渤中凹陷孔店组致密砂砾岩微裂缝形成机制及影响因素

Formation Mechanism and Influencing Factors of Micro⁃Fractures in Tight Glutenite of Kongdian Formation in Bozhong Sag

渤中凹陷西南部多口井钻遇巨厚孔店组砂砾岩,整体埋深大,储层明显致密化,部分井段产能不足10 m^(3)/d,少部分井产能较高,储层差异演化机制是制约勘探的核心问题.针对储层形成机制,开展了薄片、扫描电镜、压实模拟等实验.研究表明,微裂缝发育程度决定了储层物性差异,储层裂缝发育程度受砾石成分构成及杂基含量的影响.以花岗岩为母岩的低杂基砂砾岩,钾长石裂缝发育,溶蚀作用强,储层物性好;高杂基砂砾岩,裂缝不发育,溶蚀作用弱,测试产能低;混杂了大量碳酸盐岩砾石的砂砾岩,储层早期胶结作用强,整体致密,压裂改造后效果仍较差.压实应力物理模拟实验表明,在模拟埋深2500 m以下,砾石出现粒内缝,并随着模拟埋深加大不断增加;裂缝发育的类型、特点与研究区微裂缝特征可比对,证实压实成缝是研究区裂缝形成的重要机制.钾长石与斜长石裂缝生成有很大差异性:钾长石更容易发育破裂缝,经后期流体溶蚀改造,形成粒内溶蚀扩大缝;斜长石由于易发生高岭土化、钠黝帘石化等次生改造作用,改变了矿物的力学性质,不易产生裂缝.

Many deep wells in the southwestern part of the Bozhong sag reveal thick glutenite of Kongdian Formation.Due to the large burial depth,the glutenite reservoir is obviously densified,and the production of some wells is less than 10 m^(3)/d.A part of wells have high productivity,and the differential evolution mechanism of reservoirs is a key issue that restricts exploration.Aiming at the differential evolution mechanism of the reservoir,this study carried out experiments such as thin section,scanning electron microscope,compaction simulation.It is found that the degree of development of micro-fractures determines the physical properties of reservoirs.And the differential evolution of the reservoir is significantly affected by the composition of gravel and the content of matrix.The glutenite whose parent rock comes from granite,with low matrix content,develops potash feldspar fractures,strong dissolution,and good reservoir properties.The glutenite with high matrix is characterized by underdeveloped fractures,weak dissolution,and low productivity.The glutenite mixed with a large amount of carbonate gravel has strong cementation in the early stage,and the productivity is still poor even after fracturing.The physical compaction simulation shows that intragranular fractures appear in the gravel below simulation burial depth 2500 meters,which increase with the increase of simulation burial depth;the types and characteristics of fractures can be compared with the characteristics of micro-fractures in the study area.This confirms that glutenite with low matrix content can generate a large number of compaction fractures.Potassium feldspar and plagioclase are quite different in their ability to form fractures:potash feldspar is more prone to form fractures,and which is further corroded by later fluids,forming dissolution enlarged fractures.Plagioclase is prone to kaolinization,sodium zoisite and other secondary transformations,which changes the mechanical properties of minerals,and is not prone to form fractures.