Natural gas hydrates are solids formed by the combination of water and gases, which may be hydrocarbons or not. It has the appearance of snow or dry ice and crystallizes in the form of nodules, layers or within faults and in the porous space of marine sediments. They are distributed along the continental margins around the world or in permafrost zones, located in the polar circles. Hydrates originate through the movement of gaseous molecules during migration within the sedimentary column or in the water, through an exothermic reaction that freezes the water immediately surrounding each gas molecule. This molecule, usually methane, is then trapped within a crystalline structure composed of a trap of water molecules. For this reason, hydrates are also known as methane clathrates. However, other natural components such as ethane, propane and carbon dioxide can be observed in this form. The maximum temperature for this structure to be stable depends on the combination of temperature and pressure in the gas hydrate stability zone and, secondarily, on the composition of the gas and the salinity of the water contained in the pores of marine sediment. Methane, trapped as a hydrate, may be biogenic or thermogenic. Experimental studies indicate that 1 m3 of methane hydrate, dissociated under pressure and atmospheric temperature, releases 164 m3 of natural methane, in addition to 0.8 m3 of fresh water. For this reason, estimates of the amount of natural gas contained in hydrates far exceed the known reserves of natural gas in the world, ranging from 105 trillion cubic feet (TCF) to more than 3x109 TCF. The volume of carbon contained in this form is estimated to be twice the total amount of all the earth's fossil organic carbon, including oil, gas, and coal. Gas hydrates have been attracting interest as a potential energy resource, in addition to being considered as a possible cause of greenhouse effect and of instability of marine slopes. However, little is known about the factors controlling the formation and stability of hydrates on the marine seafloor, although significant advances have been achieved thanks to the continued study of the subject by academies and research institutions. The interaction between gas hydrates dissociation and methane plumes at the seawater column is a natural phenomenon that modifies seafloor scenario, transforming the landscape by the precipitation of carbonates and pyrite on the shallow sedimentary pores, resulting in nucleous of hardgrounds for living benthic organisms, known as chemosynthetic communities. For this reason, methane seeps related with gas hydrates dissociation creates a micro environment for living species, important for the marine ecosystem. This is an open and exciting study field for geologists, geochemical researchers and biologists.

1. June 21st
, 2017
10:50h ~11:20h
Prof. Dr. ANTONIO FERNANDO MENEZES FREIRE
fernando_freire@id.uff.br
Gas HydratesGas Hydrates
and theand the
Gas Seeps PhenomenonGas Seeps Phenomenon

4. MIGRATION THROUGH CARRIER BEDS
https://walrus.wr.usgs.gov/seeps/what.html
Generation - Migration - Retention

5. LATERAL AND VERTICAL MIGRATION
http://www.petro-ed.com/articles
Microseeps
(ppm ou ppb)
Generation - Migration - Retention

6. MIGRATION THROUGH FAULTS
Damaison & Huizinga, 1994
Generation - Migration - Retention
Microseeps
(ppm ou ppb)

7. GAS SOURCES
Gas Seeps

9. Afloramento de
GH no fundo do mar
Foto: ROV Hyper Dolphin JAMSTEC
Foto: USGS
Hardage and Roberts, 2006Gas Clathrate
•Natural gas hydrates or methane hydrates are
solids that form from a combination of water and
one or more hydrocarbon (CH4, C2H6) or non-
hydrocarbon gases (CO2, H2S, H2, N2). In
physical appearance, it resemble packed snow
or ice.
•Gas hydrates are stable under specific
pressure-temperature conditions. Under
appropriate pressure, they can exist at
temperatures significantly above the freezing
point of water.
Gas Hydrates
https://rosetta.jpl.nasa.gov/rosetta-science-blog/water-ice-what-kind-evidence-clathrates-67p

10. http://forum.arctic-sea-ice.net/index.php?topic=1389.0
Gas Hydrates
• It needs a source of methane
(thermogenic or biogenic methane);
• When CH4 arrives to appropriated P and T
conditions (GHSZ), the exothermic gas
molecules movement freezes the
surround water and a gas clathrate is
formed trapping the gas molecules inside
it.
http://www.alternativesjournal.ca/energy-and-resources/more-methane-surprises
Base of the Gas Hydrates Stability Zone - BGHSZ
(within the sedimentary column)
150 ~ 1000 mbsf
Top of the Gas Hydrates Stability Zone - TGHSZ
(within the seawater column)
100 ~ 400 m water depth
Gas Hydrates Stability Zone - GHSZ

