Insights into the Regional Evolution of the Outer Fold-and-Thrust Belt, Niger Delta, from Combining New Techniques in 3-D Sequential Geomechanical Restoration with Decompaction

We propose to use new techniques in 3D restoration to fully restore a complex model of compressional structures including a detachment fold, a forethrust fault-bend fold, and a structural wedge in series located in the outer fold-and-thrust belt, deepwater Niger Delta. The structural growth history is recorded by growth stratigraphy and erosional surfaces that record the kinematics of deformation. Beyond the problem of the mesh generation that such a system raises, the vertical variations in mechanical properties, the flexural-slip folding, and the non-cylindrical nature of the structures make the 3D restoration of this system challenging.

We address these challenges by employing a new 3D geomechanical restoration method with an implicit meshing method that facilitates the mesh generation of models including thin layers, unconformities, and/or pinch-out. In addition, we use a transversely isotropic material property in the geomechanical restoration that has been shown to be a reasonable approach for modeling flexural-slip folding without explicitly including slip surfaces in the 3D model. Moreover, accounting for decompaction during sequential restoration improves the assessment of the basin history. An exponential porosity-depth relationship is used to compute the decompaction in 3D after each restoration step.

We combine these new techniques to restore our model sequentially, using transverse materials and an appropriate set of boundary conditions, and apply decompaction after each restoration step. The results are compared to kinematic restorations of regional transects. In the case of inconsistencies between the kinematic and mechanical techniques, such as extremely different regional shortening or fault slip amounts, additional displacement constraints are set to better constrain the 3D geomechanical restoration. The outcomes of the restoration, such as strain distribution and 3D gradients of fault slip, allow us to enhance our understanding of the regional evolution of the Niger Delta toe and demonstrate the capabilities of geomechanical restorations in addressing complex, 3D deformations with sediment compaction.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California