In the public's imagination, hydrates are seen as either a potential new source of energy to be exploited as the world uses up its reserves of oil and gas or as a major environmental hazard. Scientists, however, have expressed great uncertainty as to the global volume of hydrates and have reached little agreement on how they might be exploited. Both of these uncertainties can be reduced by a better understanding of how hydrates are held within sediments. There are conflicting ideas as to whether hydrates are disseminated within selected lithologies or trapped within fractures comparable to mineral lodes. To resolve this, hydrates have to be examined at all scales ranging from using seismics to microscopic studies. Their position within sediments also influences the stability of methane hydrate in responding to pressure and temperature and how the released gas might transfer to the ocean, atmosphere, or to a transport mechanism for recovery. These results also run parallel with the studies of carbon dioxide hydrate, which is being considered as a potential sequestion medium. Over recent decades hydrates have been gradually making their way up the scientific agenda, receiving correspondingly greater wider societal interest with time. However, what is most notable to the casual observer is not the fact that it is broadly recognized that hydrates have the potential to be a major environmental hazard or a major new energy source, but that nobody is exactly shouting about it. There are several reasons for this. Geological investigations suggest that hydrates were at least partners in crime in many climatic disasters, such as the great methane outbursts that caused the mass extinctions at the end of the Permian (Erwin 1994; Krull & Retallack 2000) or Paleocene (Dickens et al. 1997; Zachos et al. 2005). Whilst people are interested in such events, the immediate instability of possibly ‘life-threatening’ natural hydrates does not seem to be their immediate concern; it is clear that they occurred a long time ago, since when environments have greatly changed, and it is also apparent that they were largely precipitated by external triggers (White 2002; Maclennan & Jones 2006). Likewise, although they see the potential to produce energy-providing methane from hydrates, initial difficulties in doing so have dampened interest. In both cases – as a hazard or resource – the lack of societal focus largely stems from the body language of the scientific community, which is itself highly uncertain of the importance of hydrates. Scientists are not certain enough of their ground to allow a clear direction to be mapped in relation to minimization of the risk of mass hydrate destablization or widespread exploitation. To a large degree this uncertainty centres on our poor fundamental understanding of the occurrence and stability of sediment-hosted hydrates. There is no better illustration of this than the widely fluctuating predictions of global hydrate reserves we have seen in recent years, where estimates vary between 1015 and 1019 m3 of methane gas at STP. Milkov (2003) describes how improved understanding of the distribution and concentration of gas hydrates in marine sediments has led to a readjustment of global estimates downwards over each subsequent decade, although the estimate of 1015 m3 is challenged by Klauda & Sandler (2005), who suggest that a total volume of 1017 m3 is likely with 1016 m3 located on continental margins. Although there is starting to be a consensus in the order of 1016 m3 (e.g. Kvenvolden 1998, 2000; Makogon et al. 2007), a range of values are still used and such apparent uncertainty in hydrate abundance hardly conveys the message to the wider community that the scientists know what they are talking about. To move this debate along, we considered the source of our greatest uncertainties and found that these largely centre around how hydrates are physically stored in sediments at a range of scales. At present our understanding is extremely crude. We have very little knowledge about how hydrates are stored in sediments of different grain size or texture, whether they dominantly are separated by water films in inter-grain pores, or whether they coat grains, how bacteria control authigenic fixation, and whether the mineralogy of host-sediment influences the microscale sediment–hydrate association. At a larger scale, we have a very poor understanding of the distribution of hydrates within individual beds, let alone complex heterolithic sequences. Big questions prevail. Do hydrates form pods that are little influenced by lithology as in many ore bodies, or are they dominantly stratiform and follow rules of behaviour that are analogous to hydrocarbons in reservoirs? How much are sediment-hosted distributions controlled by structural settings, whether tectonic, gravitational or through diagenetic changes in the sediment (for instance the development of cavities or veins through shrinkage)? It is clear that our ability to describe such relationships is very immature, even before we start to make sense of changing pressure, temperature and salinity regimes, as well as the variation in natural supply of methane from external sources. To review our current understanding, and in order to encourage debate about these issues, we decided to convene a meeting to which interested scientists could look at these many challenges in a fairly relaxed setting. Consequently, with the support of the Geological Society Hydrocarbons Group, a two-day meeting ‘Sediment-Hosted Gas Hydrates: New Insights on Natural and Synthetic Systems’ took place at Burlington House, Piccadilly, London on 25–26 January 2006. This was based on 35 presentations and posters and brought together over about 100 international hydrate scientists spanning the hazard and resource communities, as well as those with very different experience, for instance those involved principally with laboratory experimentation, mixing with those of geophysical field studies or geochemical mapping. The main theme of the meeting was the nature of the primary hydrate–sediment relationships that control hydrate stability. This largely addressed the distribution of natural sediment-hosted hydrates, but also covered research into synthetic systems. The latter are of interest as they provide analogues of natural environments, but in very well calibrated, controlled laboratory settings where textural relationships and processes can be mapped and measured. Synthetic sediment-hosts are also likely to be of interest in the future as a possible store of greenhouse gases; the possibility that hydrates could be used to store large volumes of human-generated carbon dioxide was discussed at some length in the meeting. The structure and content of this volume largely reflects the structure and interests of the meeting and addresses sediment-hosted hydrates in natural and synthetic systems separately. |
