Fold and Thrust Belts

Volumetric restoration can provide crucial insights into the structural evolution of three-dimensional (3-D) petroleum systems. A major limitation to its widespread application is the need to include complex architectures and realistic mechanics such as flexural slip. We apply an implicit approach that allows for, including unconformities, thin and/or pinched-out layers in the models but that cannot explicitly localize slip along horizons. To take advantage of this approach while accounting for flexural slip in 3-D restoration, we investigate new geomechanical properties. We consider flexural slip folding as a result of stacked rigid and thin weak layers, which can be modeled using transversely isotropic properties. We compare restorations of an anticline using transversely isotropic properties, isotropic properties, and a stack of rigid isotropic layers with nonfrictional slip between the layers. Our results show that transversely isotropic properties reasonably approximate flexural slip folding. We use these new tools to model the evolution of a complex system located in the Niger Delta toe. The system includes a detachment fold, a fault-bend fold, and a structural wedge formed in series. Growth stratigraphy and erosional surfaces delimit the kinematics of deformation. Regional erosive surfaces, 3-D gradients of fault slip, and vertical variations in mechanical strength motivated the use of our new restoration techniques. Restoring two growth units results not only in reinforcing the interpretation that the area is behaving as a deforming thrust sheet at critical taper, but also in highlighting coeval activity on both the hinterland structures and the toe of the thrust belt.