Shear localization as a key control on mass-transport complexes seal integrity: insights from geophysical datasets

Zhenghao Han, Nan Wu, Jinfeng Ren, Zenggui Kuang

Mass-transport complexes (MTCs), existing in all sedimentary basins worldwide, can serve as effective seals for hydrocarbons and carbon dioxide storage due to shear-induced overcompaction. However, localized seal failure is occasionally observed in specific part of the MTCs, leading to hydrocarbon and carbon dioxide leakage and posing potential threats to seabed stability. Due to the scarcity of borehole data that penetrate MTCs, their petrophysical properties remain poorly constrained, and the factors controlling their seal integrity are still not well understood. This study integrates 3D seismic and well log data to investigate the petrophysical characteristics of three stacked MTCs in the Qiongdongnan Basin, northern South China Sea, aiming to identify the key factors that control their seal integrity. Seismic interpretation reveals that the seal integrity of MTCs tends to fail in areas where frontal ramps or remnant blocks are present, whereas the remaining parts remain intact and effectively seal the underlying free gas and gas hydrate. Petrophysical analyses show that MTCs with frontal ramps or remnant blocks exhibit significantly lower compaction (i.e., reduced density and resistivity, increased porosity and permeability), suggesting a strong link between seal failure and the presence of these structures. Numerical simulations indicate that during MTC emplacement, shear localization develops in the lowermost part, forming a narrow, highly deformed band along the base of MTCs. The presence of remnant blocks and ramps and associated topographic highs can interrupt shear localization, causing segmentation of the basal shear zone. This segmentation reduces shear strain and compaction above these topographic highs, thereby compromising the seal integrity of the MTCs. Therefore, shear localization is the key factor controlling the seal integrity of MTCs. Topographic highs along the basal shear surface can disrupt this process, significantly affecting hydrocarbon distribution and the feasibility of carbon dioxide storage.