11. Gas hydrate nodules recovered from piston cores in the Japan Sea
Photo: Freire, 2010
GAS HYDRATES IN MARINE SEDIMENT
Gas Hydrates

12. • Concentrated in
fractures and faults;
•Dispersed in
sediments as nodules
or blocks.
Tomography
Holland, 2008
Gas Hydrates
Photo: Freire, 2010
GAS HYDRATES IN MARINE SEDIMENT

13. Photo: ROV Hyper Dolphin, 2007
Gas Hydrates
GAS HYDRATES BUBBLES IN THE SEAWATER COLUMN

15. https://woodshole.er.usgs.gov/project-pages/hydrates/seeps.html
Onshore and Offshore Seeps
Gas Seeps

16. http://www.whoi.edu/page.do?pid=110417&cid=2345&cl=2383&article=2441&tid=5782
SEEPS AND SEAFLOOR ALTERATIONS
Gas Seeps

17. ROV Hyper Dolphin, 2007
GAS HYDRATE BUBBLES
Gas Seeps

18. ROV Hyper Dolphin, 2007
Gas Seeps
GAS HYDRATE BUBBLES

19. Bacterial Matts and Benthic Organisms
ROV Hyper Dolphin, 2007
GAS HYDRATE BUBBLES
Gas Seeps

20. ROV Hyper Dolphin, 2007
Gas Seeps
Bacterial Matts and Benthic Organisms
GAS HYDRATE BUBBLES

21. GIANT PLUMES IN THE SEAWATER COLUMN (MULTIBEAM)GIANT PLUMES IN THE SEAWATER COLUMN (MULTIBEAM)
http://www.noaanews.noaa.gov/stories2012/images/seeps1.jpg
Gas Seeps
600 m high

22. MOUNDS, POCKMARKS AND GIANT PLUMES
https://www.sciencedaily.com/releases/2017/06/170601151803.htm
The massive craters were formed around
12,000 years ago, but are still seeping
methane and other gases.
Credit: Illustration: Andreia Plaza
Faverola/CAGE
Gas Seeps

23. Photo: ROV Hyper Dolphin, 2007Photo: ROV Hyper Dolphin, 2007
ANAEROBIC OXIDATION OF METHANE
(AOM)
CH4+SO4
2 HCO3
-
+ HS-
+ H2O
Modified from Ussler & Paull, 2008
0 500 1000 1500 2000 2500 3000
0 105 15 2520 30
0
200
400
600
800
1000
1200
A
CH4 (mM)
Depth(cm)
SO4
CH4
Sulfate-Mathane interface
SO4 (mM)
Gas Seeps

24. http://oceanexplorer.noaa.gov/facts/hydrates.html
CHEMOSYNTHETIC COMMUNITIESCHEMOSYNTHETIC COMMUNITIES
http://www.forbes.com/forbes/welcome/
http://oceanexplorer.noaa.gov/okeanos/explorations/ex1202/background/seeps/welcome.html
TubewormsTubeworms Deep Water Coral and SpongesDeep Water Coral and Sponges
BivalvesBivalves
Gas Seeps

25. A possible mechanism for initiation of land sliding involves a breakdown in the base of the hydrate layer,
caused by a reduction in pressure due to a sea-level drop, such as occurred during the LGM.
Landslides can trigger tsunamis and other impacts.
Modified from Kvenvolden (1999)
High Sea Level
Methane release to the atmosphere
120m
20m
BGHSZ MTD
Reduction of
hydrostatic
pressure
Low Sea Level
Gas Hydrate Layer
SLOPE INSTABILITY RELATED TO GAS HYDRATES DISSOCIATION
Gas Seeps

26. https://woodshole.er.usgs.gov/project-pages/hydrates/seeps.html
Gas Seeps
SLOPE INSTABILITY RELATED TO GAS HYDRATES DISSOCIATION

