Relationship between fluid-escape pipes and hydrate distribution in offshore Sabah (NW Borneo)

Journal article

Fluid-escape pipes represent seismic evidence for the focused cross-stratal migration of fluids. In natural gas hydrate systems, these features serve both as conduits for methane-rich fluids and as preferred locations for the formation of gas hydrates. In this study, 3D seismic, well-log and core data from offshore Sabah (NW Borneo) are used to investigate the controls on the occurrence of fluid-escape pipes and their impact on hydrate distribution in a system dominated by the vertical leakage of thermogenic hydrocarbons.

The pipes are observed within a gas hydrate stability zone (GHSZ) that extends 100 m below a bottom simulating reflector (BSR), located at 155 m below the seafloor (mbsf). Pipes are restricted to an area with evidence of free gas-bearing sediments, suggesting a causative link where the free gas promotes the build-up of critical fluid pressures. The stacking of the upper terminus of fluid-escape pipes at discrete stratigraphic intervals suggests that fluid flow to the seabed has been episodically enhanced. Possible triggers for cyclical increases of pore fluid pressures are sea-level and temperature fluctuations, tectonic activity and gas leakage from deep reservoirs.

This fluid flow system further impacts the gas hydrate distribution. The fluid-escape pipes can be locations where hydrates occur at high concentrations up to the seafloor if the pipe is presently active. Therefore, the observed up-bending of the stratigraphic reflections along the pipes are interpreted as a combination of a net volume increase of the host sediment owing to hydrate formation and seismic velocity pull-up effects. Away from the pipes, hydrates do not occur until 65–152 mbsf and are present only at low to moderate concentrations. At this site of focused fluid flow, fluid-escape pipes constitute, by volume, only 7–11% of the gas hydrate occurrence zone. Nevertheless, we predict that they could host between 20 and 50% of the whole hydrate volume. It is therefore likely that, in similar systems, a volumetrically significant portion of the total hydrate reservoir is hosted within fluid-escape pipes. The distribution of these features should thus be considered as a critical parameter for hydrate volume estimates.

Authors: Paganoni, M; Cartwright, J; Foschi, M; Shipp, C; Van Rensbergen, P

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Marine Geology
• Volume: 395
• Pages: 82-103
• Publication date: 2017-10-09
• Acceptance date: 2017-09-22
• DOI: 10.1016/j.margeo.2017.09.010
• ISSN: 0025-3227
• Keywords: bottom-simulating reflector; free gas zone; gas hydrates; fluid-escape pipes; chimneys; fluid flow