Inclined-Shear Restoration of Growth Folds

Abstract

We present a technique of progressively restoring seismic, well, and outcrop data, as well as structural interpretations of contractional growth folds. Data restorations allow viewing of seismic, well, and outcrop information in the restored state, providing a basis to evaluate whether structural interpretations are balanced and geologically reasonable. Validating structural interpretations helps to reduce the exploration risks associated with trap definition; moreover, sequential restorations reveal folding kinematics and define the geometry of structures at various stages in geologic history. These sequential restorations may be used to define the timing of trap development relative to hydrocarbon migration and to determine how traps may have been modified by subsequent deformation. To restore fold growth, we use the approximation of inclined-shear restoration vectors parallel to the axial surfaces of the folds and propagate them homogeneously downward in the section. This choice of restoration vectors is commonly based on direct observation of axial surface orientations in data and generally satisfies appropriate balancing constraints. Inclined-shear restoration works well for simple kink-band migration folds and gives good approximations of restored geometries for folds developed by limb rotation and some more complex folding mechanisms. We present examples from southern California, the Spanish Pyrenees, and Angola.