14 Gas Hydrates in Marine Sediments
490
14.3.4 Methane Transport and Hydrate
Formation
If methane, either from biogenic or thermogenic
sources, is present in high enough concentration
to stabilize the hydrate structure at thermodynamically favourable conditions (Figs. 14.3 and
14.4), it will combine with water to form hydrate.
For methane hydrate to occur, the rain rate of
carbon to the seafloor must be high enough to
supply the required methane via degradation of
organic matter in the sediment. Hydrate stability
requires gas concentration in the hydrate at least
two orders of magnitude greater than gas
solubility in the liquid phase. Thus, methane
generation and transport processes are key factors for constraining global hydrate inventories.
Hydrate Formation by in situ Biogenic Methane
Generation and Transport in Advecting Fluids
The amount of biogenic methane is essentially
controlled by both the availability and reactivity
of organic matter in the upper hundreds of meters
of the sedimentary sequence. Davie and Buffett
(2001, 2003) demonstrated the critical need for
quantitative models of biogenic methane production to describe the distribution of gas
hydrate in the top few hundred meters of sediment. Key parameters are rates of sedimentation,
quality and quantity of the organic matter and
biological activity rates. They show that hydrate
accumulation from in situ production in sediment
with a TOC of 1.5%, will be less than 7% of the
pore volume
If in situ production of biogenic methane is
not adequate to support observed accumulations
within the GHSZ, then additional methane must
migrate from below. Paull et al. (1994), proposed a
mechanism to concentrate methane via recycling
at the base of the GHSZ in the Blake Ridge
hydrate-bearing province. Progressive burial and
subsidence through geologic time shifts the base
of the GHSZ upward, so that deep-seated hydrate
decomposes. As hydrate dissociates, the methane
solubility is surpassed, and free gas permeates
fissures in the overlying hydrate stability layer,
enhancing gas hydrate contents via precipitation
of the “recycled” methane.
Davie and Buffett (2001) also show that both
in situ methane production and transport in
upward migrating saturated fluids are needed to
explain the dissolved chloride profiles observed
in Blake Ridge sediment. Similarly, Hensen and
Wallmann (2005) show that, although organic
carbon degradation in the upper sediments of the
Costa Rica margin can account for 0.4 to 1.1 % of
hydrate content of the sediment, it alone cannot
explain the hydrate distribution in this region.
Furthermore they show that fluid flow may
increase the total amount of hydrate that can be
formed from the organic reservoir in this margin
by more than 50%.
Fluid flow can scavenge methane from a broad
region, thus it is expected that active margins
with pervasive fluid transport would have higher
abundance of gas hydrate. Nevertheless, even
the small rates of fluid flow in passive margins,
play a controlling role on the accumulation of gas
hydrate (Egeberg and Dickens 1999). In fact,
using a mechanistic model for the distribution of
hydrate in marine sediment, Buffet and Archer
(2004) conclude that the global inventory of gas
hydrate is particularly sensitive to both, methane
generation from organic matter and the rate of
fluid flow.
Methane Transport in the Gas Phase
Most disseminated hydrate in marine sediment is
thought to occupy less than 8% of the pore space
of sediments integrated over the GHSZ. However,
there are regions where massive hydrate is known
to form near or at the seafloor. These shallow
hydrate deposits are usually associated with
areas of fluid venting and gas ebullition
(Mazurenko and Soloviev 2003). Geochemical
modelling of the shallow hydrate at the summit of
southern Hydrate Ridge demonstrates the need
for methane transport in the gas phase. Because
of the low solubility of methane in water,
advection of methane-saturated water is not
enough to sustain the rapid hydrate growth in
this system (Torres et al. 2004). In general,
methane hydrate will only form large concentrated deposits where gas flow is present.
Methane concentration increases with depth
in the sediment due to a combination of processes including microbial generation, methane
recycling at the base of the GHSZ, and thermochemical generation at depth. When methane
concentration in the pore water exceeds saturation, methane gas will exolve. However, the
difficulty of nucleating bubbles of small size in
fine-grained porous media can lead to significant
supersaturations. Clennell et al. (2000) provide a
490
14.3.4 Methane Transport and Hydrate
Formation
If methane, either from biogenic or thermogenic
sources, is present in high enough concentration
to stabilize the hydrate structure at thermodynamically favourable conditions (Figs. 14.3 and
14.4), it will combine with water to form hydrate.
