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

Abstract

Sedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and hydrate removal from hydrate stability zone (HSZ) and further affect the accumulation of hydrate in submarine sediments. Most of previous models assumed a constant sedimentation rate and thus may result in inaccurate estimates of total amount of hydrate within the HSZ. In this study, we developed a hydrate accumulation model that is capable of handling multiple sedimentation stages. Site 997 of ODP Leg 164, with data indicating its recent history of four sedimentation stages of different rates, is chosen to investigate the significance of sedimentation rate variation experienced at this location. We examined the effect of varying sedimentation rates on hydrate accumulation by taking in consideration sediment compaction, in situ methanogenesis, and composition and component transport processes. Simulation results suggest that the history of hydrate accumulation at Site 997 is characterized by a sequence of increase, decrease, and then increase till the present day. At present, the hydrate deposit has accumulated to 8.30 × 10 4 mol/m 2 , and the average hydrate saturation near the base of the HSZ is 6.3%, which is in general agreement with published estimates in the literature. The accumulation of hydrate at Site 997 was significantly affected by variable sedimentation rates, and nearly one half of the hydrate deposit was accumulated during the last 2.5 Myr. Plain Language Summary Methane hydrate forms in submarine sediments where thermodynamic conditions are satisfied and adequate methane is available. Three main mechanisms have been proposed for adequate methane availability: in situ methanogenesis, advection of methane-bearing fluids, and diffusion of methane. However, in actual environments off continental margins, a variable sedimentation rate may significantly affect the above mechanisms and thus change the amount of methane supply. Therefore, the formation of methane hydrate is influenced eventually. Meanwhile, the dissociation of methane hydrate changes simultaneously with a variable sedimentation rate. Hence, it is unclear how will hydrate reservoirs respond to different sedimentation rate values. In this study, we selected Site 997 of the ODP Leg 164 due to the significant variations in sedimentation rate and developed a hydrate accumulation model reflected several sedimentary stages. We highlighted the importance of consideration of variation of sedimentation rate in the estimation of total amount of hydrate within the HSZ and drawn a conclusion that dissociation of hydrate is more sensitive to the changes in sedimentation rate compared to hydrate formation.

A New Dynamic Modeling Approach to Predict Microbial Methane Generation and Consumption in Marine Sediments

“…By decreasing POC initial age (increasing the sedimentation rate), the organic matter becomes more reactive, and the POC degradation rate increases (according to the results, the rate of youngest POC is almost 3.73 and 1.93 times higher than the rate of the oldest and middle-aged one, respectively) results in increasing sulfate reduction and methanogenesis rates (Figure 7), and consequently, shallowing the SMT and increasing methane production (Figure 6). As methane generation increases, pore water is saturated at shallower depths, hydrate formation commences in shallower depths, and the thickness of hydrate (the supersaturation zone of methane) increases (276, 330, and 366 m for the oldest, middle-aged, and youngest, respectively), which is in general agreement with other studies [9,60]. Nevertheless, the gas hydrate average concentration decreases with an increase in sedimentation rate.…”

A Numerical Model for Determining Deep Methane Flux Linked to the Free Gas Zone: Application to the Ocean Drilling Program Site 995 and Implications for Regional Deep Methane Flux at the Blake Ridge

“…This result is roughly consistent with the interstitial fluid velocity at the nearby Site 997 which was obtained by Davie and Buffett (2003b). The porosity profile (φ) as a function of depth was determined by fitting an empirical exponential function to the measurement data (Zheng et al, 2020). Finally, the porosity exhibits a limited variation in Domains 1 and 2, which represents a nearly complete compaction below BHSZ.…”

“…Some site-specific parameters needed to be stated before application to Site 995 include the fluid flow rate, the porosity, the sedimentation rate and the density of the gas bubbles. The rate of upward fluid flow was predicted by fitting the computed chloride profiles to the chloride measurements at ODP Site 995 (Paull et al, 1996;Dickens, 2001;Zheng et al, 2020). Accordingly, the value of the flow rate was determined to be 0.125 kg/m 2 /a, which was described as the mass flux.…”

“…However, we applied a lower upward methane flux (0.015 mol/m 2 /a) comparing with that at Site 995 on hydrate accumulation and found that the methane hydrate occurrence was extremely difficult to extend to the BHSZ, despite the evolution time is long enough. The detailed model system for hydrate accumulation is described by Zheng et al (2020). Finally, the actual hydrate occurrence thickness was thinner than that of the GHSZ.…”