Fracturation associated to fault-related folds: 3D analogue models

3D analogue models have been used to investigate the distribution pattern of fractures during the evolution of fault-related folds. In the upper part of the brittle crust, folds are often associated to fractures. The timing and distribution of these fractures may have significant implications for hydrocarbon exploration and production in folded and fractured reservoirs. Successful development of fractured reservoirs in anticlinal traps requires knowledge of the spatial variability of joint orientations throughout a fold, as the production may strongly depend on the permeability created by systematic joint sets. We present the results of two sets of sandbox experiments that reproduce geometry and kinematics of a fault- propagation fold and a fault-bend fold, in order to analyze the relationship between fracturing and evolution of folding. In both experiments, an initial 6 cm thick layer sand pack was constructed manually, trying to minimize local inhomogeneities and packing anisotropy. Coloured (but mechanically equivalent) layers were used as marker horizons within the sand-pack to facilitate identification of faults and displacement measurement in cross-section view. Finally, at the top of the multilayer, a thin layer of SI-CRYSTAL powder, 0.5 cm thick, has been added. The strong cohesion of this material allows fractures to open, without collapsing. As SI-CRYSTAL powder and dry sand have Mohr-Coulomb failure envelopes, they are both suitable for modelling the brittle upper crust. During the experiments, a moving piston caused horizontal progressive shortening within the multilayer. Two digital cameras continuously monitored in map and in section view the evolution of the models. Moreover, a topographic measurement device (a structured light 3D scanning system) allowed to acquire the topography in the different shortening steps movements. At the end of each experiment (21,4% shortening), the model was cut in order to obtain internal vertical sections. All these steps allowed to reconstruct in 3D and analyse the two fault-related folds, to highlight differences and analogies in the fracture development and distribution through time/shortening. The data show a main fracture pattern parallel to fold axes, but interesting and significant differences occur in the number of fractures, their average length, continuity and other parameters that have been automatically detected using the 3D models. 3D reconstruction of models and their quantitative analysis are therefore fundamental tools for hydrocarbon exploration and evaluation in folded reservoirs as it allows to predict the spatial distribution of fracture in the different types of folds, related to their amount of shortening.