| The methane concentration in marine sediment determines methane hydrate accumulation.In situ biogenic methane,diffusion of dissolved methane and advection of methane-carrying pore fluids through marine sediment are sources of methane in hydrate system.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 as described above and the hydrate dissociation rate.Therefore,sedimentation is the key control factor of methane hydrate accumulation.In addition,the methane concentration in the methane-bearing pore fluids from deep also affect the dynamic hydrate accumulation.Previous studies on the formation of methane hydrate primarily consider a constant sedimentation rate and assume a specific methane concentration in upward fluid,and thus may result in inaccurate understanding of methane hydrate accumulation process.Hence,a hydrate accumulation model that is capable of handing multiple sedimentation stages and a calculation model of deep methane influx are established,to clarify the controlling effect of variable sedimentation rates and methane influx on dynamic hydrate accumulation.The models are then applied to Blake Ridge Site ODP997,ODP995and ODP994.Combining the obtained methane influx and the hydrate distribution characteristics at Blake Ridge,we give insight into the influence of deep methane influx and variable sedimentation rate on hydrate accumulation,and reveal regional deep methane flux at Blake Ridge.We simulated the evolution of hydrate reservoir at Site 997 based on the hydrate accumulation model which is affected by variable sedimentation rate.The simulation of ODP997 site shows that the methane hydrate is mainly formed during the latest2.5Ma,the average hydrate saturation at the base of methane hydrate stability zone is in good agreement with the estimated value based on the measured data.The total resources of methane hydrate in sediments are significantly affected by the sedimentation rate.The stable and slow sedimentation rate favors hydrate accumulation.Furthermore,a numerical model beneath the base of hydrate stability zone(BHSZ)by relating the free-gas zone(FGZ)to deep methane flux through analytical expressions is established to obtain the methane flux information.This model is then applied to ODP Site 995,and the results indicate that we can take advantage of the properties of the FGZ to limit the methane flux.The values will be smaller if the base of the free gas zone is shallower or the critical gas saturation is larger.Assuming that the present-day free-gas zone at Site 995 is in steady-state,a deep methane flux of0.0231mol/m~2/a is determined.The second free-gas interval at about 700mbsf at Site995 has also been obtained using this methane flux,which is coincident with the seismic profiles.This result verifies the accuracy of this model and indicates that the methane flux is likely to originate from the second free gas layers.This result is then extended to explore its influence on hydrate accumulation.We obtain that the deep methane influx determines the hydrate distribution.The smaller the deep methane influx,the longer the time for hydrate occurrence zone to reach the BHSZ.If the methane influx is extreme low,the hydrate distribution is difficult to extend to the BHSZ though a large evolution time is applied.Therefore,it is highly suspected that a thin zone of methane hydrate distribution at Site 994 is resulting from a low methane flux.Combined with the previous conclusions on the high methane flux at Site 997,we speculate that the methane influx at Blake Ridge appears to be laterally variable,from a low value at edge flank Site 994,normal at flank Site 995 to a high value at crest Site 997. |
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