For methane hydrate to occur, the rain rate of
carbon to the seafloor must be high enough to
supply the required methane via degradation of
organic matter in the sediment. Hydrate stability
requires gas concentration in the hydrate at least
two orders of magnitude greater than gas
solubility in the liquid phase. Thus, methane
generation and transport processes are key factors for constraining global hydrate inventories.
Hydrate Formation by in situ Biogenic Methane
Generation and Transport in Advecting Fluids
The amount of biogenic methane is essentially
controlled by both the availability and reactivity
of organic matter in the upper hundreds of meters
of the sedimentary sequence. Davie and Buffett
(2001, 2003) demonstrated the critical need for
quantitative models of biogenic methane production to describe the distribution of gas
hydrate in the top few hundred meters of sediment. Key parameters are rates of sedimentation,
quality and quantity of the organic matter and
biological activity rates. They show that hydrate
accumulation from in situ production in sediment
with a TOC of 1.5%, will be less than 7% of the
pore volume
If in situ production of biogenic methane is
not adequate to support observed accumulations
within the GHSZ, then additional methane must
migrate from below. Paull et al. (1994), proposed a
mechanism to concentrate methane via recycling
at the base of the GHSZ in the Blake Ridge
hydrate-bearing province. Progressive burial and
subsidence through geologic time shifts the base
of the GHSZ upward, so that deep-seated hydrate
decomposes. As hydrate dissociates, the methane
solubility is surpassed, and free gas permeates
fissures in the overlying hydrate stability layer,
enhancing gas hydrate contents via precipitation
of the “recycled” methane.
Davie and Buffett (2001) also show that both
in situ methane production and transport in
upward migrating saturated fluids are needed to
explain the dissolved chloride profiles observed
in Blake Ridge sediment. Similarly, Hensen and
Wallmann (2005) show that, although organic
carbon degradation in the upper sediments of the
Costa Rica margin can account for 0.4 to 1.1 % of
hydrate content of the sediment, it alone cannot
explain the hydrate distribution in this region.
Furthermore they show that fluid flow may
increase the total amount of hydrate that can be
formed from the organic reservoir in this margin
by more than 50%.
Fluid flow can scavenge methane from a broad
region, thus it is expected that active margins
with pervasive fluid transport would have higher
abundance of gas hydrate. Nevertheless, even
the small rates of fluid flow in passive margins,
play a controlling role on the accumulation of gas
hydrate (Egeberg and Dickens 1999). In fact,
using a mechanistic model for the distribution of
hydrate in marine sediment, Buffet and Archer
(2004) conclude that the global inventory of gas
hydrate is particularly sensitive to both, methane
generation from organic matter and the rate of
fluid flow.
Methane Transport in the Gas Phase
Most disseminated hydrate in marine sediment is
thought to occupy less than 8% of the pore space
of sediments integrated over the GHSZ. However,
there are regions where massive hydrate is known
to form near or at the seafloor. These shallow
hydrate deposits are usually associated with
areas of fluid venting and gas ebullition
(Mazurenko and Soloviev 2003). Geochemical
modelling of the shallow hydrate at the summit of
southern Hydrate Ridge demonstrates the need
for methane transport in the gas phase. Because
of the low solubility of methane in water,
advection of methane-saturated water is not
enough to sustain the rapid hydrate growth in
this system (Torres et al. 2004). In general,
methane hydrate will only form large concentrated deposits where gas flow is present.
Methane concentration increases with depth
in the sediment due to a combination of processes including microbial generation, methane
recycling at the base of the GHSZ, and thermochemical generation at depth. When methane
concentration in the pore water exceeds saturation, methane gas will exolve. However, the
difficulty of nucleating bubbles of small size in
fine-grained porous media can lead to significant
supersaturations. Clennell et al. (2000) provide a