Three-dimensional Seismic Analysis and Modelling of Marine Hydrate Systems Offshore of Mauritania

Abstract

Marine hydrates, which lock-up vast quantities of methane, are considered to be a prospective alternative energy source, a slow tipping point in the global carbon cycle and a probable trigger for submarine failures. In this thesis marine hydrate systems offshore of Mauritania and associated structural and sedimentary features are investigated by utilising two surveys of high-quality three-dimensional (3-D) seismic data. Interpreting them provides new insights into marine hydrate systems and how they respond to changes in ambient conditions.

In one region of one of the 3-D seismic surveys, a shear zone covering 50 km2 is identified immediately above the hydrate bottom simulating reflector (BSR). It is considered to be the initial stages of a failure that did not result in widescale downslope transport of the succession. Due to this failure not going to completion, some free gas remains trapped at the level of the BSR. At this level the presence of free gases is supported by the continuous high-amplitude reflections. It is proposed that buoyancy built up by the inter-connected gas accumulation increases the pore pressure of the overlying hydrate-bearing to the level such that its base was critically stressed. In this research there is no seismic evidence for failures triggered by hydrate dissociation but the role of free gas in priming submarine failures is examined.

Whether marine hydrates can release significant amounts of methane into the atmosphere is inconclusive. In this research a proposed model indicates that methane was re-captured in the hydrate stability zone after being liberated. Ocean warming since the last glacial maximum (LGM) gave rise to the shoaling of the base of the hydrate stability zone (HSZ). Gases released from hydrate accumulating at the base entered the HSZ, driven by buoyancy built up in the gas accumulation. The hydrate seal was breached and this is manifested by 15 gas chimneys in seismic data. Hydrates then re-formed at a specific level within the HSZ. This study implies that not all of methane would enter the ocean after released from hydrates and therefore the contribution of marine hydrates to the atmospheric methane budget may be not that much as it was predicted before.

Gas venting is an effective way to transport methane at depth vertically to the ocean and an example of it is found in the feather edge of marine hydrate. This venting was possible due to the presence of faults above a salt diapir and is manifested by a series of pockmarks and mounds at the seabed. The BSR at this site is convex upwards and hence formed a trapping geometry for underlying free gases. Numerical model shows that this up-convex geometry is caused by the salt diapir having a higher thermal conductivity. Permeable migration conduits along the faults and excess pore pressure at the top of the trap allow for the happening of the venting. Compared with the neighbouring area where the BSR can be well observed, the region affected by diapirism has a limited scale of the observable BSR. This absence is proposed to result from the formed trap intercepting methane-rich pore fluid that would migrate landwards along the level of the base of the HSZ.