Joint Oceanographic Institutions, United States

Ocean Drilling Program (ODP) Leg 204 to Hydrate Ridge, located on the continental slope offshore Oregon (USA), was the first drilling expedition dedicated to understanding gas hydrate processes in accretionary complexes and provided a testbed for a number of different techniques for estimating the gas hydrate content of sediments. It was also the first time that (1) digital infrared scans of core temperature were systematically recorded for all cores from within or near the gas hydrate stability zone, facilitating identification of gas hydrate samples for further study, (2) hydrate-bearing cores were recovered and logged at in situ pressure, and (3) ODP allowed acquisition of logging-while-drilling data prior to coring at a given site, providing an initial comprehensive estimate of gas hydrate distribution that was used to design the subsequent coring program. Gas hydrate estimates based on a variety of geophysical and geochemical techniques indicate a heterogeneous distribution of gas hydrate, which results in part because of two distinct regimes for delivery of gas to the gas hydrate stability zone. In the "reaction regime," which is pervasive throughout the study region, the average gas hydrate content of the sediments is relatively low (2%-8% of the pore space), no gas hydrate is present in the upper approximately 30 meters below seafloor (mbsf) because the methane content of the pore water is below saturation, and the fine-scale distribution of gas hydrate depends strongly on lithology. Superimposed on the reaction regime is a "transport-dominated regime" in which gas is focused into a stratigraphically controlled conduit and is transported as free gas to the structural summit. At the summit, high gas pressure drives free gas into and through the gas hydrate stability zone, resulting in a shallow deposit in which gas hydrate comprises approximately 25% of the total sediment volume to a depth of approximately 25 mbsf. Geochemical data indicate that most of the gas that forms the summit deposit has migrated from greater depth and has either a thermogenic or altered biogenic character, and modeling suggests that abundant free gas is needed to form gas hydrate in these conditions. Although this deposit contains only approximately 2% of the estimated total volume of methane trapped in gas hydrate within the study region, it may be particularly susceptible to destabilization in response to oceanographic change.