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

Abstract

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 km 2. In Zone B, the free gas is sub-circular in plan view and covers an area of 30.58 km 2 for a volume of sediment approaching 1.5 km 3. 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 failure on continental slopes worldwide.

True Volumes of Slope Failure Estimated From a Quaternary Mass‐Transport Deposit in the Northern South China Sea

“…Shear compaction during slope failure can cause overconsolidation of emplaced MTDs. This has been confirmed by identifying losses in porosity and water content in failed sediment (Piper et al, ; Shipp et al, ), by core observations (Strasser et al, ; Tripsanas et al, ), and by observations of trapped free gas underneath failed material (Sun, Alves, et al, ). Multiple DSDP/ODP/IODP wells have been drilled through failed sediment (e.g., Dugan, ; Piper et al, ; Strasser et al, ).…”

Different origins of seafloor undulations in a submarine canyon system, northern South China Sea, based on their seismic character and relative location

“…Sun et al, 2012;2017). Gas chimneys, pipes and faults can act as pathways for the vertical migration of free gas and fluids (Sun et al, 2017). The presence of free gas and fluid increases overpressure in shallow strata, and reduces its shear strength until the creeping movements occur (e.g.…”

Gas migration pathways and slope failures in the Danube Fan, Black Sea

“…Such failures are also found along the shelf break in the Danube Fan (Ker and Riboulot, 2015). Gas hydrates have frequently been linked to submarine slope failures, and it has been proposed that dissociation of hydrates following a shift in the GHSZ due to sea level lowering or seafloor warming may be a factor in slope failure events through generating overpressure and causing sediment destabilisation (Crutchley et al, 2007;Dondurur et al, 2013;Horozal et al, 2017;Li et al, 2016;Mountjoy et al, 2014;Sun et al, 2017). In addition, there is some debate regarding the impact that the presence of gas hydrates has on the stability of sediments.…”

“…Based on the observation of the position of the BSR corresponding with the base of MTD 4 deposits beneath the S2 slump scar in the seismic data, it would appear that the sediments of MTD 4 also facilitate the migration of gas, with discontinuities in the chaotic seismic facies likely providing flow conduits that result in the formation of gas migration features and flares (Thöle et al, 2016). Upward migration of gas through the sediments is also prevented by MTD deposits in some cases (Sun et al, 2017). This would explain the capped structures that are observed upslope of the S2 slump ( Figs.…”