Quantitative analysis of fracture evolution in fold: Insights from FDEM modelling

Abstract

Fractures associated with buckle folds play a vital role in hydrocarbon migration and accumulation in reservoirs. However, the prediction of fracture initiation and propagation during buckle folding is technically challenging. In this study, a series of buckle folds with different initial geometries, which represent different deformation stages of folds, are simulated to study the fracturing mechanism during buckling. To obtain a better understanding of the progressive fracturing process, a 2D modelling approach using an improved combined finite-discrete element method (FDEM) is utilized. The influence of fold geometries with significant influence on fracture development are considered, including layer thickness and wavelength. The results show that the development of fractures can be directly observed through the improved FDEM modelling. It is concluded that the development and distribution of major fracture sets are different at various periods during the evolution history of the fold. Tensile fractures initiate more easily in tighter folds, while shear fractures develop more easily in gentle folds. The pressure from the overburden or the constrains of the surrounding rock is critical for the developments of fractures at hinges. As the fold becomes tight, the density of fractures decreases, whereas the length of single fractures increases. The results further show that the layer thickness and wavelength are vital for the initiation and propagation of major fracture sets. In summary, the FDEM modelling approach can well explain most of the fracture sets occurring in the buckle fold. This study has provided a deeper insight into the fracture evolution in fold.