Generalization of gas hydrate distribution and saturation in marine sediments by scaling of thermodynamic and transport processes

Abstract

Gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins. These compounds form in pore space between the seafloor and a sub-bottom depth where appropriate stability conditions prevail. However, the amount and distribution of gas hydrate within this zone, and free gas below, can vary significantly at different locations. To understand this variability, we develop a one-dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time. The model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments. The effect of salinity on gas hydrate distribution is also included.The simulations delineate basic modes of gas hydrate distribution in marine sediment, including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone, for various methane sources. The results are scaled using combinations of dimensionless variables, particularly the Peclet number and Damkohler number, such that the dependence of average hydrate saturation on numerous parameters can be summarized using two contour maps, one for a biogenic source and one for upward flux from a deeper source. Simulations also predict that for systems at steady state, large differences in parameters like seafloor depth, seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables, although important caveats exist. Our model presents a unified picture of hydrate accumulations that can be used to understand well-characterized gas hydrate systems or to predict steady-state average hydrate saturation and distribution at locations for which seismic or core data are not available.

A transfer function for the prediction of gas hydrate inventories in marine sediments

“…11a), most likely because for such high POCar (sedimentation rates >70-100 cm/kyr) the residence time of organic matter within the GHSZ decreases significantly. Overall, this effect is in agreement with results of Davie and Buffet (2001) and Bhatnagar et al (2007).…”

“…The input and degradation of POC are critical parameters in this regard as they are the driving force for CH 4 formation (Davie and Buffett, 2001). Bhatnagar et al (2007) performed a rigorous numerical model study of LGF systems and found out that GH formation mainly depends on (i) the thickness of the GH stability zone (GHSZ), (ii) the Damkohler number, representing the ratio of methane production to methane diffusion, and (iii) the first Peclet number, representing the ratio of fluid advection to methane diffusion. While the GHSZ can be easily calculated, specifically the estimation of the rate of methane production is problematic, which makes the result of Bhatnagar et al (2007) difficult to use without applying models of the same complexity.…”

Down the Rabbit Hole: toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events

“…Second, the 7 × 10 6 km 3 estimate pertains to bathymetric conditions during the last glacial maximum. The rationale for using this bathymetry to discuss gas hydrate accumulation comes from considerations of sea level (hydrostatic pressure) and the relatively slow cycling time of CH 4 in gas hydrate systems (>10 000 yr; Davie and Buffett, 2001;Bhatnagar et al, 2007). Sea level during the Holocene is much higher than that spanning most of the late Pleistocene.…”

Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane?

“…These assumptions turn equation (3) into an ordinary differential equation that we solve with a Laplace transform method (see Appendix A). This analytic solution is useful because the results are easily reproducible and because it complements the numerical solutions most commonly employed in this kind of modeling [e.g., Davie and Buffett, 2001;Bhatnagar et al, 2007;Sivan et al, 2007]. While numerical solutions are more flexible and do not require the assumptions made here, an analytic treatment is adequate for our purpose, which is to quantify the major effects of diffusion, burial, and solute generation or consumption on steady state concentrations and isotopic compositions.…”

“…[55] The argument of the exponential in equation (6) is a dimensionless Peclet number, which quantifies the relative importance of diffusion and burial by sedimentation at a rate w in a layer of thickness z SMT [Boudreau, 1997;Bhatnagar et al, 2007]:…”