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Numerical models for the formation of marine gas hydrate : constraints on methane supply from a comparison of observations and numerical models Davie, Matthew K.

Abstract

Methane hydrate is a chemical compound composed of a rigid network of water molecules that enclose methane gas. Although this abundant form of hydrate is stable under pressure and temperature conditions present in seafloor sediments of most of the world's oceans, hydrate occurrences are confined to regions where the concentration of methane is sufficient to exceed the local solubility. Consequently, marine hydrates are generally restricted to continental margins where conversion of high inputs of organic carbon or focusing of methane bearing fluids supply the methane required for hydrate formation. Empirical extrapolation of hydrate volumes from known locations to all continental margins yields a methane abundance which exceeds all other fossil fuel resources combined. Consequently, methane hydrates have generated interest in their potential use as an energy resource and as a mechanism for climate change. However, fundamental questions about marine hydrate remain unresolved. Specifically, the source of the methane and the mechanism of methane supply required for hydrate formation is not well known. A better understanding of the source and mechanism of methane supply would help to refine global estimates and identify regions of high hydrate potential. In this thesis, I address these issues by developing numerical models for the formation of marine gas hydrate and comparing the predictions with observations from known hydrate locations. I first develop a time-dependent model to account for the important processes during hydrate formation in marine sediments. The model predicts the methane and chlorine concentration in the pore water, organic carbon available for methanogenesis, hydrate and free gas volumes, and interstitial fluid velocity. Application of the hydrate model to both the Blake Ridge and Cascadia margin allows for a quantitative appraisal of the mechanisms of the methane supply. I find that observations of chlorinity, hydrate and free gas from both the Blake Ridge and Cascadia margin can be explained equally well by either migration of methane bearing fluids from deep sources or in situ biogenic production of methane in the shallow sediments. In particular, observations at the Blake Ridge are reproduced by either a saturated deep source of methane migrating upward at 0.26 mm/yr or in situ production of methane requiring 50% conversion of organic carbon along with an upward fluid velocity of 0.08 mm/yr. In a similar manner, the chlorinity data at the Cascadia margin can be explained by either a saturated deep source of methane with an upward fluid velocity of 0.42 mm/yr or in situ production of methane together with an upward fluid velocity of 0.35 mm/yr. As a result, additional model constraints are needed to differentiate between the two mechanisms of methane supply. Measurements of sulfate concentration provide an additional constraint on the source of methane because sulfate measurements can be related to the methane concentration in the pore water. To exploit the sulfate measurements I develop a steady state model to describe the formation of marine hydrate, which includes a thin sulfate reducing zone. Constraints imposed by sulfate and chlorine measurements at the Blake Ridge favour in situ methane production (located in the lower part of the HSZ) together with an incoming methane bearing fluid at 0.23 mm/yr. Comparison of model predictions with observations at the Cascadia margin requires a shallow in situ methane source together with a deep methane bearing fluid migrating upward at a velocity of 0.36 mm/yr. The volume of hydrate predicted at the Cascadia margin is substantially smaller than previous estimates from seismic velocity and resistivity modelling, but agrees well with hydrate estimates from drill core temperature measurements. In addition, the predicted sulfate profiles at both the Blake Ridge and Cascadia margin indicate that the process of anaerobic methane oxidation is the primary pathway of sulfate depletion at these locations of hydrate occurrence.

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