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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
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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.
2.6
SUMMARY