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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
49
tration of gas hydrates was generally low – typically 5 per cent
or less of pore space, with local increases up to 10 per cent or
more correlated with fine, vertical-scale increases in sediment
grain size (Ginsburg
et al.
2000). Although the resource con-
centration is low, the large area and significant thickness of the
gas hydrate occurrence at the Blake Ridge results in very large
cumulative in-place resources of gas (Dickens
et al.
1997), po-
tentially exceeding 28 trillion cubic metres.
A recent gas hydrate drilling expedition within the Shenhu
region of the South China Sea (Expedition GMGS-1) provid-
ed additional insight into gas hydrate occurrences in mud-
rich systems (Yang
et al.
2008; Figure 2.11). Analysis of log
data suggested potential gas hydrates at the base of the gas
hydrate stability zone (GHSZ) at five of eight sites drilled.
Figure 2.10:
Gas hydrates in clay-rich sediments. Gas hydrates have been discovered
in rich deposits characterized by dense arrays of nodules and hydrate-filled fractures in
clay-rich sediments. Left: CT image of a core samples collected offshore India in 2006.
The hydrate-filled veins appear as white (from a paper by Rees
et al.
2011, published by
JMPG, permission would be needed). Above pictures, gas hydrate samples (courtesy
NGHP Expedition-01 science party).
Cores acquired at these five locations confirmed significant
gas hydrates near the base of the GHSZ at three locations. At
each well, degassing of pressurized core samples confirmed
gas hydrate at saturations routinely of 20 per cent, with local
increases to more than 40 per cent in thin zones (Wu
et al.
2010). Notably, analysis of X-ray radiographs from Shenhu
showed the gas hydrates were primarily in disseminated,
pore-filling mode. Only minor macroscopic lenses, nodules,
or fracture-fills typically seen in rich, fine-grained occurrences
were observed. Such high saturations are unique, so far, for
fine-grained gas-hydrate systems lacking macro-scale fractures
and may reflect locally high concentrations of silt-sized parti-
cles, in particular biologic fragments (foraminifera tests) that
might enhance permeability above normally expected levels in
predominantly fine-grained sediments (Wu
et al.
2010).