3D seismic analysis of subsurface gas migration and the gas hydrate system offshore Mauritania

Abstract

3D seismic data are used in this thesis to investigate fluid migration processes and the gas hydrate system offshore Mauritania. The studied interval was deposited during the Neogene, characterised by fine-grained hemipelagic sediments, polygonal faults, mass transport complexes (MTCs), high amplitude anomalies (HAAs), and a widespread bottom simulating reflection (BSR).
In the study area, there are many localized HAAs above some MTCs. Their spatial relationship indicates that there was upward gas migration up the margins or local faults associated with the MTCs. One possible gas source could be dissociated hydrates due to the upward resetting of the BSR, but there may be other sources like in situ biogenic gas. At shallower depth, a gravity-driven fault below the BSR acted as a conduit for gas recycling after hydrate dissociation. There is a close spatial relationship among the fault location, a breached gas accumulation and a shallower free gas zone (FGZ), suggesting the process of gas recycling between hydrates and another gas trap. The BSR is investigated which is composed of alternating high and low amplitudes and characterized by amplitude bands on the BSR map. The high amplitude bands are interpreted to be caused by free gas trapped below the base of gas hydrates, potentially in spaced beds of coarser sediments. These amplitude bands provide evidence of lateral changes in the ambient conditions that control the depth of the BSR, including the variable water depth of the slope, impingement of salt diapir and mud volcano, and canyon cutting at the seabed.
Generally this thesis exemplifies the potential of 3D seismic data in studying the gas hydrate system and related processes. For example, mapping the BSR on 3D seismic data can show lateral changes of the BSR at different depths from only one map, representing an effective method to study the base of gas hydrates.