Site Summary – Green Canyon Block 955
The gas hydrates JIP site selection team identified numerous potential targets in Green Canyon block 955. Three of these sites were drilled in Leg II. The wells are located in over 6,500 ft of water near the foot of the Sigsbee Escarpment. The locations are near a major embayment into the Escarpment (“Green Canyon”) which has served as a persistent focal point for sediment delivery into the deep Gulf of Mexico.
Topographic map of the seafloor in the Green Canyon area. Block 955 lies just seaward of the Sigsbee Escarpment in ~6,500 feet of water
Green Canyon block 995 includes a prominent channel/levee complex that has transported and deposited large volumes of sandy sediment from the canyon to the deep Gulf of Mexico abyssal plain. The southwest corner of the block includes a recently developed structural high caused by deeper mobilization of salt. The crest of the structural high is cut by complex network of faults that can provide pathways for migrating fluids and gases. Geophysical data reviewed during assessment of the site revealed a complex array of geophysical responses near the inferred base of gas hydrate stability. Some of these responses are suggestive of free gas and some indicative of gas hydrate, but all are limited to depths that are near or below the inferred base of gas hydrate stability.
The Green Canyon site combines many of the features required for the formation of significant gas hydrate accumulations, including sources of gas and migration pathways (the faults) for that gas into the gas hydrate stability zone as well as porous sands within the stability zone in which gas hydrate can accumulate. A motivation behind the JIP’s selection of the site for Leg II drilling was to test the hypothesis that gas hydrate accumulations within sands at the base of gas hydrate stability restrict the vertical migration of gas into shallower units within the structure.
Three wells were drilled in Green Canyon from April 22 to April 28. The first (GC955-I) was drilled very close to a late-stage channel axis (to maximize the occurrence of sand reservoirs) in a location with muted geophysical indications of gas hydrate. This first well encountered more than 300 ft of porous sands as predicted; however the sands contained primarily water – with only modest indications of gas hydrate. The well also flowed water from these thick porous sand zones, requiring roughly a day of effort to control the flow.
The second well (GC955-H) was drilled about 1 mile southwest of the I-location in a structurally higher position on the domal structure. The well targeted strong geophysical anomalies with features suggestive of gas hydrate at a projected depth of 1,355 ft below the sea floor. Surprisingly, the logging-while-drilling data indicated the presence of a thick zone of gas hydrate-filled fractures in the mud-rich sediments from 500 to 1000 ft below the seafloor. At a depth of 1,230 ft, the sediments turned sandy, and at 1,305 ft the well encountered the top of a slightly more than 100-ft-thick gas hydrate-bearing sand in three zones of 88 ft., 13 ft, and 3 ft thick. Below the gas hydrate, the sediments that had been assessed as having a modest risk of free gas were found to be primarily water-bearing.
Based on the success of the “H” well, the JIP decided to drill the GC955-Q well further up on the structure into a different fault block that exhibited a thick sequence of high-amplitude geophysical responses. At a depth of 1,405 ft, gas hydrate-bearing sand was encountered to a depth of at least 1,458 ft (the deepest data point provided by the LWD tools) when drilling was halted at 1,498 ft below the seafloor when gas began to flow from the well. Well-control measures were quickly implemented using heavy muds. After stopping the flow, the logging-while-drilling assembly was removed, and the well re-entered and cemented.
The JIP's discovery of thick gas hydrate-bearing sands at GC955 validates the integrated geological and geophysical approach used in the pre-drill site selection, and provides increased confidence in assessment of gas hydrate volumes in the Gulf of Mexico. It is expected that further evaluation of the complex geology of these sites, including both fracture-filling and pore-filling gas hydrates in numerous fault blocks (with potentially varying geochemical conditions and gas hydrate/free gas configurations) will add significantly to the understanding of the nature and occurrence of gas hydrate-saturated sands in the marine environment
Map view of the top of the highly faulted buried channel (transparent) showing gas hydrate targets below and wells Q, H, and I.
Helix crew handling drill pipe on the rig floor of the Q4000