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FROZEN HEAT
40
2.5.1
PORE-FILLING GAS HYDRATES IN
SAND RESERVOIRS
This most promising form of deposit, in terms of production
potential, has been observed widely across the globe. Gas-
hydrate-bearing sands have been discovered offshore Korea,
where they are currently under evaluation as future produc-
tion test sites (Lee, S-R.
et al.
2011; Moridis
et al.
, 2013), and
have been reported as well from the Cascadia margin (Riedel
et al.
2006), the permafrost of Siberia (Makogon 1981), and
elsewhere. The best-studied occurrences are permafrost-as-
sociated sands on the Alaska North Slope (Collett
et al.
2008)
and the Mackenzie Delta of Arctic Canada (Dallimore
et al.
1998; Dallimore and Collett 2005), in the extensive deep-
water turbidites of the Nankai Trough, and the deeply buried
sands of the northern Gulf of Mexico.
The Alaska North Slope has a long history of oil and gas ex-
ploration. Gas hydrates were first inferred in 1972 during
initial exploration of the Prudhoe Bay oil field. Drilling data
from more than 1 000 wells in the area indicate that gas hy-
drates likely occur throughout the Alaska North Slope. They
have been confirmed within a thick sequence of sand reser-
voirs below the base of permafrost throughout a broad area
known as the Eileen Trend (Collett 1993). A second trend,
the Tarn Trend, was discovered in the early 1990s overly-
ing the Kuparuk River oil field (Collett 2002). In this case,
gas-hydrate-bearing sands are present largely within the low-
ermost permafrost-bearing section. Inks
et al.
(2009) used
standard industry seismic data to interpret more than a doz-
en specific gas hydrate prospects within the Milne Point unit
at the northern end of the Eileen Trend. In February 2007,
the most promising of these, the Mount Elbert Prospect, was
drilled, logged, and cored (Hunter
et al.
2011), confirming the
occurrence of a sand reservoir with gas hydrate saturations
ranging from 50 to nearly 80 per cent. In 2011, the Ignik Si-
2.5
CASE STUDIES OF GAS
HYDRATE OCCURRENCES
kumi #1 research well confirmed similar occurrences of gas
hydrates in four separate sand reservoirs (Schoderbek and
Boswell 2011) in the western Prudhoe Bay unit.
The geology of the shallow sediments of the Alaska North
Slope, and of many other Arctic regions in which gas hydrates
occur, is dominated by sediments deposited in shallow-water
marine, coastal, and terrestrial environments. These conti-
nental deposits generally include significantly greater pro-
portions of sand-sized sediments than are typically found in
deep-water settings. Virtually all known gas hydrate occur-
rences in the Arctic are associated with sands. Prudhoe Bay
gas hydrates are charged primarily by the upward migra-
tion (aided by many faults) of gas leaking from the deeper
Prudhoe Bay oil and gas fields. It appears likely that gaseous
methane began to charge sand reservoirs prior to the evolu-
tion of gas hydrate stability conditions roughly 1.6 million
years ago (Collett 1993). Conversion to gas hydrates occurred
after the climate cooled dramatically during glacial times, ag-
grading thick occurrences of terrestrial permafrost.
Collett (1995) assessed Alaska North Slope in-place gas re-
sources from gas hydrates at 16.7 trillion cubic metres. Sub-
sequently, using information from the 2007 Mount Elbert
well and recent advances in numerical modelling (Anderson
et al.
2010), Collett
et al.
(2008) provided the first assessment
of technically recoverable resources from gas hydrates, indi-
cating a mean of 2.4 trillion cubic metres from Alaska North
Slope sand reservoirs using existing technologies.
Compared to the data available about permafrost gas hydrates
on the Alaska North Slope and the Mackenzie Delta, very lit-
tle is known about gas hydrates in the vast deep-water ba-
sins of the world. Perhaps the best-characterized occurrences
in sand reservoirs are those located in the eastern Nankai
Trough, off the southeastern coast of Japan (Figure 2.7). The