Abstract

Buckle folds of single-layer and multilayer sedimentary strata characterized are among the most common structural traps for hydrocarbon reservoirs. The spatio-temporal evolution of the stress state in folded reservoirs is of significant importance for the understanding and predicting patterns of fractures associated with buckle folds. This paper compares the evolution of the principle stress state associated with buckle folding by studying the deformation process of two different geometries: single-layer and multilayer buckle folds. A 3D finite element modeling approach is applied to simulate the buckle folding of single-layer and multilayer stacks with Maxwell visco-elastic rheology. It is concluded that the stress state within the folding layer(s) are determined by the buckling process, fold geometry and material parameters. This study shows that the presence of the competent layers has a governing influence on the amplification rate of fold amplitude and the evolution of the strain/stress field. The lowest magnitude of effective minimum principle stress is found at the top of the hinge of the bottom competent layer in the three-layer system and central competent layer in the five-layer system. Moreover, layer-parallel extensional strain and low magnitudes of layer-parallel principle stress is observed between the limb and the hinge of the central competent layer in the five-layer system. Little differential stress develops throughout the folding layer in the less competent layer(s). This study also shows that tensile fractures perpendicular and parallel to the fold axis at the hinge and in the limb can be explained by the developed overpressure in low permeability rock. In summary, this study shows that strain distribution and stress evolution within buckle folds are directly dependent on the number of competent layer(s) and the distribution of material parameters.

1. INTRODUCTION

Fold systems in deformed rock are considered as common structural trap systems for petroleum reservoirs. Understanding the mechanics of folding and associated natural fracture systems is essential for a better understanding of hydrocarbon migration and production [1]. In nature, multilayer folds are more frequent than single-layer folds [2]. The structural development of multilayer systems during buckle fold evolution have been extensively studied by either considering the competent and incompetent layers as separate objects [e.g., 3, 4] or treating the multilayer system as a homogeneous but anisotropic body by applying average properties [e.g., 5-7]. More recent studies have applied different approaches including thin-beam equations [e.g., 3, 8 and 9] and fluid dynamics [e.g. 5, 10-13] to study folding in multilayer system. The observed fold shapes and the tectonic history of multilayer system are found to be dependent on various parameters, such as the amount of bulk shortening [14], layer thickness [12], the rheology [15], the viscosity ratio [16,17], and the type of active folding [2]. Moreover, a detailed analysis on the strain evolution of a two-layer fold system has been performed by using finite strain ellipses and the two layers are found to form parallel folds with changing thickness between them [18].

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