The USGS Gas Hydrates Project has long maintained informal databases of global gas hydrate occurrences. In 2011, an effort was initiated to formalize the database activity. Our goal is to develop a database with full, traceable referencing to published reports, as well as information about the location, depth, and type of gas hydrate occurrence. In building the database, we used information and conducted quality control on older databases started within the Gas Hydrates Project. We then added newer information gleaned from the peer-reviewed literature and from reports on particular drilling programs.
The database effort will eventually lead to a Web-based map maintained within a Geographic Information Systems (GIS) framework. The map will be periodically updated to reflect new additions to the database. The database is currently under development and not yet ready for release. However, two new maps show the distribution of recovered gas hydrate and inferred gas hydrate based on references compiled through the middle of December 2013.
Map of locations where gas hydrate has been recovered (white diamonds) and inferred (red circles). Inferences of gas hydrate are most often based on discovery of a seismic interface (the bottom simulating reflector) in marine sediments, but sometimes reflect the analysis of borehole logging data or other information that points to the occurrence of gas hydrate. Data from the USGS Gas Hydrates Project database, which is being built now. Map may be used with attribution to "US Geological Survey" or "USGS Gas Hydrates Project."
Same as above, but blue diamonds mark the locations of recovered gas hydrate and the background is a plain map. Map may be used with attribution to "US Geological Survey" or "USGS Gas Hydrates Project."
It is important to note that these maps are inherently biased. Samples are only recovered where gas hydrate occurs close to the seafloor and is accessible with a piston corer or where it is retrieved by deep drilling. Most gas hydrate occurs deep in the marine sediments and cannot be recovered by piston cores. Thus, the "recovered" data tend to place too much emphasis on locations with high gas flux and even seafloor seeps, where gas hydrate is likely to be within reach of a piston corer. The inferred locations also have a bias. Each time that researchers visit a new location on the continental margins, they may find like bottom simulating reflectors (BSRs). The presence of a BSR means that gas hydrate exists in the overlying sedimentary section, but the absence of a BSR in an adjacent location does not mean that gas hydrate is lacking.
These maps differ from many published in the gas hydrates literature in that they focus on actual occurrences. In the literature, many maps show prospective occurrences (predicted) based on an analysis of the probable pressure-temperature stability conditions and sometimes the availability of methane or organic carbon from which methane can be produced my microbes.
There has been only a limited number of deep drilling programs that have sampled gas hydrates or conducted geophysical tests (borehole logging) that allow gas hydrate occurrences to be studied indirectly. This led to the development of a different kind of map based on type of gas hydrate 'reservoir.' Here the term reservoir does not have an energy resource connotation. Instead, it refers to the type of host rock in which the gas hydrates mostly occur in the formation. This designation has relevance with respect to the so-called resource pyramid of Boswell and Collett , shown below. For more information, see the overview article on this map.
Gas hydrate reservoir types at the location of deep drilling programs through 2011. Image attribution to Ruppel et al. (2011), citation below
Simplified hydrate resource pyramid, after Boswell and Collett (2011).