Deep methane pulsing as a mechanism for concentrating microbial methane hydrate in marine sediments

Naturally occurring gas hydrate accumulations in marine sediments are typically broadly characterized as originating from either thermogenic or microbial methane sources. Gas sources and hydrate reservoirs are linked through corresponding migration mechanisms; often, analyses of gas sources are used to infer migration mechanisms, or vice versa. Gas hydrate is commonly observed to preferentially accumulate in the pore space of coarse-grained sediment strata in marine environments while remaining largely absent or in a disseminated fracture morphology in surrounding mud sequences. This preferential accumulation phenomenon is typically attributed to either preferential flow in coarse-grained strata due to their higher permeability than muds (advective mechanism) or to thermodynamic inhibition to formation in muds because of their small pore size (diffusive mechanism). Accumulation of gas hydrate from a deep thermogenic source originating below the gas hydrate stability zone is often associated with an advective migration mechanism, while concentrated hydrate deposits from locally sourced microbial methane are usually explained with either a diffusive or recycling mechanism. This study presents the development of a multiphase flow and reactive transport simulator, PFLOTRAN, to explicitly track methane that is derived from microbial sources and from deep thermogenic sources. Simulations demonstrate that a pulse of methane-rich fluid of thermogenic origin updip along a high permeability sediment layer can result in a significant region of concentrated hydrate with a predominantly microbial signature. This finding is important to consider when inferring the nature and distribution of a gas hydrate system from observational data. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energys National Nuclear Security Administration under contract DE-NA-0003525. SAND2021-9493 A