Prediction of the Methane Supply and Formation Process of Gas Hydrate Reservoir at Odp1247, Hydrate Ridge, Offshore Oregon

Abstract

The formation of gas hydrate reservoir in marine sediments is mainly controlled by methane supply and sedimentary burial. Based on the mass conservation of methane in gas hydrate system, a numerical model of gas hydrate formation was established considering the methane supplied by dissolved methane diffusion, pore water advection, and in situ methanogenesis. A case study of ODP site 1247 at the Hydrate Ridge, offshore Oregon shows that dissolved methane transported by molecular diffusion and pore water advection is the major supply of methane for gas hydrate formation, while in situ methanogenesis contributes little to the gas hydrate reservoir. The gas hydrate reservoir was also evaluated considering the changes of sedimentation rate since 1.67 Ma. Our model results show that the variations of sedimentation rate lead to little change in the size of gas hydrate reservoir at ODP 1247. The calculated hydrate saturation amounts to ∼0%∼3%, which is consistent with the measured values using pressure coring.

A Numerical Model to Estimate the Effects of Variable Sedimentation Rates on Methane Hydrate Formation—Application to the ODP Site 997 on Blake Ridge, Southeastern North American Continental Margin

“…Moreover, the rate of the already accumulated hydrate moving out of the base of HSZ also varies accordingly (Haacke et al, 2007;Karstens & Haflidason, 2018). Zheng et al (2017) presented a model for methane hydrate accumulation in marine sediment, which is capable of handling a transient sedimentation rate, and applied it to investigate the mechanisms of methane supply and hydrate accumulation in marine sediment at ODP Site 1247. The authors found that, although the multistage sedimentation rates at Site 1247 within a rather small range, the estimated hydrate saturation levels still deviate from that obtained if an averaged sedimentation rate was used.…”

“…Above the HSZ, the amount of dissolved methane is balanced by supply through advective and diffusive methane transport and consumption via micro organic oxidation and approaches zero when completely oxidized by microorganisms at the SMI (Borowski et al, 1996;Davie & Buffett, 2003a, 2003bBhatnagar et al, 2011;Zheng et al, 2017; Figure 3). Below the HSZ, methane hydrate is being carried out through the bottom of HSZ by the downward-moving sediment.…”

“…Potential effects of thermal, mechanical, or salinity variation on methane hydrate formation or dissociation are neglected due to their relatively small influence (Davie et al, 2004;Liu & Flemings, 2007;Paull et al, 1996;Rueff et al, 1988;Xu et al, 2001;Xu & Ruppel, 1999;Zheng et al, 2017).…”

“…In equations 12 and 13, C poc (wt-%) is concentration of particulate organic carbon (POC), C DIC (mol/kg), and C w m (mol/kg) represent the concentration of inorganic carbon and dissolved methane, respectively, K c Figure 3. Schematic illustration showing the methane hydrate system within the marine sediments (adapted from Zheng et al, 2017). The free gas zone is located below the base of the hydrate stability zone (BHSZ); there, the methane exists in the form of both gas and liquid and the dissolved methane concentration equals the gas-liquid solubility.…”