Free gas accumulations in basal shear zones of mass-transport deposits (Pearl River Mouth Basin, South China Sea): An important geohazard on continental slope basins

• Published in: Marine and petroleum geology 2017-03, Vol.81, p.17-32
• Main Authors: Sun, Qiliang, Alves, Tiago, Xie, Xinong, He, Jiaxiong, Li, Wei, Ni, Xianglong
• Format: Article
• Language: English
• Subjects: Fluid migration; Free gas; Geohazards; Mass-transport deposits; Seal units; South China Sea
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2016.12.029

Free gas is an important trigger of instability on continental slopes, and resulting mass-wasting strata can potentially form competent seals to hydrocarbon accumulations. This work uses two high-quality 3D seismic volumes to investigate fluid accumulations at the base of mass-transport deposits in the Pearl River Mouth Basin, South China Sea. In parallel, IODP/ODP borehole data are used to document the petrophysical character of mass-transport deposits formed in similar continental-slope environments to the South China Sea. The interpreted data show gas accumulations as comprising enhanced seismic reflections that are discordant, or vertically stacked, below mass-transport deposits with chaotic seismic facies. Gas was accumulated in basal shear zones of mass-transport deposits in response to differences in capillary pressure and porosity. Free gas in Zone A covers an area of at least 18 km2. In Zone B, the free gas is sub-circular in plan view and covers an area of 30.58 km2 for a volume of sediment approaching 1.5 km3. This work is important as it shows that vertical migration of gas is not significant in mass-transport deposits from the Pearl River Mouth Basin, but up-dip migration along their basal shear zones is suggested in multiple locations. As a result, free gas can pinch-out laterally to extend 1–2 km beyond these same basal shear zones. As a corollary, we show that free gas accumulations below mass-transport deposits comprise an important geohazard and should be taken into account when drilling continental-slope successions in both the South China Sea and continental margins recording important mass wasting. Strata charged with free gas form weak layers, hinting at a novel trigger of retrogressive slope failures on continental slopes worldwide. •Free gas trapped underneath mass-transport deposits is firstly documented in the South China Sea.•Seal capacity of mass-transport deposits is attributed to its intensive consolidation.•Free gas under the mass-transport deposits serves as an important geohazard on continental slope basins.•Retrogressive slope failure is possibly caused by the lateral migration of free gas.