Hydrogeological structure model of the fault zone in the karst area of north China

The fault zone has a controlling effect on the mechanical properties and fluid flow characteristics of the crust. It is an extensively distributed and very important structural pattern in the upper crust, and widely participates in the crustal activity process. At present, the structural composition and fluid flow pattern of fault zones are hot and difficult points in foreign research, and the research methods and understandings are different in various fields. In the field of petroleum geology, mature quantitative evaluation techniques for fault sealing have been developed in hydrocarbon migration and accumulation. In contrast, research of hydrogeological properties of faults is still in the stage of qualitative evaluation of mechanical properties on the water conductivity or water resistance of faults. Studies on fault zone structure, permeability anisotropy and other aspects have not yet been carried out. At present, many scholars and teams at home and abroad have done much meaningful research and discussion on the internal structure characteristics of the fault zone and its influence on fluid flow. This paper summarizes the results of foreign studies on the hydrogeological properties of fault zones in terms of structural composition, fault zone evolution, permeability factors, etc. The hydrogeological structure model of the fault zone in the northern part of the Taihang Mountains is introduced by taking the fault that is developed in the carbonates of the Cambrian Zhangxia formation with clastic water-bearing rocks as an example.

In previous studies on the composition and permeability structure of fault zone, its composition is divided into fault core and fracture zone. The structure of fault core and fracture zone determines the heterogeneity and anisotropy of permeability structure of fault zone. In the fault zone, the thickness ratio between fault core and fracture zone provides a convenient and generalized framework for describing the hydrogeological characteristics of fault zone. If the permeability of each component of the fault zone is combined with geological maps, geological profiles, or three-dimensional structural models of the fault zone, the permeability structure of the fault zone can be obtained from field outcrops. The permeability characteristics of each component of the fault zone are the most important to study the fluid flow properties of the fault zone. The permeability characteristics are mainly affected by the structural characteristics of the fault zone, the properties of the fluid passing through the fault zone, the tectonic stress, the scale of the fault slip and other factors. The geometric structure and permeability of the core and fracture zone are the main controlling factors to characterize the hydrogeological properties of the fault zone, which is characterized by the type of water-blocking and water-guiding system of fluid flow. According to the thickness ratio of its component parts, the fault zone can be divided into four types: single fault, dispersed deformation zone, local deformation zone and composite deformation zone. Meanwhile, the corresponding permeability structures are local water conduction, dispersed water conduction, local water resistance, and composite water-resistance. In order to accurately reflect the hydrogeological properties of the fault zone, in this study, we should take the different positions of the fault zone into full consideration when constructing the hydrogeological structure model, and to generalize the permeability structure. The structure and hydrogeological properties of the studied fault zone will be different due to the different scale and precision of the selected fault zone and the different development sites. Taking the northern Cambrian carbonate aquifer as an example, there are clastic rocks with different thickness and relative water isolation, where lithology is dominated by thin layer shale. At different positions of the same fault zone, the different distribution characteristics of the water-proof layer and the fracture mechanism result in significant differences in the permeability and hydrogeological characteristics.

How to establish the characteristics of the spatial structure and permeability of the fault zone is the basis and premise for the accurate evaluation of the fluid flow characteristics of the fault zone, which requires geologists to have the professional knowledge of structural geology, rock mechanics, numerical simulation and other fields. Through the sharing of data in various fields and the integration of multidisciplinary methods, accurate and typical permeability structure models of fault zone are established so as to promote the study on development of fault zone and fluid flow characteristics.