27. Modified from Matsumoto et al., (2009)
Step 1: stable
GH formation
Low Pressure
(unstable)Transition
interglacial glacial
Step 2: transition
GH growth and concentration
(mounds formation)
Step 3: unstable
GH dissociation
(pockmarks formation)
High standHigh stand
High Pressure
(stable)
Low standLow stand
MOUNDS, POCKMARKS AND GAS HYDRATES DISSOCIATION
Gas Seeps

28. ROV Hyper Dolphin, 2007
Giant Pockmark: 500m in diameter and 40m deep
Gas Seeps
MOUNDS, POCKMARKS AND GAS HYDRATES DISSOCIATION

29. Modificado de Matsumoto et al., 2008
Giant Mounds and Pockmarks in the Japan Sea: 500m in diameter
Pockmarks
Mounds
Gas Seeps
MOUNDS, POCKMARKS AND GAS HYDRATES DISSOCIATION

31. GAS SAMPLING METHODS
Gas Seeps Geochemistry
Niskin bottles
Piston corer
Push corer
・ Seawater (collected by Niskin Bottles)
・ Seafloor sediment (collected by Piston corers & Push corers)
Ishizaki, 2007
Addition of HgCl2 solutions in order to sterilize microbes.

32. Ishizaki, 2007
GAS ANALYSIS METHODS
Gas Seeps Geochemistry
Gas Chromatography
Concentrations of dissolved
hydrocarbon gases
(CH4, C2H6, C3H8…)
Mass Spectrometry
•δ13
CCH4
•δDCH4
Sediment 3cc + MilliQ water (30cc vial)
Seawater (100ml vial)
Headspace
(N2 or He)
Ultrasonic vibration for 20 min.

33. Ishizaki, 2007
GAS ORIGIN INTERPRETATION
EXAMPLE: JAPAN SEA
Gas Seeps Geochemistry
Adapted from Bernard et al., 1976.
・ Mud gas recovered from plume-
sites and hydrate-dissociated gas are
shown within thermogenic origin in
Bernard diagram ．
・ Collected thermogenic gases had
more C1 than common thermogenic
gas.
→ Fractionation caused by
・ migration (Schoell, 1983).

34. Plot of C1/(C2 + C3) versus d13C of methane from gas hydrate samples collected
in piston cores PC67 and PC76 indicating a biogenic origin for the gas (Adapted
from Bernard et al., 1976). Miller et al., 2015
Gas Seeps Geochemistry
GAS ORIGIN INTERPRETATION
EXAMPLE: PELOTAS BASIN

35. Ishizaki, 2007
GAS ORIGIN INTERPRETATION
EXAMPLE: JAPAN SEA
Gas Seeps Geochemistry
Adapted from Withicar, 1995.
Main
Study Area
MOUNDS, POCKMARKS AND
GIANT PLUMES ARE IN
THE MAIN STUDY AREA!

36. Ishizaki, 2007
GAS ORIGIN INTERPRETATION
EXAMPLE: JAPAN SEA
Gas Seeps Geochemistry
Sampling needs to be made direct
on the gas seep or gas plume and deeper
than the methanogenesis zone !!!!
THERMOGENIC
BIOGENIC
50
100
150
200
250
300
350
400
-48 -46 -44 -42 -40 -38 -36
-188 -187 -186 -185 -184 -183 -182
PC 706
13Cδ
Dδ
depth[cmbsf]
δ 13
CCH4
δ DCH4

37. GAS ORIGIN INTERPRETATION
DEPENDS ON A CORRECT GAS SAMPLING LOCATION
Gas Seeps Geochemistry
Good location
for gas sampling
in the seawater
Good location
for gas sampling
in marine sediment
(mud gas)
• In other words, we should “see”
what we are really sampling.
ROV is the best way to do that.
• Piston Cores are better to be used in
regional surveys, followed by ROV surveys.
• Before to infer the origin of a gas seeps you
should be right about the sampling location!

38. Thank you for your attention!Thank you for your attention!
Prof. Dr. ANTONIO FERNANDO MENEZES FREIRE
fernando_freire@id.uff.br
http://www.dot.uff.br/
http://www.geofisica.uff.br/