Book Description

Natural gas hydrate has been studied for its natural gas storage capacity, its significant role in global carbon cycles and its possible link to continental margin submarine landslides. However, the role of gas hydrate systems in broad global processes is still not well understood due to the large uncertainties in the estimate of global distribution and concentration of gas hydrate. For a given marine geosystems, the location and volume of gas hydrate accumulations heavily rely on the methane migration mechanisms, which are important links between the gas source and gas hydrate reservoir in natural gas hydrate systems. Numerical models are a great tool to explore methane migration mechanisms for natural gas hydrate accumulations by providing a framework that combines the geophysical, geochemical, geobiological, and geological measurements. The goal of this dissertation is to apply methane transport-reaction models to better understand the mechanisms and key factors that control gas hydrate accumulations in different marine settings. In Chapter 2, I focus on gas hydrate formation in thin, isolated, coarse- grained layers and in surrounding fine-grained hydrate-free zones. I use the available geophysical logging data measured in Walker Ridge 313 (WR 313) Hole H and Hole G in the northern Gulf of Mexico, combined with advection- diffusion-reaction model to explore the hydrate formation mechanisms in the 3 m-thick Red Sand and the 4 m-thick Purple Sand. The advection-diffusion- reaction model involves aqueous methane diffusion, methane transport with water advection, and methane generation. I confirm that a short-range methane diffusion mechanism is able to explain the hydrate accumulations in thin layers and the hydrate-free zones in the surrounding muds. I show the key parameters that control the total amount of methane and hydrate formation within coarse- grained layers as well as the thickness of hydrate-free zones in the surrounding fine-grained muds. This work is published in Geochemistry, Geophysics, Geosystems, titled Factors Controlling Short-Range Methane Migration of Gas Hydrate Accumulations in Thin Coarse-Grained Layers. In Chapter 3, I describe in detail the time-dependent calculation of dissolved methane concentration and gas hydrate content in the advection- diffusion-reaction model. The partial differential equations for the mass balance of sedimentary organic carbon, methane, and hydrate are solved in terms of material derivatives. The material derivatives for each component are then solved applying a semi-Lagrangian finite difference method combined with a fractional step method, which is an effective mathematical method for solving advection problems. In addition, I show the key assumptions made in the development of the methane hydrate formation model in this work. In Chapter 4, I focus on high saturations (79-93% in pore volume) of hydrate formation in thick, coarse-grained sediments at Green Canyon 955 (GC 955) in the northern Gulf of Mexico. I use 1D and 2D numerical models and compare the predicted gas hydrate formation in coarse-grained layers with different mechanisms. I show that short-range methane diffusion and upward water advection are not enough to form high saturations of hydrate in the GC 955 reservoir, but the free gas flow is required to form high saturations of hydrate at GC 955. This work has been accepted to the special volume of American Association of Petroleum Geologists (AAPG) Bulletin, titled Methane Migration Mechanisms for The GC 955 Gas Hydrate Reservoir, Northern Gulf of Mexico. In Chapter 5, I focus on the gas hydrate system on the Hikurangi Margin, offshore New Zealand, where the reservoir is cm-thick, coarse-grained silts bounded with fine-grained muds that contain no hydrate. I show the estimation of the base of gas hydrate stability zone (GHSZ) applying temperature measurements from coring and logging and the hydrate phase boundaries calculated from Sloan and Koh (2008) and Tishchenko et al., 2005. The estimated depth to the base of GHSZ are compared to that applied in Pecher et al. (2018), Screaton et al. (2019), and Sultan (2019). In addition, I apply previously developed 1D microbial methanogenesis advection-diffusion- reaction model to test methane migration mechanisms to form hydrate reservoir at Hikurangi Margin. In Chapter 6, I summarize the effects of different methane transport mechanisms that are short-range diffusion, upward water advection, and free gas flow on gas hydrate accumulations in heterogeneous marine sediments. I conclude that hydrate distribution and volume in marine settings depends heavily on the methane migration mechanisms, but it is also controlled by the amount of methane supply and sediment heterogeneity.