Geologic framework of the 2005 Keathley Canyon gas hydrate research well, northern Gulf of Mexico

This paper is not subject to U.S. copyright. The definitive version was published in Marine and Petroleum Geology 25 (2008): 906-918, doi:10.1016/j.marpetgeo.2008.01.012.The Keathley Canyon sites drilled in 2005 by the Chevron Joint Industry Project are located along the southeastern edge of an intraslope minibasin (Casey basin) in the northern Gulf of Mexico at 1335 m water depth. Around the drill sites, a grid of 2D high-resolution multichannel seismic data designed to image depths down to at least 1000 m sub-bottom reveals 7 unconformities and disconformities that, with the seafloor, bound 7 identifiable seismic stratigraphic units. A major disconformity in the middle of the units stands out for its angular baselapping geometry. From these data, three episodes of sedimentary deposition and deformation are inferred. The oldest episode consists of fine-grained muds deposited during a period of relative stability in the basin (units e, f, and g). Both the BSR and inferred gas hydrate occur within these older units. The gas hydrate occurs in near-vertical fractures. A second episode (units c and d) involved large vertical displacements associated with infilling and ponding of sediment. This second interval corresponds to deposition of intercalated fine and coarse-grained material that was recovered in the drill hole that penetrated the thin edges of the regionally much thicker units. The final episode of deposition (units a and b) occurred during more subdued vertical motions. Hemipelagic drape (unit a) characterizes the modern seafloor. The present-day Casey basin is mostly filled. Its sill is part of a subsiding graben structure that is only 10–20 m shallower than the deepest point in the basin, indicating that gravity-driven transport would mostly bypass the basin. Contemporary faulting along the basin margins has selectively reactivated an older group of faults. The intercalated sand and mud deposits of units c and d are tentatively correlated with Late Pleistocene deposition derived from the western shelf-edge delta/depocenter of the Mississippi River, which was probably most active from 320 ka to 70 ka [Winker, C.D., Booth, J., 2000. Sedimentary dynamics of the salt-dominated continental slope, Gulf of Mexico: integration of observations from the seafloor, near-surface, and deep subsurface. In: Proceedings of the GCSSEPM Foundation 20th Annual Research Conference, Deep-water Reservoirs of the World, pp. 1059–1086]. The presence of sand within the gas hydrate stability zone (in units c and d) is not sufficient to concentrate gas hydrate even though dispersed gas hydrate occurs deeper in the fractured mud/clay-rich sections of units e and f.Partial support for the field and interpretive aspects of this project were provided by the Department of Energy, National Energy Technology Lab (NETL)

Sand wave fields beneath the Loop Current, Gulf of Mexico: reworking of fan sands

Extensive fields of large barchan-like sand waves and longitudinal sand ribbons have been mapped by deep-towed SeaMARC IA sidescan sonar on part of the middle and lower Mississippi Fan that lies in about 3200 m of water. The area is beneath the strongly flowing Loop Current. The bedforms have not been adequately sampled but probably consist of winnowed siliciclastic-foraminiferal sands. The size (about 200 m from wingtip to wingtip) and shape of the large barchans is consistent with a previously observed peak current speed of 30 cm/s, measured 25 m above the seabed. The types of small-scale bedforms and the scoured surfaces of chemical crusts, seen on nearby bottom photographs, indicate that near-bed currents in excess of 30 cm/s may sometimes occur. At the time of the survey the sand transport direction was to the northwest, in the opposite direction to the Loop Current but consistent with there being a deep boundary current along the foot of the Florida Escarpment. Some reworking of the underlying sandy turbidites and debris flow deposits is apparent on the sidescan sonar records. Reworking by deep-sea currents, resulting in erosion and in deposits characterised by coarsening upwards structures and cross-bedding, is a process that has been proposed for sand found in cores in shallower parts of the Gulf of Mexico. This process is more widespread than hitherto suppose