张性断裂带内部结构特征及油气运移和保存研究

断裂带是一个宽度、长度和高度均与断距呈正比关系的三维地质体,具有典型的二分结构:即断层核和破碎带。断层核由多种类型的断层岩和后期胶结物组成,具有分选差,粘土含量高,颗粒粒径小等特征,表现为具有比围岩更低的孔渗性。破碎带同围岩相比发育大量的裂缝,裂缝的密度随着离断层核距离的增大而逐渐减小,孔渗性较高。断层岩类型取决于断移地层的岩性、成岩程度和断裂变形时期。对于同生断层而言,泥岩和不纯净的砂岩主要发生泥岩涂抹作用;纯净砂岩发生解聚作用,形成颗粒重排的变形带。中成岩阶段发生断裂变形,泥岩发生泥岩涂抹作用,不纯净的砂岩发生碎裂作用和层状硅酸盐涂抹作用,形成碎裂岩和层状硅酸盐-框架断层岩;纯净砂岩主要发生碎裂作用,形成碎裂岩。晚成岩阶段发生断裂变形,碎裂作用成为主要的变形机制,泥岩破碎形成大量断层泥,不纯净的砂岩和纯净的砂岩均形成碎裂岩,其中纯净砂岩形成的碎裂岩由于石英的压溶胶结变得更致密。因此不同成岩阶段、不同岩性形成的断层岩类型不同,泥岩涂抹的排替压力高于层状硅酸盐-框架断层岩和碎裂岩,即使都是碎裂岩,其渗透率相差7个数量级。从断裂带结构看油气运移和保存,断层垂向封闭主要靠剪切型泥岩涂抹的连续性,侧向封闭能力取决于断层岩物性,物性很高的碎裂岩自身封闭能力很差,依靠两盘岩性对接封闭油气,最小断距决定油水界面位置。物性很低的断层岩一般能封住一定高度的油气柱,其是断裂带中泥质含量的函数。断层在储盖层段变形机制差异,决定了断裂输导与封闭油气的耦合,即破碎带双向输导充注,盖层段剪切型泥岩涂抹顶部封闭,断层核遮挡成藏。

Internal structure of normal fault zone and hydrocarbon migration and conservation

Fault zone is a three-dimensional geologic body,of which width,height and length have a positive correlation with fault displacement.Fault zone has a typical binary structure: it consists of fault core and damage zone.Fault core is made up of diverse types of fault rocks and later-stage cementing materials.Fault core is characteristic of poorly sorted,higher clay concentration,small particle and lower porosity and permeability than those of the surrounding rocks.Comparing with surrounding rock,the damage zone is developed with abundant fractures,and as the distance away from fault core increases,the density of fractures is progressively decreasing.And porosity and permeability of the damage zone is higher than those of the surrounding rock.Types of fault rocks depend upon the lithology of the faulted rock formation,its diagenetic grade and epoch of the ruptural deformation.As to the syngenetic fault,in mudstone and impure sandstone mainly clay smear occurs,while in pure sandstone,disaggregation is generated forming deformation zone with particle rearrangement.In the middle diagenetic stage,ruptural deformation happens,and at the same time,clay smear is generated in mudstone,cataclasis and phyllosilicate smearing happened in impure sandstone forming cataclasites and phyllosilicate-framed fault rock,and cataclasis mainly occurs in pure sandstone forming cataclasites.In the late diagenetic stage,ruptural deformation happens;cataclasis becomes the principal deformation mechanism;mudstones are broken forming large amount of fault clay;both impure sandstone and pure sandstone are forming cataclasites.The cataclasites derived from pure sandstones are tighter because of the pressure solution and cementation of quartz.Hence types of fault rocks are different owing to various diagenetic stages and different lithology.The entry pressure of the clay smear is higher than phyllosilicate–frame fault rock and cataclasites,and disparity of their permeability is of 7 orders of magnitude even they are all cataclasites.Judging from the fault zone structures,we can analyze the migration and reservation of oil/gas.Vertical fault sealing mainly relies on shear clay smear,while capability of lateral fault sealing is determined by physical properties of the fault rock.Cataclasites with high physical property may have poor capability of self-sealing,it seals oil/gas mainly depending on the juxtaposition seal of hanging wall and footwall,and oil/water boundary is decided by the minimum fault displacement.Ordinarily,fault rock with low physical property can seal specified height of hydrocarbon column,and the height is a function of shale content in the fault zone.Differences of fault deformation mechanism in reservoir and cap rocks decide the coupling of fault conducting and sealing for oil/gas,in other words,the damage zone is filled up and conducts in two-ways,shear clay smear in cap rock forms the top-seal,fault core shields oil/gas and finally forms the reservoir.