贡献。在构造高部位碳酸盐胶结程度较低或断层较发育的区域次生孔隙较发育,在构造高部位侧翼的厚层滩坝砂岩次生孔隙也较发育。依据次生孔隙的结构特征、胶结物的性质和产状、碎屑颗粒的性质将博兴洼陷滩坝砂岩划分为4种成岩相:其中不稳定碎屑溶蚀成岩相控制的区域次生孔隙最为发育,可成为较好的油气聚集区;碳酸盐胶结和压实-溶蚀成岩相控制的地区次生孔隙发育次之,但也可以成为油气聚集区;压实成岩相基本不具储集性能。

Based on the data analysis of casting lamella, scan electron microscope and reservoir pressure, the mechanism of secondary pore of the beach-bar sandstones are researched in the upper Es4 of the Palaeogene in Boxing Sag. These secondary pores mainly include intergranular pore, intragranular pore, moldic pore, oversized pore and a little fracture pores. The selective dissolution is the primary cause for the sec ondary pores during the diagenetic process. The upcurrent of deep level promotes the migration of pore water and creates the condition for the formation of the secondary pores. The aluminosilicate and the carbonate are dissolved by CO2 and the organic acid, forming the large-scale effective secondary pores in the area. The transformation of clay mineral also contributes to the pore formation. The beach-bar sandstone can be divided into four types of diagenetic facies. The areas controlled by unstable clastic dissolution facies have massive secondary pores and the reservoir quality is best in the studied area. The sandstones with carbonate cementation facies and compaction-dissolution facies have the lesser porosity.