FROZEN HEAT | Volume 2

2.6 SUMMARY

Nearly two decades of drilling and coring programs have confirmed that gas hydrates occur in substantial volumes in nature. However, the form in which these resources occur varies widely, largely influenced by the nature of the enclos- ing sediment. Because of these variations – which include gas hydrate concentration, burial depth, and many other factors – only a subset of the global in-place resource is po- tentially technically recoverable through the application of known technologies. This subset consists primarily of gas hydrates housed in sand-rich sediments. Total resource vol- ume in sand reservoirs remains as poorly constrained as the global in-place estimates, but may be on the order of 285 to more than 1 400 trillion cubic metres of gas (Boswell and Collett, 2011). Large volumes are also likely present in muddy systems, particularly in association with chimney structures, but the lack of any feasible production approach for such de- posits means that these resources cannot currently be con- sidered part of the recoverable resource base.

Given the limited amount of exploration conducted to date, even within the best-studied locations, it should not be as- sumed that any of the specific cases referred to in this chapter typify the nature of gas hydrate resources in the regions where they were found. For example, both the Gulf of Mexico and Nankai Trough, discussed above with reference to sand reser- voirs, contain large resource volumes within thick accumula- tions at low concentrations in mud-rich sediments. The Ul- leung Basin, presented as an example for chimney structures, has confirmed pore-filling gas hydrates in discrete turbidite sands (Bahk et al. 2011a), which are currently under evaluation as future production test sites (Moridis et al. , 2013). Similarly, the large, diffuse deposit on the Blake Ridge – once thought to be typical of all marine gas hydrates – may not even be repre- sentative of gas hydrate occurrence along the eastern coast of North America. Recent work by Frye et al. (2011) shows great potential for sand-rich sediment in the shallow section of the hydrate stability zone just north of the Blake Ridge.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 53

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