Large uncertainties about the energy resource potential and role in global climate change of gas hydrates result from uncertainty about how much hydrate is contained in marine sediments. During Leg 204 of the Ocean Drilling Program (ODP) to the accretionary complex of the Cascadia subduction zone, we sampled the gas hydrate stability zone (GHSZ) from the seafloor to its base in contrasting geological settings defined by a 3D seismic survey. By integrating results from different methods, including several new techniques developed for Leg 204, we overcome the problem of spatial under-sampling inherent in robust methods traditionally used for estimating the hydrate content of cores and obtain a high-resolution, quantitative estimate of the total amount and spatial variability of gas hydrate in this structural system. We conclude that high gas hydrate content (30–40% of pore space or 20–26% of total volume) is restricted to the upper tens of meters below the seafloor near the summit of the structure, where vigorous fluid venting occurs. Elsewhere, the average gas hydrate content of the sediments in the gas hydrate stability zone is generally <2% of the pore space, although this estimate may increase by a factor of 2 when patchy zones of locally higher gas hydrate content are included in the calculation. These patchy zones are structurally and stratigraphically controlled, contain up to 20% hydrate in the pore space when averaged over zones ~10 m thick, and may occur in up to ~20% of the region imaged by 3D seismic data. This heterogeneous gas hydrate distribution is an important constraint on models of gas hydrate formation in marine sediments and the response of the sediments to tectonic and environmental change.

Hydrate content expressed as % of pore space from IR. Depth, sediment (m) is BSR depth (m). Hydrates from Delta T: Results are shown for integration over the gas hydrate occurrence zone (GHOZ), defined as the depth range over which indicators for gas hydrate were observed, and the gas hydrate stability zone (GHSZ), defined as the region from the seafloor to the BSR. At Site 1252, the projected depth of the BSR was used. For the summit region, the GHOZ and the GHSZ are the same because indicators of gas hydrate presence extend from the BSR to the seafloor. At these sites, the gas hydrate content of the shallow zone of massive hydrate is given separately. Hydrates from PCS: Each number represents the average of one to three samples spaced at unequal intervals. Each sample averages gas hydrate content over ~1 m. No PCS data were acquired in Holes 1245B, 124B, 1248C or 1252A. Hydrates from Cl-: These estimates represent averages of 15-30 samples/hole spaced at unequal intervals. Each sample averages gas hydrate content over 5-10 cm of core length. Uncertainties due to uncertainties in the baseline are estimated to be +/- 0.5%. Hydrates from RAB: Averages were calculated from pore water saturation estimates based on electrical resistivity data and tabulated at ~15-cm intervals and include hundreds of data points. These data require no correction for core recovery. No RAB data were acquired at Site 1252. Note that all RAB data are from the "A" hole at a given site, except from Site 1244, where they are from the “B” hole. Holes characterized by RAB data are ~50 m away from other holes at that site. All measurements in the GHOZ or GHSZ were included in the average, including data points indicating no gas hydrate

Supplement to: Tréhu, Anne M; Long, Philip E; Torres, Marta E; Bohrmann, Gerhard; Rack, Frank R; Collett, Tim S; Goldberg, D S; Milkov, Alexei V; Riedel, Michael; Schultheiss, P; Bangs, N L; Barr, Samantha R; Borowski, Walter S; Claypool, George E; Delwiche, Mark E; Dickens, Gerald Roy; Gràcia, Eulàlia; Guerin, Gilles; Holland, M; Johnson, J E; Lee, Young-Joo; Liu, C-S; Su, Xin; Teichert, Barbara M A; Tomaru, Hitoshi; Vanneste, M; Watanabe, Mahito; Weinberger, J L (2004): Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraints from ODP Leg 204. Earth and Planetary Science Letters, 222(3-4), 845-862