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Corso Idrati di metano nel sottosuolo
Docente Alberto Malinverno
CFU e ore 5

Struttura e durata Aula C, Via Botticelli 23, 9.30-12.30 e 14.00-16.30
Programma 1. Gas hydrate (GH) stability and methane supply
History of discovery
Seismic evidence
Early drilling observations (DSDP)
GH structure: burning ice (video)
Thermodynamic stability
Pore pressure, lithostatic pressure, and effective pressure
Ingredients: water, methane, P and T for stability
Where are gas hydrates: permafrost and continental margins
The GHSZ on continental margins: depth interval
The importance of C-org supply
Methane concentration and solubility
GHOZ versus GHSZ
Interest in GH
Climate change and GH formation/dissociation: the GH reservoir in the C cycle
GH dissociation and submarine slope failure
GH as an energy resource

2. Physical properties of continental margin sediments that host GH
Sediments on continental slopes (include volcanic sands)
Physical properties of sediments
Index properties (density, porosity)
Electrical resistivity
Elastic / acoustic
Minerals associated to GH: authigenic carbonates
Carbonate ion from methane and sulfate by anaerobic oxidation of methane: CH4 + SO4-- => HCO3- + HS- + H2O
Minerals associated to GH: magnetic sulfides (greigite and pyrrhotite)

3. Measuring GH
Surface seismic data and the bottom-simulating reflector
Effect of GH and gas bubbles on P-wave velocity
Electromagnetic measurements on the seafloor
Degassing of cores (video)
Sediment textures (moussy and soupy intervals)
X-ray tomography of samples frozen in liquid nitrogen
Pressure cores: degassing, X-ray tomography (video)
Infrared photography
Pore water chemistry
Well logging
Results: Blake Ridge, Cascadia, India
EXERCISE: Gas hydrate saturaition from chlorinity
EXERCISE: Gas hydrate saturation from resistivity and porosity

4. GH textures in sediments
Low-flux and high-flux end member gas hydrate distributions
Methane transport by
Gas bubble migration
Gas bubble migration in high-permeability channels (eg., fault planes, mud diapirs)
Massive GH (at or near the seafloor)
Low flux: lithology influences pore habit
Disseminated in the pore space (coarse-grained sediments)
In veins and fractures (fine-grained sediments)
Inhibition of GH formation in small pores
State of stress in the subsurface and hydraulic fracturing

5. Climate change and GH formation/dissociation
Pore pressure, lithostatic pressure, and effective pressure
Change in seafloor T and heat conduction in the sediment column
Gas hydrate dissociation from the base of the GHSZ
Effect of change in sea level
Effect of change in seafloor temperature
Computing the GH stability curve for the fresh water-methane system
Effect of salinity on the GH stability curve
EXERCISE: Modeling the effect of change in seafloor P, T on the base of the GHSZ (Matlab)
Computing the methane solubility in the pore water
EXERCISE : Modeling the effect of change in seafloor P, T on methane solubility and the top of the GHOZ (Matlab)
Gas hydrate dissolution from the top of the GHOZ
Sediment compaction and fluid expulsion due to compaction
Overpressure due to high sedimentation rates in fine-grained sediments
State of stress in the subsurface and hydraulic fracturing
EXERCISE: Overpressure due to gas hydrate dissociation
Overpressure and submarine sliding
Storegga slide interpretation.

6. Biogenic production of methane in sediments
Biogenic and thermogenic methane
Isotopic and chemical evidence for biogenic methane
Organic carbon to the seafloor and consumption by aerobic bacteria
Vertical zonation of dominant anaerobic bacterial populations
Sulfate reduction
Shallow methane production within the GHSZ vs. methane advection from depths beneath the GHSZ
Bacterial consumption of methane (methanotrophy): anaerobic oxidation of methane (AOM)
Evidence for AOM
Methane in the atmosphere oxidizes to CO2 in ~ 10 yr in the presence of sunlight
Methane in the water column is oxidized bacterially to CO2 in 1.5-50 yr
CO2 equilibrates between atmosphere and ocean in a few 100 yr
After few 100 yr CO2 distribution will be the same regardless of the source (atmosphere or ocean)

7. Modeling the formation of gas hydrates
Diagenetic modeling
Steady-state and time-dependent modeling
Simple calculation of dissolved methane concentration due to biogenic in situ methane production
Approximate calculation of GH saturation (as in Paull et al.)
Complex models that account for gas hydrate formation
EXERCISE: Biogenic in situ methane production and methane solubility (Matlab)
Example: GH in thin sands from shallow biogenic methane

8. Estimating the total amount of C stored globally in gas hydrates
Observation-based estimates (eg., Milkov)
Model-based estimates (eg., Buffett and Archer)
EXERCISE: Estimate the worldwide quantity of marine gas hydrates based on observations (Volume and saturation of GHSZ)

9. Gas hydrates and the geological record
The long-term carbon cycle
Stable C-isotopes and delta-C13
delta-C13 of different carbon reservoirs (Table 1 of Cramer and Kent, 2005)
Mechanisms to explain the PETM delta-C13 anomaly: terrestrial Corg, gas hydrates, cometary impact, methane from contact metamorphism of C-rich sediments
EXERCISE: Quantify C-isotope effect of GH dissociation
The gas hydrate explanation of the PETM and its limitations
The Quaternary "clathrate gun" hypothesis
Possible episodes of gas hydrate dissociation in the Mesozoic and at the Permo-Triassic boundary