FROZEN HEAT | Volume 2

Beaufort Sea

Nankai Trough is a subduction zone where the Philippines Sea Plate to the east is being overridden by the Eurasian Plate to the west. This deep basin has collected thick sections of sediment eroded from the Japanese Islands – including exten- sive turbidite channel complexes and other sand-rich strata. Exploration drilling conducted in the eastern Nankai Trough in 1999 provided the world’s first confirmation of substantial gas-hydrate-bearing sand reservoirs in a deep-water setting (Tsuji et al. 2004). Guided by a range of advanced geophysical studies, additional drilling in 2004 permitted the delineation of more than ten separate accumulations of gas hydrates in deep-water sands (Tsuji et al. 2009; Fujii et al. 2009). The reservoirs in the Nankai Trough are characterized by thick sections of interbedded deep-water sands and muds (Takano et al. 2007; Noguchi et al. 2010) with individual gas-hydrate- bearing sand layers typically less than a metre thick (Fujii et al. 2008; 2009). Analysis of data acquired during the 2004 drilling and cor- ing programs (Takahashi and Tsuji 2005) and associated geophysical programs demonstrated that conventional oil and gas data sets and concepts could be applied to the prob- lem of deep-water gas hydrate detection and characterization (Saeki et al. 2008). Fujii et al. (2008) conducted an assess- ment of gas hydrate resources in the most extensively stud- ied area in the eastern Nankai Trough, an area estimated to represent perhaps ten per cent of the total prospective area for gas hydrates in waters around Japan. The assessment re- vealed a mean estimate of gas-in-place of approximately 1.1 trillion cubic metres within a region totalling 7 000 square kilometres, with 550 billion cubic metres occurring at high concentrations in sand reservoirs. Kurihara et al. (2010) re- ported numerical simulations of production potential and determined that the technically recoverable portion of this resource is likely large, constituting 50 per cent or more of in-place resources, depending on production method and location-specific geology. In 2012, Japan re-initiated drilling and sampling activities in the Nankai Trough in preparation for the first field trials (which began in 2013) of gas hydrate production from a deep-water sand reservoir (Yamamoto et al. 2011). The 2013 drilling program included a rigorous re- view of baseline environmental conditions and monitoring of environmental impacts that might be associated with gas hydrate production (Arata et al. 2011).

Mallik

Richards Island

Mackenzie Delta

MALLIK Road INUVIK-TUK Road 20 km

Inuvik

Figure 2.5: The Mallik site in Canada’s Arctic. This has been the site of dedicated gas hydrate programs since 1998. The map (above) shows the route of the temporary ice road (red line) that provides access to the site near the shore of the Mackenzie delta. The photo (below) shows a sample of gas-hydrate-bearing coarse-grained sandstone recovered from the site in 1998. (Courtesy: Geological Survey of Canada.)

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 41