3D structure and kinematics of salt walls and megaflaps driven by differential loading: lessons from analogue modeling and field examples

The two dimensional geometry and evolution of salt walls driven by differential loading could be considered well established from previous studies. However, little is known about the internal deformation at the flanking edges and the 3D architecture at their terminations. This induces risk in hydrocarbon exploration of poorly imaged areas that could be significantly reduced with a better understanding of near-salt deformation and salt-sediment interaction in three dimensions. Using an experimental approach based on sandbox models and comparing their results with the southeastern termination of the Gypsum Valley salt wall in the Paradox Basin, we investigate both the internal deformation and the 3D structure of rectilinear salt walls emphasizing the development of faults and folds, including megaflaps, around their terminations. The experimental program includes 5 experiments simulating the salt, pre-kinematic overburden and syn-kinematic detrital deposits with a pure polymer, a sand/clay mixture and silica sand, respectively. It includes experiments with different salt/pre-kinematic overburden thickness ratios and salt wall edge geometries.

The experimental results document that in rectilinear salt walls driven by differential loading the plan-view geometry of the salt walls depends (amongst other factors) on the salt thickness. Hence, salt wall terminations developed over a constant-thickness salt layer are abrupt with a rectangular form; and those developed over a progressively decreasing salt layer thickness are gradual with a plan-view triangular shape. They also show that:

• At the salt walls edges, the pre-kinematic overburden is folded by limb rotation in the younger minibasin side and by limb lengthening with vertical shearing in the older minibasin side.
• The prekinematic overburden is internally almost underformed in the younger minibasin side where megaflaps developed. Instead, it is much strongly deformed (faulted) and broken by extensional faults in the opposite older minibasin side. This could have strong implications for hydrocarbon exploration since: 1) the megaflaps lack any significant secondary porosity and have lateral continuity of bedding; and 2) the prekinematic package of the older minibasin is compartmentalized and affected by internal deformation that can significantly increase its porosity and permeability.
• At the ends of the salt walls, faults are mainly parallel or perpendicular to the diapir trend, being predominantly transversal in the abrupt terminations and longitudinal in the gradual ones.
• Radial faults are rare and only present at the corners of abrupt salt wall terminations.