Active hinge folding-related deformation and its role in hydrocarbon exploration and development: insights from HCA modeling

Thrust-related folding is an important and efficient mechanism for generating wide, deformed rock panels at shallow structural levels. In these environmental conditions, three basic forms of deformational features commonly occur: pressuresolution seams (stylolites), extensional fractures (joints and veins), and small-scale faults. Two end-member mechanisms of thrust-related folding can be identified: active-hinge folding and fixed-hinge folding. The spatial distribution of longitudinal deformational features (LDF) induced by fixed-hinge folding is confined within the axial-surface zone, and its intensity is roughly proportional to the fold interlimb angle. Conversely, the spatial distribution of LDF induced by active-hinge folding is arranged in well-defined rock volumes-the deformation panels, each of which is affected by a characteristic LDF intensity. Deformation panels result from the development and interference of deformation domains, which correspond to the rock volumes that underwent deformation during their rolling across the active axial surfaces. The geometry of deformation domains is therefore defined by the architecture of the axial surfaces and the amount of fault displacement. The spatial distribution of deformation panels bears important insights for predicting the distribution of secondary permeability in folded carbonate reservoirs where, in the past, the permeability in the anticlinal crests has been overestimated. The development of deformation panels and deformation domains related to longitudinal deformational elements has been numerically simulated by a hybrid cellular automata (HCA) modeling technique for fault-bend and décollement folding. Model results show that, in many cases, active-hinge anticlines have a crestal zone that is unaffected by folding-related, longitudinal deformational features, and that deformation panels develop along the corresponding limbs. Our numerical results compare favorably with the spatial distribution of deformation in field examples and in analog models. The numerical technique we developed provides an efficient tool for evaluating the spatial distribution of fracture porosity and permeability in folding-related fractured reservoirs.