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Freire ALAGO 2017-06-21


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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.

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Freire ALAGO 2017-06-21

  1. 1. June 21st , 2017 10:50h ~11:20h Prof. Dr. ANTONIO FERNANDO MENEZES FREIRE Gas HydratesGas Hydrates and theand the Gas Seeps PhenomenonGas Seeps Phenomenon
  2. 2. MIGRATION THROUGH CARRIER BEDS Generation - Migration - Retention
  3. 3. LATERAL AND VERTICAL MIGRATION Microseeps (ppm ou ppb) Generation - Migration - Retention
  4. 4. MIGRATION THROUGH FAULTS Damaison & Huizinga, 1994 Generation - Migration - Retention Microseeps (ppm ou ppb)
  5. 5. GAS SOURCES Gas Seeps
  6. 6. 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
  7. 7. 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. 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
  8. 8. Gas hydrate nodules recovered from piston cores in the Japan Sea Photo: Freire, 2010 GAS HYDRATES IN MARINE SEDIMENT Gas Hydrates
  9. 9. • 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
  10. 10. Photo: ROV Hyper Dolphin, 2007 Gas Hydrates GAS HYDRATES BUBBLES IN THE SEAWATER COLUMN
  11. 11. Onshore and Offshore Seeps Gas Seeps
  13. 13. ROV Hyper Dolphin, 2007 GAS HYDRATE BUBBLES Gas Seeps
  14. 14. ROV Hyper Dolphin, 2007 Gas Seeps GAS HYDRATE BUBBLES
  15. 15. Bacterial Matts and Benthic Organisms ROV Hyper Dolphin, 2007 GAS HYDRATE BUBBLES Gas Seeps
  16. 16. ROV Hyper Dolphin, 2007 Gas Seeps Bacterial Matts and Benthic Organisms GAS HYDRATE BUBBLES
  18. 18. MOUNDS, POCKMARKS AND GIANT PLUMES 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
  19. 19. 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
  20. 20. CHEMOSYNTHETIC COMMUNITIESCHEMOSYNTHETIC COMMUNITIES TubewormsTubeworms Deep Water Coral and SpongesDeep Water Coral and Sponges BivalvesBivalves Gas Seeps
  21. 21. 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
  23. 23. 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
  24. 24. ROV Hyper Dolphin, 2007 Giant Pockmark: 500m in diameter and 40m deep Gas Seeps MOUNDS, POCKMARKS AND GAS HYDRATES DISSOCIATION
  25. 25. 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
  26. 26. 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.
  27. 27. 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.
  28. 28. 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).
  29. 29. 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
  30. 30. 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!
  31. 31. 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
  32. 32. 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!
  33. 33. Thank you for your attention!Thank you for your attention! Prof. Dr. ANTONIO FERNANDO MENEZES FREIRE