The Formation of Forced Folds and Wing‐Like Sand Intrusions Driven by Pore Fluid Overpressure: Implications From 2‐D Experimental Modeling

Sand injections form by intrusion of overpressured, fluidized sand into surrounding low‐permeable, fine‐grained rocks. Modern 3‐D seismic data revealed their abundant occurrence in many sedimentary basins and that their intrusion is typically associated with forced folding and tensile fractures of the sealing cover layer. In order to investigate the kinematic evolution of forced folds in relation to the associated propagation of fractures originating from an overpressured source layer, we performed idealized, quasi‐2‐D analog experiments. The models consist of noncohesive and cohesive granulates to mimic a sand reservoir and its overburden layer and injected air to produce fluid overpressure in the layered materials. Our results show that forced folding first induces tensile bending fractures at the base of the fold limbs at a certain critical fluid pressure. Due to further increase of the fluid pressure, the apex of these bending fractures serves as origin for branching, conical fractures characterized by shear and tensile failure. Fracture breakthrough is accompanied by a rapid uplift of the breached fold limb and a pressure drop in the reservoir layer followed by a continuous subsidence of the central forced fold. The morphology of the fracture pattern and the forced fold provides helpful implications for understanding formation processes of natural sand injections observed in seismic data and in outcrops.