Structural Controls on Hydrate Distribution and Morphology at Hydrate Ridge, Oregon

Analysis of resistivity-at-the-bit (RAB) images from nine sites drilled during Ocean Drilling Program (ODP) leg 204 to southern Hydrate Ridge reveals a complex pattern of fracture orientations that correlate with the structural location of the borehole on the ridge. Sites 1244 and1245 on the eastern and western flanks of the ridge, respectively, have concentrated zones of resistive fractures within the gas hydrate stability zone that exhibit a preferred orientation orthogonal to the regional direction of plate convergence and crustal shortening. The mean orientation of the identified fracture planes at Site 1244 strikes N10W and dips 40° E, and the mean fracture plane at Site 1245 strikes N15W and dips 33° E. Higher on the eastern ridge flank, at site 1246, fracture planes exhibit a transitional behavior from clustering on the flanks to a less well organized pattern at the ridge crest. Poles to structural planes at the ridge crest sites (1247-1250) are evenly distributed, though their typically shallow dips (i.e. steeply dipping planes > 30° ) create a girdle pattern in stereographic projections. The steeply dipping planes at the ridge crest occur in zones of chaotic resistivity in the RAB images and correlate to zones of massive hydrate in the cores. Simple planar relationships can be obscured in these zones by hydrate growth patterns and morphology. Bedding plane dips were also measured and are consistently sub horizontal at all sites. Little change in bedding dip occurs with depth at sites on the ridge crest and flanks, though the basin site shows bedding dips increase below ∼ 200 mbsf. As expected, few fracture planes were documented in the basin site images where hemipelagic sedimentation dominates. Results of the RAB image analyses indicate that the orientation and morphology of hydrate filled fractures is controlled by structural position on the ridge. At the ridge crest, undergoing extension, migrating aqueous fluid and free gas have the potential to hydrofracture the sediment creating a network of steeply dipping, but randomly oriented fractures with massive hydrate accumulations. Along the ridge flanks where fluid flow is controlled primarily by lithology, hydrate filled fractures maintain a preferred orientation that parallels that of the uplifted sedimentary section and is oriented perpendicular to the regional maximum compressive stress.