14.3 Hydrocarbon Habitats
14.3.6 Gas Hydrates
Gas hydrates, containing methane and possibly some
other natural gases, occur in enormous quantitities at
shallow burial depths below the present-day sea
floor. The amount of methane carbon stored in the
form of gas hydrates below the sea is estimated to
exceed the carbon known from fossil fuel sources
and has therefore received much attention during the
last decade.
Gas hydrates are solid, ice-like substances consisting of a lattice of water molecules which enclose gas
molecules. The gas is usually biogenic methane produced by bacterial activity within shallowly buried
sediments containing organic matter (cf. Sect.
13.4.2), but thermogenic methane and higher-order
hydrocarbons derived from deeper-buried sediment
can locally contribute to the natural gas content of
the hydrates.
The gas accumulating in the pore system of fine-grained
sediment is first dissolved in pore water which is facilitated
by increasing pressure. If the amount of methane dissolved
is in excess of the local solubility of methane, gas hydrates
(or clathrates ) can form. The hydrates fill only part of the
pore space (a few percent up to about 20-30%; methane
saturation 1-5%) besides water which is still present in thin
films around the sediment particles. This process is closely
related to the formation of ice in the small pores of soils
where part of the water cannot freeze. Similarly, the tiny
hydrate particles sometimes combine to small nodules or
lenses by exerting crystallization pressure onto the neighboring grains and ~ushing them aside. 1 m 3 of hydrate contains up to 164 m methane and 0.8 m 3 water (at standard
conditions).
Besides high pressure (around 50 bar or 500 m water
depth) gas hydrates require relatively 10w temperatures
(~7°C) to become thermodynamically stable at this depth.
However, because two principal factors, i.e., pressure and
temperature, control the stability field of the hydrates, their
occurrence in nature cannot be defined by a fixed boundary. With increasing pressure, for example at 3000 m water
depth, they are stable at temperatures weIl above 20°C. The
vertical range of gas hydrates is often 500-1000 m within a
sedimentary colurnn. Farther below, the temperature within
the sediment becomes, due to the geothermal gradient, too
high for the hydrates to remain stable. The 10wer boundary
of the hydrates is marked in seismic records by the socalled bottom simulating reflector (BSR). Below this reflector, methane is present as free gas or dissolved in pore
water.
The various aspects of the gas hydrates have been addressed in many publications (e.g., Hesse 1986; Collett
1993; Maslin et al. 1993; Kvenvolden 1993; Paull et al.
1996; van Wearing et al. 1997; Haq 1998; Sloan 1998;
Clennell et al. 1999).
The stability field of the gas hydrates and the occurrence of organic matter in most of the hemipelagic
sediments indicate that gas hydrates must be very
common on the lower continental slope and rise of
the present-day ocean basins. The water temperature
721
at a depth of 500 m is normally low, even in low-Iatitude regions such as along the western margins of the
continents. In high latitudes, gas hydrates occur at
shallower water depth (around 250 m), and in permafrost regions they can form in shelf regions and even
on land.
It is not yet clear whether or not these huge hydrocarbon reservoir can be used as an energy source in
the future. The concentration of methane per unit
volume of sediment is low and it is difficult to extract the gas from the fine-grained material deep below the sea in an economic way. However, two other
important processes, caused by the natural release of
methane from gas hydrates, become increasingly evident:
- The triggering of submarine slope failures and
- The influence of gas hydrates on rapid climate
change.
When the gas hydrates thaw, due to increasing temperature with depth or decreasing pressure in response to sea-Ievel fall, they release large quantities
of methane and fresh water. A significant drop in sea
level, for example, as known from the late Pleistocene, mnst have raised the base of the gas hydrate
stability zone by about 20 m. Such a process can diminish the shear strength of slope sediments and
therefore generate large-scale slope failures in wide
areas of the ocean. These in turn facilitate the escape
of methane into the atmosphere where this greenhouse gas, similar to carbon dioxide, may lead to
rapid warming and termination of a glacial period.
14.3.7 General Conclusions for
Petroleum Systems
All these examples have in common that the hydrocarbon source rocks reached aburial depth in the
order of 1500 to 3000 m with a temperature of at
least 60-90°C. In this depth range (except some specific cases) part of the kerogen is or was transformed
to petroleum and/or gas under the influence of both
subsurface temperature and time. Areas underlain by
source rocks undergoing such processes are called
petroleum generative depressions or "hydrocarbon
kitchens" (Demaison 1985). A generative basin may
contain one or more hydrocarbon kitchens. Hydrocarbon kitchens associated with carrier beds and reservoir rocks protected by seals are called petroleum
systems (Magoon and Dow 1994). For the effectivity
of such a system it is crucial that carrier beds and
reservoirs at higher elevation or in structural highs
are ready to store the hydrocarbons released from the
source rock.
