Environmental Engineering Reference
In-Depth Information
mately recoverable gas at 1.68 Tbbl of oil equivalent, or
about 96% of his total for ultimately recoverable crude
oil. But because only about 25% of all gas has been al-
ready produced, compared to almost 45% of oil, the
remaining reserves and undiscovered deposits of gas add
up, according to this accounting, to about 1.28 Tbbl of
oil equivalent, or roughly 30% more than all remaining
conventional oil. Limited resources would thus make it
impossible to contemplate natural gas as a long-term
substitute for oil.
Again, as with crude oil, the latest USGS (2005) as-
sessment of global resources of natural gas (Ahlbrandt
et al. 2006) is much more optimistic, putting the total
at about 415 Tm
3
, or an equivalent of roughly 2.6
Tbbl (330 Gt) of crude oil and about 50% above
the Campbell and Laherr`re figure. Breakdown of the
USGS estimates shows that only about 11% of ultimately
recoverable gas has been produced, that remaining
reserves account for some 31%, that their eventual
growth should amount to nearly 24%, and that the
undiscovered potential is almost exactly one-third of
the total. Odell (1999) forecasts global natural gas ex-
traction peaking at about 5.5 Gtoe by the year 2050
and declining to the year 2000 level only at the very be-
ginning of the twenty-first century.
All these figures refer to conventional natural gas, the
fuel that escaped from parental rocks and accumulated
in nonpermeable reservoirs. Nonconventional gas com-
prises resources that are already being recovered, above
all, methane in coalbeds, as well as much larger deposits
in tight reservoirs, high pressure aquifers, and methane
hydrates whose eventual recovery still awaits needed
technical advances (Rogner 2000). Gas in coalbeds is
absorbed into coal's structure. Gas in tight reservoirs
is held in impermeable rocks whose leakage rate is slower
than their filling rate; these would have to be fractured
inexpensively in order to allow economic extraction.
Global resources of geopressured gas were estimated to
be more than 100 times the reserves of the fuel (Rogner
2000).
Methane hydrates (clathrates) were formed by the gas
released from anoxic decomposition of organic sediments
by methanogenic bacteria and trapped inside rigid lattice
cages formed by frozen water molecules (Kvenvolden
1993; Lowrie and Max 1999). The upper depth limit
for their existence is close to 100 m in continental polar
regions and about 300 m in oceanic sediments, and the
lower limit in warmer oceans is about 2000 m. Fully
saturated gas hydrates have one CH
4
molecule for every
5.75 molecules of H
2
O, which means that 1 m
3
of hy-
drate can contain as much as 164 m
3
of methane (Kven-
volden 1993). There are no reliable estimates of the total
amount of methane in hydrates, but coastal U.S. waters
may contain as much as 1,000 times the volume of U.S.
conventional gas reserves (Lowrie and Max, 1999). The
USGS estimates the global mass of organic carbon
locked in gas hydrates at 10 Tt, or roughly twice as
much as the element's total
in all fossil fuels (Dillon
1992).
Nearly 20,000 hydrocarbon fields have been discov-
ered worldwide, but more than 70% of recoverable oil
and gas is in just 500 giant formations, each containing
at least 80 Mm
3
of oil or 85 Gm
3
of gas or any com-
bined energy equivalent and located mostly in five (of
260) producing basins: Persian Gulf-Zagros, West Sibe-
ria, Gulf of Mexico, Volga-Ural, and Maracaibo (Nehr-
ing 1978; Perrodon 1985; Tiratsoo 1986; Brooks 1990;
Downey 2001). Of the world's 15 largest fields, contain-
ing more than half of all recoverable hydrocarbons, 12
are in the Persian Gulf-Zagros basin. In 2005 the region
