Natural gas hydrate systems as pressure valves: a multi-scale study

In methane-rich geological environments, methane and water can combine at high pressures and low temperatures to generate natural gas hydrates, a solid compound that usually fills the primary and secondary pore space of the sediment. Beneath areas of gas hydrate occurrence, a zone characterised by free gas-bearing sediments is usually observed in seismic reflection data. The presence of gas hydrates is thought to profoundly influence the fluid dynamics, due to the reduction of the pore space available for fluid flow. Therefore, free gas can be mechanically trapped at the base of the gas hydrate stability zone (BGSHZ) and result in overpressure generation.

Therefore, evaluating the distribution of free gas and gas hydrates at the BGHSZ is crucial to identify the existence of a pressure valve capable of contributing to the movement of submarine landslides and induce an episodic release of free gas towards the seafloor. These events constitute major geohazard issues, as well as critical processes in the carbon cycle dynamics at the interface between the seabed and the underlying sedimentary column. The integration of three-dimensional seismic, petrophysical well-log and core data is the best approach to assess such scenario. However, to date, such integrated studies are relatively rare, and there are only a few boreholes which successfully penetrate both a submarine landslide and the BGHSZ.

The Tuaheni landslide complex (TLC) is a submarine landslide exhibiting characteristics attributable to creeping movements identified in the northern portion of the Hikurangi Margin, offshore New Zealand. Compelling seismic evidence indicates that part of the TLC is located just above the BGHSZ.
The IODP Expedition 372 will evaluate the role of gas hydrates and free gas in the genesis and movement of the TLC by acquiring well-log and core data at three sites, located in the extensional and in in the compressional domain of the slide, as well as in a 'background' site, outside the TLC. Well-log and core data will be further jointly acquired by the Expeditions 372 and 375 from a gas hydrate stability zone located in a completely different setting, at the border of the forearc basin of the Hikurangi Margin.
Seismic evidence suggests that free gas, and therefore potential overpressured zones, occur both below the TLC and at the border of the forearc basin.

The available three-dimensional seismic data will be used to map the possible location of overpressured zones, generated by the fluid flow of methane-rich fluids and the accumulation of free gas. The well-log data acquired onboard will instead provide essential information on the gas hydrate and free gas distribution, as well as on the variation of pore fluid pressure with depth.
Finally, the retrieved cores will provide clues on the porosity, permeability and compositional structure (i.e. the reservoir quality) of the gas hydrate and free gas-bearing sediments. For example, the observation of sedimentary layers with high hydrate saturation near the BGHSZ could indicate the existence of a barrier to fluid flow. Conversely, the presence of a fracture network and methanogenic minerals across the drilled intervals will suggest that overpressure has been released in the past and methane has migrated towards shallower depth.

Considering such amount of data, the principal aim of this proposed research is to assess the role of hydrates and free gas in accumulating and releasing overpressures at present-day and in the past, with a specific focus at the BGHSZ. These overpressures could trigger or influence the movement of submarine landslides but, more generally, can profoundly impact fluid-flow in shallow sediments.

The proposed research will benefit the academic community, people working in the oil and gas industry, the broader community, as well as the designated PDRA.

The academic community interested in marine geology, structural geology, fluid flow and natural gas hydrate systems will benefit by access to data from a convergent margin that has never been drilled for academic purposes.
The occurrence of natural gas hydrate along the tectonically active Hikurangi Margin has always been postulated by indirect observations (i.e. geophysical). This IODP expedition will hopefully provide the scientific community with the first samples of gas hydrates from this margin.
Moreover, the acquired data and the resulting research outputs could be used by the academic community to refine existing models and theories on (1) the generation and the distribution of overpressures in the shallow parts of sedimentary basins; (2) the interplay between submarine landslides and gas hydrates, as well as free gas accumulations and (3) the 'reservoir quality' of gas hydrate and free gas-bearing sediments.

The oil and gas industry will gain critical understanding on the geo-hazards in the shallow parts of sedimentary basins. This portion of the overburden is rarely studied in detail during exploration and production.
However, shallow free gas and gas hydrates are frequently considered hazardous elements during drilling towards deeper oil and gas reservoirs. Therefore, this expedition and the proposed research will provide invaluable information to those hydrocarbon industry people interested in the pore pressure prediction in shallow parts of basins along convergent margins, which is a critical aspect of the exploration at the scale of the prospect and subsequent drilling.
The successful accomplishment of geochemical analyses onboard and after the cruise, together with the detailed seismic interpretation of the shallow hydrocarbon migration system, will jointly provide critical clues on the petroleum system along this frontier basin, regarding industrial exploration. This results will be of great interest to industry people working with basin models and source rock assessment in this region.

The results of the IODP Expeditions 372 and 375, as well as the results of the proposed post-cruise research, will be shared with the academic and industry community through publications in peer-reviewed journals, expedition proceeding reports and conferences.

The general public constitutes a fundamental beneficiary of the expedition too. Therefore, with the aim of raising the awareness of the importance of scientific drilling activities worldwide, the critical moments of the expedition will be shared through live broadcasts from the ship and through the social networks and the internet. Additionally, the results of the expedition could be presented at schools outreach activities conducted at the University of Oxford during the post-cruise research period.

Importantly, by participating in the cruise and conducting subsequent post-cruise research, the PDRA will improve his interpretational, communication and data recovery skills. Taking part to the direct acquisition of core and well-log data, collaborating with scientists with diverse and complementary backgrounds, as well as combining multiple types of information, represent essential requisites for the formation of an independent researcher.