Lessons Learned about the Acoustic Properties of Natural Gas Hydrates from 50 Years of Scientific Ocean Drilling Expeditions within DSDP/ODP/IODP Programs

Vast quantities of methane are stored in marine gas hydrate reservoirs across the globe. Even as the world moves away from fossil fuel energy sources, understanding marine gas hydrates remains important given their contribution to the global carbon cycle and their vulnerability to changes in sea level and water temperature. Accurate quantification of the gas hydrate inventory requires an understanding of how the presence of hydrates impacts the response of geophysical exploration techniques used to detect them, such as seismic surveying and sonic borehole logging. The DSDP/ODP/IODP data repository holds a wealth of geophysical, petrophysical, geological, geochemical, and biological data collected during scientific ocean drilling expedition in the past 50 years. Of the 276 DSDP/ODP/IODP expeditions, nearly 10% uncovered direct or indirect evidence of gas hydrate. These datasets have been collated to evaluate the acoustic properties of gas hydrate-bearing sediments at different scales in the context of their morphology and distribution, as well as the lithology of their host sediment. Here, we use the IODP data to constrain the input parameters and associated uncertainties to generate a variety of rock physics models that relate p-wave velocities to hydrate content and distribution. The majority of available models describe gas hydrates that are evenly disseminated within the sediment pore structure. Simple averaging approaches and finite element modeling are applied to investigate the effect of uneven dissemination and larger scale features (nodules, veins, layers) that are common in marine gas hydrates. The modeling results demonstrate the scale dependency of the acoustic properties with respect to both data acquisition method (seismic, log or core data) and the hydrate features (disseminated, nodular or layered). Consequently, the geological setting of a given hydrate occurrence should be considered when using any rock physics model to determine its acoustic properties.