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2.5.2
GRAIN-DISPLACING, FRACTURE-
DOMINATED OCCURRENCES IN MUDDY
SEDIMENTS
Gas hydrates in the form of grain-displacing veins and nod-
ules in fine-grained sediments have been reported from cor-
ing programs offshore Japan (Fujii
et al.
2009) and Malaysia
(Hadley
et al.
2008) and are likely very common worldwide.
Perhaps the best-studied sites are the particularly thick and
rich occurrences discovered offshore India in 2006 (Collett
et al.
2006) and offshore Korea in 2007 (Park
et al.
2008).
Drilling at Site 10 in the Krishna-Godovari Basin, a part of In-
dia’s 2006 NGHP Expedition 01, showed gas hydrates occur-
ring as a pervasive network of fracture-filling veins and lenses
in mud-rich sediments (Figure 2.10). The 150-metre-thick unit
lay below roughly 20 metres of gas-hydrate-free mud-rich sed-
iments and had no clear sea-floor expression. Core samples
revealed the fossilized remains of an earlier sea-floor chem-
osynthetic community at the top of the gas hydrate deposit
(Mazumdar
et al.
2009), suggesting that a relatively recent
sea-floor slump had buried a once-active cold seep, promoting
the accumulation of sub-sea-floor gas hydrates at the site. The
gas hydrates are not evident in standard analyses of geophysi-
cal data, but advanced techniques have delineated a 1.5-square-
kilometre area inferred to represent the zone of increased
gas hydrate occurrence (Riedel
et al.
2010a, b). The site also
provided an opportunity to cross-calibrate core-based and log-
based analyses, enabling scientists to refine significantly the
models used to estimate gas hydrate saturation from log data
in fracture-dominated systems (Lee and Collett 2009; Cook
et
al.
2010). Core data confirmed that gas hydrate concentrations
are about 25 per cent of the pore space, on average, throughout
the gas hydrate deposit. Prior to the drilling at Site 10, it was
widely believed that gas hydrates could not accumulate to val-
ues much in excess of 10 to 15 per cent in muddy sediments,
and that whatever gas hydrates occurred in such settings
would generally be dispersed within the sediment pore space.
The surprising findings at Site 10, therefore, fundamentally
changed the view of fine-grained gas hydrate systems.
Confirmation of the potential global abundance of rich gas-
hydrate occurrences as fracture-fill in muddy sediments was
obtained in the Ulleung Basin, offshore Korea, in 2007 (Ex-
pedition UBGH1). Among other targets, this program tested
several chimney structures (Figure 2.11), anomalous verti-
cal features of reduced seismic amplitude that are observed
worldwide in areas of significant gas seepage (Riedel
et al.
2002; Wood
et al.
2000; Westbrook
et al.
2008). UBGH1
provided both well-log and core data through two chimneys
(Park
et al.
2008), confirming significant fracture-filling
gas-hydrate occurrence. A second expedition (UBGH2), con-
ducted in 2010, tested several more chimney structures with
similar results. Abundant chimney structures, perhaps more
than 1 000, have been identified in the Ulleung Basin alone
(Horozal
et al.
2009; Kang
et al.
2011), and it now appears
likely that virtually all these structures represent significant
occurrences of grain-displacing gas hydrates. Preliminary
analyses of logging-while-drilling and core data show that
concentrations are quite variable, but likely similar to those
seen offshore India (about 25 per cent of pore space).
While fracture-filling gas hydrate deposits probably represent
significant global in-place resources, no promising produc-
tion strategies have yet been proposed. Challenges include
the production difficulties (many of which are related to the
geomechanical stability of the formation and of the wellbore
assembly) and the potential environmental impact associated
with extraction from such shallow, highly unconsolidated, and
low-permeability sediments.
2.5.3
PORE-FILLING GAS HYDRATES IN
MUDDY SEDIMENTS
Perhaps the bulk of global gas hydrate in-place resources occurs
in low concentrations, dispersed within the pores and grains of
clay-rich sediments. Such accumulations exist broadly across
the globe, their presence commonly betrayed by conspicuous
geophysical responses such as bottom-simulating reflectors
(BSRs) and blanking zones (Tucholke
et al.
1977; Text Box 2.3).
The investigation of such features and their potential links to gas
hydrates turned the attention of the first dedicated gas-hydrate
scientific field program (IODP Leg 164) to the Blake Ridge, off-
shore eastern North America, in 1995 (Paull
et al.
1996).
At the Blake Ridge, drilling confirmed the widespread occur-
rence of gas hydrates throughout a thick (approximately 200
metres) and very fine-grained sediment section. The concen-