507
hydrate in nature requires comprehensive interdisciplinary studies that inlude laboratory experiments,
numerical modelling and field observations. Because
the factors that influence gas hydrate stability and
the processes that occur as a consequence of gas
hydrate formation are highly dynamic, these interrelationships can only be understood through timeseries monitoring of complementary parameters over
space and time though the installation of seafloor
observatories. Efforts to establish such observatories
are underway at a few key gas hydrate locations. These
and other ongoing studies may well provide key
answers to our current challenges of evaluating the
role of these deposits in the global energy resources
of the future, and on the global carbon cycle, climate
change, and perhaps biotic evolution through our
planet’s geologic history.
Acknowledgements
This research used data provided by the Ocean
Drilling Program (ODP), which is sponsored by the US
National Science Foundation and participating
countries under management of Joint Oceanographic
Institutions (JOI), Inc. Funding for this research was
provided by the U.S. Science Support Program (USSP)
grant F001557, WCNURP grant PF806880 and by NSF
grant OCE-9731157. The Federal Ministry of Education
and Research (BMBF, Berlin) supported the studies
by grant 03G0604A (collaborative project METRO).
This is publication GEOTECH-194 of the program
GEOTECHNOLOGIEN of the BMBF and DFG and
publication No 0336 of the Research Center Ocean
Margins of the University of Bremen.
14.7 Problems
Problem 1
The thermodynamically defined gas hydrate zone
in sediments of the deep sea is much thicker than
in sediments underlying shallow water, such as
those in the upper continental slope (see Fig.
14.4). Does this mean that gas hydrate concentrations are higher in the deeper ocean than on the
upper slope?
Problem 2
Discuss different methane sources in sediments.
Is there a difference in hydrate formation whether
the gas is biogenic or thermogenic in origin?
Problem 3
A sample collected from 90 meters below seafloor
on Hydrate Ridge has an in situ methane concentration of 300 mM. Based on the phase boundary
diagram shown in Fig.14.5B, do you expect
hydrate to be present in this sample? If a sample
with the same methane concentration was recovered from 250 mbsf, would there be hydrate in
it? Why/why not. Discuss also a methane
concentration of 10 mM in samples from 90 and
250 mbsf using Fig 14.5 B.
Problem 4
A water sample recovered from a gas-bearing region
has a chloride concentration of 530 mM and sulfate
concentration of 26.5 mM. Do you expect methane
hydrate to be present?
Problem 5
Pore water samples from a hydrate-bearing core were
shown to have a dissolved chloride concentration of
507 mM. If the background concentration at this site
is known to be 550 mM, calculate the percent of the
pore space that is occupied by gas hydrate, assuming full occupancy of the hydrate structure.
Problem 6
How could you explain the formation of shallow
brines are formed in marine pore water, where there
is no association with evaporites?
14.7
Problems
hydrate in nature requires comprehensive interdisciplinary studies that inlude laboratory experiments,
numerical modelling and field observations. Because
the factors that influence gas hydrate stability and
the processes that occur as a consequence of gas
hydrate formation are highly dynamic, these interrelationships can only be understood through timeseries monitoring of complementary parameters over
space and time though the installation of seafloor
observatories. Efforts to establish such observatories
are underway at a few key gas hydrate locations. These
and other ongoing studies may well provide key
answers to our current challenges of evaluating the
role of these deposits in the global energy resources
of the future, and on the global carbon cycle, climate
change, and perhaps biotic evolution through our
planet’s geologic history.
Acknowledgements
This research used data provided by the Ocean
Drilling Program (ODP), which is sponsored by the US
National Science Foundation and participating
countries under management of Joint Oceanographic
Institutions (JOI), Inc. Funding for this research was
provided by the U.S. Science Support Program (USSP)
grant F001557, WCNURP grant PF806880 and by NSF
grant OCE-9731157. The Federal Ministry of Education
and Research (BMBF, Berlin) supported the studies
by grant 03G0604A (collaborative project METRO).
This is publication GEOTECH-194 of the program
GEOTECHNOLOGIEN of the BMBF and DFG and
publication No 0336 of the Research Center Ocean
Margins of the University of Bremen.
14.7 Problems
Problem 1
The thermodynamically defined gas hydrate zone
in sediments of the deep sea is much thicker than
in sediments underlying shallow water, such as
those in the upper continental slope (see Fig.
14.4). Does this mean that gas hydrate concentrations are higher in the deeper ocean than on the
upper slope?
Problem 2
Discuss different methane sources in sediments.
Is there a difference in hydrate formation whether
the gas is biogenic or thermogenic in origin?
Problem 3
A sample collected from 90 meters below seafloor
on Hydrate Ridge has an in situ methane concentration of 300 mM. Based on the phase boundary
diagram shown in Fig.14.5B, do you expect
hydrate to be present in this sample? If a sample
with the same methane concentration was recovered from 250 mbsf, would there be hydrate in
it? Why/why not. Discuss also a methane
concentration of 10 mM in samples from 90 and
250 mbsf using Fig 14.5 B.
Problem 4
A water sample recovered from a gas-bearing region
has a chloride concentration of 530 mM and sulfate
concentration of 26.5 mM. Do you expect methane
hydrate to be present?
Problem 5
Pore water samples from a hydrate-bearing core were
shown to have a dissolved chloride concentration of
507 mM. If the background concentration at this site
is known to be 550 mM, calculate the percent of the
pore space that is occupied by gas hydrate, assuming full occupancy of the hydrate structure.
Problem 6
How could you explain the formation of shallow
brines are formed in marine pore water, where there
is no association with evaporites?
14.7
Problems