Leg 204 Preliminary Report

During Leg 204, we cored and logged nine sites on the Oregon continental margin to determine the distribution and concentration of gas hydrates in an accretionary ridge and adjacent slope basin, investigate the mechanisms that transport methane and other gases into the gas hydrate stability zone (GHSZ), and obtain constraints on physical properties of gas hydrates in situ. A three-dimensional seismic survey conducted from 19 June through 3 July 2000 provided images of potential subsurface fluid conduits and indicated the depth of the GHSZ throughout the survey region. After coring at the first site, we acquired logging-while-drilling (LWD) data at all but one site to provide an overview of downhole physical properties prior to coring. The LWD data confirmed the general position of key seismic stratigraphic horizons and yielded an initial estimate of gas hydrate concentration through the proxy of in situ electrical resistivity. These records proved to be of great value in planning subsequent coring. We also tested the use of infrared thermal imaging of cores as a new and effective tool to identify gas hydrates while the cores were on the catwalk as rapidly as possible after core retrieval. The thermal images were used to estimate the distribution and texture of hydrate within the cores. Geochemical analyses of interstitial waters and of headspace and void gases provided additional information on the distribution and concentration of hydrate within the stability zone, the origin and pathway of fluids into and through the GHSZ, and the rates at which the process of gas hydrate formation is occurring. Biostratigraphic and lithostratigraphic descriptions of cores, measurement of physical properties, and in situ pressure core sampling and thermal measurements complement the data set, providing ground-truth tests of inferred physical and sedimentological properties. Among the most interesting preliminary results are the following: 1. Gas hydrates are distributed within a broad depth range within the GHSZ. 2. Within this broad range, lithology is an important factor influencing hydrate concentration. 3. Different physical and chemical proxies for hydrate distribution and concentration give generally consistent results. 4. The gas hydrate system at Hydrate Ridge contains significant concentrations of higher-order hydrocarbons. 5. Fractionation and mixing signals provide important constraints on gas hydrate dynamics. 6. Very high chloride concentration values extend to a depth of 20–30 meters below seafloor near the summit, indicating that hydrate formation here must be very recent and rapid. 7. Gas hydrate concentration is significantly greater beneath the ridge than beneath the adjacent slope basin. 8. In the slope basin, hydrates are concentrated just above the bottom-simulating reflector (BSR). 9. The BSR represents a discontinuity in several chemical constituents of pore waters and gas voids, indicating that hydrate formation has profound geochemical effects.