Shear Localisation and Its Impact on Mass‐Transport Complexes Seal Potential: Insights From Geophysical Datasets

Mass‐transport complexes (MTCs), the deposits of submarine slope failures and common features of sedimentary basins worldwide, can act as effective seals for hydrocarbons and carbon sequestration due to shear‐induced overcompaction. However, seal failure is occasionally observed in specific parts of MTCs, leading to hydrocarbon and carbon dioxide leakage and posing potential threats to seabed stability. In this study, we combine 3D seismic and well log data from Qiongdongnan Basin, northern South China Sea, to investigate the mechanisms that influence and control the seal potential of MTCs. We identified three vertically stacked MTCs overlying a gas hydrate bearing interval. Seismic interpretation reveals that MTCs seal tends to fail in intervals where MTCs overlies the frontal ramp or remnant block, whereas the remaining intervals effectively seal the underlying gas hydrate. Petrophysical analyses show that MTCs intervals overlying frontal ramps or remnant blocks exhibit significantly lower density and velocity, higher porosity and permeability, indicating reduced compaction in these intervals. Numerical simulations indicate that during MTCs emplacement, shear localisation normally develops in the lowermost part, forming a narrow (10%–20% MTCs total thickness), highly deformed basal shear zone. However, shear localisation is disrupted by the remnant block or frontal ramp, leading to low shear strain and thus low seal potential in MTCs intervals overlying the remnant block or frontal ramp. Therefore, we propose that shear localisation is a key mechanism controlling the seal potential of MTCs. Disruption of this process during emplacement can significantly compromise MTCs seal potential, with important implications for understanding hydrocarbon distribution and for assessing the feasibility of submarine carbon sequestration using MTCs as natural seals.