The Sites

Locations of six sites evaluated as possible targets for JIP Leg II drilling. Background colors indicate depth to salt, with areas of shallow salt in yellow/orange and small circular areas of blue representing salt-withdrawal mini-basins. Areas in red indicate seismic indications of base of gas hydrate stability.

Walker Ridge 313 is characterized by an inferred ponded sand delivery system within the Terrebonne "mini-basin". Evidence for gas hydrates in the stratigraphic section include a series of anomalous, shingled seismic reflections indicative of stratal-bound free gas ("bright spots") that closely align with the predicted depth of gas hydrate stability. These anomalous features include both increased amplitude anomalies indicative of free gas below the base of gas hydrate stability (BGHS), as well as reversed polarity of those reflections above the BGHS. Low thermal gradients within the mini-basin place the Leg II drilling targets at about 2,700 feet below the seafloor. GC781/825 targets a geologic setting similar to WR313, but is located just landward of the Sigsbee Escarpment.

Green Canyon 955 features a well established and persistent sand delivery system through the Green Canyon channel at the Sigsbee Escarpment. Wells drilled previously in the lease block show significant sand development within the GHSZ and minor indications of gas hydrate. The primary target for Leg II is a large closed structure with abundant seismic evidence of gas and migration pathways into a suspected sand-rich lithofacies near the BGHS. Additional potential targets in GC955 include areas proximal to the primary paleo-channel-levee system.

East Breaks 992 wells will target anomalous seismic reflections that occur well above the interpreted BGHS within the Diana sub-Basin. These anomalies coincide with the top and base of a 130’-thick, resistive sand logged in an existing well . The Alaminos Canyon 21/65 sites provide additional targets in the Diana Basin within a large area of seismic anomalies analogous to those targets in the EB992 wells.

West-to-east seismic section through the EB992 area. Red line denotes location of an 1995 exploration well that encountered a 130’-thick resistive sand. The top and base of that sand, and the correlation of that sand to the seismic anomalies are noted. Data courtesy WesternGeco.

Each site evaluated for Leg II drilling has indications of (1) the existence of gas in reservoirs below the base of gas hydrate stability or visible in the shallow stratigraphic sections; (2) conduits for gas migration from potential sources and into the hydrate stability zone, through either faults or high-permeability lithologies like sands; and (3) the presence of coarse-grained sediments that could serve as reservoirs for high concentrations of gas hydrate.

In addition, the sites represent a variety of Gulf of Mexico geologic settings : if Leg II is able to drill its planned complement of sites, the expedition will provide a good test of current concepts for the occurrence of gas-hydrate-bearing sands in the Gulf of Mexico.

Diagrammatic depiction of sand depositional environments in the deepwater Gulf of Mexico (Courtesy G. St. C. Kendall). WR313, EB992, and AC21/25 target sands within the interiors of mini-basins formed on the Gulf of Mexico shelf by salt tectonics. GC955 focuses on leveed channels on the abyssal plain just beyond the Sigsbee Escarpment. The setting for GC781 is not as well defined, although the targets sit just landward of the Escarpment.