The largest petroleum accumulations tend to occur
in reservoir rocks located close to the center of the
14.3.6 Gas Hydrates
Gas hydrates, containing methane and possibly some
other natural gases, occur in enormous quantitities at
shallow burial depths below the present-day sea
floor. The amount of methane carbon stored in the
form of gas hydrates below the sea is estimated to
exceed the carbon known from fossil fuel sources
and has therefore received much attention during the
last decade.
Gas hydrates are solid, ice-like substances consisting of a lattice of water molecules which enclose gas
molecules. The gas is usually biogenic methane produced by bacterial activity within shallowly buried
sediments containing organic matter (cf. Sect.
13.4.2), but thermogenic methane and higher-order
hydrocarbons derived from deeper-buried sediment
can locally contribute to the natural gas content of
the hydrates.
The gas accumulating in the pore system of fine-grained
sediment is first dissolved in pore water which is facilitated
by increasing pressure. If the amount of methane dissolved
is in excess of the local solubility of methane, gas hydrates
(or clathrates ) can form. The hydrates fill only part of the
pore space (a few percent up to about 20-30%; methane
saturation 1-5%) besides water which is still present in thin
films around the sediment particles. This process is closely
related to the formation of ice in the small pores of soils
where part of the water cannot freeze. Similarly, the tiny
hydrate particles sometimes combine to small nodules or
lenses by exerting crystallization pressure onto the neighboring grains and ~ushing them aside. 1 m 3 of hydrate contains up to 164 m methane and 0.8 m 3 water (at standard
conditions).
Besides high pressure (around 50 bar or 500 m water
depth) gas hydrates require relatively 10w temperatures
(~7°C) to become thermodynamically stable at this depth.
However, because two principal factors, i.e., pressure and
temperature, control the stability field of the hydrates, their
occurrence in nature cannot be defined by a fixed boundary. With increasing pressure, for example at 3000 m water
depth, they are stable at temperatures weIl above 20°C. The
vertical range of gas hydrates is often 500-1000 m within a
sedimentary colurnn. Farther below, the temperature within
the sediment becomes, due to the geothermal gradient, too
high for the hydrates to remain stable. The 10wer boundary
of the hydrates is marked in seismic records by the socalled bottom simulating reflector (BSR). Below this reflector, methane is present as free gas or dissolved in pore
water.
The various aspects of the gas hydrates have been addressed in many publications (e.g., Hesse 1986; Collett
1993; Maslin et al. 1993; Kvenvolden 1993; Paull et al.
1996; van Wearing et al. 1997; Haq 1998; Sloan 1998;
Clennell et al. 1999).
The stability field of the gas hydrates and the occurrence of organic matter in most of the hemipelagic
sediments indicate that gas hydrates must be very
common on the lower continental slope and rise of
the present-day ocean basins. The water temperature
721
at a depth of 500 m is normally low, even in low-Iatitude regions such as along the western margins of the
continents. In high latitudes, gas hydrates occur at
shallower water depth (around 250 m), and in permafrost regions they can form in shelf regions and even
on land.
It is not yet clear whether or not these huge hydrocarbon reservoir can be used as an energy source in
the future. The concentration of methane per unit
volume of sediment is low and it is difficult to extract the gas from the fine-grained material deep below the sea in an economic way. However, two other
important processes, caused by the natural release of
methane from gas hydrates, become increasingly evident:
- The triggering of submarine slope failures and
- The influence of gas hydrates on rapid climate
change.
When the gas hydrates thaw, due to increasing temperature with depth or decreasing pressure in response to sea-Ievel fall, they release large quantities
of methane and fresh water. A significant drop in sea
level, for example, as known from the late Pleistocene, mnst have raised the base of the gas hydrate
stability zone by about 20 m. Such a process can diminish the shear strength of slope sediments and
therefore generate large-scale slope failures in wide
areas of the ocean. These in turn facilitate the escape
of methane into the atmosphere where this greenhouse gas, similar to carbon dioxide, may lead to
rapid warming and termination of a glacial period.
14.3.7 General Conclusions for
Petroleum Systems
All these examples have in common that the hydrocarbon source rocks reached aburial depth in the
order of 1500 to 3000 m with a temperature of at
least 60-90°C. In this depth range (except some specific cases) part of the kerogen is or was transformed
to petroleum and/or gas under the influence of both
subsurface temperature and time. Areas underlain by
source rocks undergoing such processes are called
petroleum generative depressions or "hydrocarbon
kitchens" (Demaison 1985). A generative basin may
contain one or more hydrocarbon kitchens. Hydrocarbon kitchens associated with carrier beds and reservoir rocks protected by seals are called petroleum
systems (Magoon and Dow 1994). For the effectivity
of such a system it is crucial that carrier beds and
reservoirs at higher elevation or in structural highs
are ready to store the hydrocarbons released from the
source rock.
The largest petroleum accumulations tend to occur
in reservoir rocks located close to the center of the