断层内部结构及其对封闭性的影响

先前的研究多考虑断层封堵和开启的2种极端状态,近来的研究认为,在多数情况下断层处于2种之间的状态,只有在静止期具有封闭能力的断层,才有可能对油气起封堵作用。分析断层对流体运移的影响,需要分析断层在演化过程中的内部结构特征。断层可以划分出破碎带、诱导裂缝带和围岩3部分,断层岩和伴生裂缝构成破碎带的主体部分。常见的断层岩包括断层角砾岩、断层泥和部分碎裂岩,它们充填在断层裂缝空间中,断层内部结构受断层形成时的构造应力性质、断层活动强度和围岩岩性因素的控制。从动态角度看,随着断距增加,断层活动伴随着裂缝的发育和岩石的破碎混杂,可用泥质源岩层厚度和断距的比值来划分不同的发育阶段。断层活动期为油气运移通道,在静止时表现出差异性的封闭,通常用断层渗透率和排替压力2个参数来定量评价断层的封闭程度。断层岩渗透率主要受断距、泥质含量、埋深等因素的控制;断层排替压力的预测方法有2种:一种是从断层岩成岩角度分析的"等效埋深法",另一种是分析实测排替压力与主控地质因素的"拟合法"。通过简化的断层模型,建立了渗透率、排替压力与主控因素的预测关系。和储层类似,流体在断层中的运移遵循多孔介质的渗流特征。利用断层两侧的流体压力和油气柱高度并不能直接评价封闭性能,还必须考虑油气充注史和流体压力变化历史。

Review on Fault Internal Structure and the Influence on Fault Sealing Ability

Recent studies have transformed the old polarized view of faults as either leaks or seals into realistic notions of more complex fault-fluid flow behavior.Only the sealing fault in inactive period could barrier oil and gas migration.Fault structure and the stage of fault growth control the mechanics and fluid flow properties in the crust.Usually,the internal structure of a fault can be divided into host rock,induced fracture zone with main slip plane and fault damage zone which include fault rocks and associated crack.Fault rocks filling in crack space include breccia,fault gouge and part of calaclastic rocks.The development of fault architecture in a layered sandstone and shale sequence is distributed in a consistent pattern determined by three factors: ① the relative contributions of different faulting mechanisms to faults growth and slip;② the intensities of fault activity;③ the distribution of rock types.Considering the physical process responsible for fault development,fault throw increases during activity,accompanying with fracture formation and rock clastation and mixture.The stages of faults growth can be measured by ratio between shale source rock thickness and fault throw.In an active period,faults become the main channels for vertical hydrocarbon migration.In a static period,faults show variability and dynamic characteristics of sealing,which could be measured by permeability and displacement pressure.Fault rock permeability in a layered sandstone and shale sequence has a favorable correlation with fault throw,shale content and the burial depth.Two methods are used to predict displacement pressure in faulted belt: ① evaluating the diagenesis of fault rocks that has an equivalent effect as buried depth of sedimentary rocks;② using the actual measured displacement pressure to match with key geological factors.Through the simplified fault model,we establish relationship between key geological factors and permeability and displacement pressure of fault rocks.Fluid migration in fault zone follows porosity seepage characteristics as it does in reservoir.Cross-fault pressure and petroleum column height can not be converted to seal capacities simply because charge history and sealing type influence sealing.