Modelling approaches to understanding fold development: implications for hydrocarbon reservoirs.

A mechanics-based approach to the evolution of buckle folds is proposed. Existing analytic and simple numerical models are used to demonstrate that dramatic changes in layer parallel stress occur in developing folds and that the bulk effective rheology of folding rocks is strongly reduced in relation to neighbouring rocks with no folds. A methodology for estimating regions of folds more or less likely to suffer fracture is set out. In this methodology a simple abstraction of the natural fold is identified and the stress history during its development is calculated using linear viscosity as a proxy for competence. This simple and computationally cheap approach allows all salient features of natural multilayer folds to be recreated in numerical experiments. The final step in the method is to use the stress conditions for three types of fracture failure, tensile failure, dilatant shear failure and grain crushing to define potential functions that indicate increased or decreased probability of failure. Results give predictions that agree well with observation in the simple cases studied. A novel method for approximating three-dimensional deformations is used to model the propagation of folds in the axial direction and it is found that this occurs rapidly and allows constructive and destructive interference of propagating folds below a certain amplitude. This provides a means by which perturbations may interact over a certain distance. Such a mechanism was an implicit requisite of classic fold theories, but its identity has been obscure for three decades. It is shown that many small perturbations are needed to give patches of coherent folds and that few, large perturbations give single arc folds.