Pure Void Space and Fracture Pore Space in Fault-Fractured Zones

Xiaogang Li,Andrew V Zuza,Zhiwu Li,Gang Chen,Shaohua Xu,Shihu Wu,Chen Wu,Guoqiang Xu,An Yin,Yiwen Li

doi:10.3389/feart.2021.683439

Abstract

Fault-fractured pore space is complex and difficult to predict and evaluate. For a single independent ramp-flat fault-bend fold structure, the pure void space between two fault walls equals the integrated fracture pore spaces within the fault damage zone if it were concentrated on the fault plane. Using an area balancing technique and geometrical relationship, we have developed a two-dimensional (2D) model to calculate the pore space of fractures associated with fault development. The development and distribution of fault detachment voids or fault fracture pore space are controlled by the physical properties of the deforming medium, mechanics of deformation, and geometry of a fault-ramp structure. We demonstrate how concordant or discordant folding of the fault wall rock affects the nature of fault-fracture pore space. The pure void space and fracture pores in the fault zone can be quantitatively described by the following parameters: initial ramp angle and height, overlap ramp length, throw and slipping displacement, stack thickness, curvature and derivation of the angle between bed and fault plane (Rθ), and dip isogons.Rθreflects the conformity of two opposite fault sections and the folding accordance of two walls, and it is a key element for the development and distribution of fracture pore space in a fault zone. Furthermore, we observed natural outcrops supporting and validating our model assumptions in the foreland fault system, Central China.

Highlights

The relationship between the ramp angle and the fracture porosity based on Equation (6) indicates that the ramp angle is inversely proportional to the fracture porosity
For a fault-ramp structure developed within rigid strata, pure void space is predicted to form along the fault depending on the relative curvature of the hanging wall and footwall wall rock
This void space equals the sum of fracture pore space and filledin void space around the fault zone; we quantify this void space assuming simple area balance

Summary

INTRODUCTION

Understanding and quantitatively predicting the formation, location, orientation, intensity, porosity, and permeability of natural fractures within a fault damage zone (Caine et al, 1996; Kim et al, 2004; Choi et al, 2016; Peacock et al, 2017) are important for hydrocarbon exploration and production planning activities (Aydin, 2000; Nelson, 2001; Smart et al, 2009; Feng et al, 2018) and for site selection for large engineering projects and earthquake prevention and disaster reduction (Beach et al, 1999; Scholz, 2002; Huang and Li, 2009). The height of the largest void we have observed in outcrops or subsurface drilling, which filled by calcite, was ∼1 m (see the below as an outcrop case), but a restored “fault detachment void space” extracted from the fracture pores should exist between two translated walls (Figure 1C) In such a case, the top crest was arched (instead of a flat top crest in Figure 1B), the footwall maintains the original form on cross section, and there is no deformation on the in-line section; the stack thickness (Hs) is larger than the sum of throw and rock thickness (Hs > Hf + Hr); the front displacement is less than the back displacement (D1 < D2) because of the slip displacement consumed in the fold, and such displacement is closely related to the fault detachment voiding. It has the three basic structure configuration types: (1) both of the walls deformed into anticline (Figure 3D); (2) both of the two walls

D External configuration

DISCUSSION

CONCLUSIONS

DATA AVAILABILITY STATEMENT