Cao Yungchen; Chen Duofu (2011): Numerical simulation of gas hydrate crystallization from dissolved methane in marine sediments. American Geophysical Union, Washington, DC, United States, In: Anonymous, AGU 2011 fall meeting, 2011, georefid:2012-049887

Methane hydrate can only form under low temperature and high pressure when dissolved methane concentration exceeds that in equilibrium with gas hydrate. There are two types of methane sources in marine gas hydrate systems: in-situ biogenic methane produced by microbial breakdown of organic matter and dissolved methane from deep sediment (Davie and Buffett, 2003, J. Geophys. Res., 108(B10), 2495). Thus, the occurrence of gas hydrate from dissolved methane mainly depends on the production rate of biogenetic methane and fluid flux that also influences chlorinity and sulfate profile. A numerical model was developed to simulate gas hydrate accumulation at IODP site 1327. The hydrate crystallization rate was determined by the amount of methane that exceeds the methane solubility: R (sub h) =kDelta X S (sub w) phi , where R (sub h) [kg/m (super 3) /year] is the hydrate formation rate, k is the rate constant (k =2000 kg/m (super 3) /year), phi is the porosity of the sediment, and Sw is volume fraction of the pore space filled with water. Delta X [mol/kg] is the difference between the dissolved methane concentration and the local solubility of methane hydrate (Chen et al., 2006, Terr. Atmos. Ocean. Sci., 17(4): 723-737). The chlorinity vs. depth profile shows a continuous freshening trend with depth and the depth of sulfate/methane interface (SMI) is 9.5 mbsf at IODP site 1327 (Riedel et al., 2006, IODP, doi:10.2204/iodp.proc.311.2006). The measured chlorinity at IODP site 1327 is used to obtain fluid flux (q (super w) =0.2 kg/m (super 2) /year). To best fit the measured sulfate profile, the biogenetic methane production rate is adjusted to 4X10 (super -6) mole/m (super 3) /year. Using water flux, in-situ biogenic methane production rate and the methane concentration derived from our simulation, we estimate the rates of methane supplied by fluid advection, diffusion and in situ production of methane, respectively. The methane supplied by water advection from BHSZ is 0.0292 mole/m (super 2) /year (77.7%), which is obtained based on the water flux and methane concentration at base of the hydrate stable zone (BHSZ). A supply of methane due to diffusion is calculated to 8X10 (super -3) mole/m (super 2) /year (20.2%) from the gradient of methane concentration at BHSZ. On the basis of the biogenetic methane production rate, the in situ source methane between SMI and BHSZ is 8X10 (super -4) mole/m (super 2) /year (2.1%). Thus, the supply rate of methane to HSZ is 0.038 mole/m (super 2) /year. The deep methane source beneath hydrate stable zone (HSZ) serves as main contributor. Those dissolved methane could crystallize as gas hydrate at HSZ or consumed at SMI. The results show that the rate of gas hydrate formation at IODP site 1327 is 0.019 mole/m (super 2) /year which occupies 50% of the whole methane supplied to HSZ. The other 50% of the methane is transported to shallow depth by advection and/or diffusion, and further was consumed through anaerobic methane oxidation at SMI.
West: -126.5200 East: -126.5200 North: 48.4200 South: 48.4200
Expedition: 311
Site: 311-U1327
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