AN INTEGRATED GEOLOGICAL, GEOPHYSICAL, AND GEOCHEMICAL ANALYSIS OF SUBSURFACE GAS HYDRATES IN THE NORTHERN GULF OF MEXICO

D. R. Hutchinson¹, C. D. Ruppel², J. Pohlman³, P. E. Hart⁴, B. Dugan¹, F. Snyder⁵, R. B. Coffin^3
1 USGS, 384 Woods Hole Rd, Woods Hole, MA, 02543
² School of Earth and Atmospheric Sciences, Georgia Tech, Atlanta, GA, 30332
³ Naval Research Lab, 4555 Overlook Ave. SW, Washington, DC, 20375
⁴ USGS, 345 Middlefield Road, Menlo Park, CA, 94025
⁵ Schlumberger Reservoir Services, 3600 Briarpark Drive, Houston, TX 77042

As commercial hydrocarbon exploration and production move into deep water in the Gulf of Mexico, drilling is occurring in regions where gas hydrates are theoretically stable. Whereas initial concern about drilling through the hydrate stability zone centered on environmental safety, borehole stability, and slope stability issues, recognizing that gas hydrates may be a viable future energy resource has also engendered industry interest for their economic potential. The upper slope of the northern Gulf of Mexico contains well-documented gas hydrate mounds and leaky hydrocarbon reservoirs underlie the entire margin. Despite these seemingly favorable conditions, there are very few observations of bottom simulating reflections (BSR) or other indicators of subsurface hydrate. To investigate this enigma, researchers from government, academia, and industry have developed a multidisciplinary strategy to study and eventually drill gas hydrates in the subsurface of the northern Gulf of Mexico.

During spring and summer, 2003, three data sets comprising 700 km of high-resolution multichannel seismic-reflection data, 13 heat flow penetrations (up to 6 m long), and 19 piston cores (4-6 m long) were collected around lease block Keathley Canyon 195 (KC195) in the northern Gulf of Mexico in about 1300 m water depth. The seismic data, combined with an existing prestack, migrated, 3D seismic cube provide a detailed image of the structural and stratigraphic setting around KC195. Nine sites have coincident thermal and geochemical measurements, which place constraints on the geologic and hydrologic near-surface processes. In addition, single-channel chirp reflection profiles were acquired along several of he heat flow and piston core sites.

A distinct, although weak, BSR near KC195 can be mapped on both the 3D and the higher resolution 2D profiles and therefore provides good evidence for gas hydrate occurrence near the base of the gas hydrate stability zone. The seismic data reveal that the BSR extends across about 15 km² of the eastern flank of a salt-withdrawal minibasin, crossing a well-layered and unconformity-rich stratigraphy that dips into the minibasin. The thickness of basin sediments above the BSR ranges from about 200 m to more than 450 m. Bright amplitudes adjacent to and beneath the BSR are interpreted to represent coarser sediments that contain free gas trapped beneath the hydrate stability zone. The eastern edge of the minibasin is bounded by a structural high created by salt diapirism. Numerous faults, disrupted reflections, amplitude blanking, sea-floor mounds and diffractive surfaces indicate intense deformation within this structural high. Pockets of flat-lying sediments also occur amidst the disrupted zones, indicating that deformation by salt tectonism is localized and heterogeneous. Geochemical measurements on sediments from 0-6 m below the sea floor indicate that methane in the sediments is biogenic (d13C less than 70 ‰ and C1/C2 ratios generally greater than 1000). Consistent with diapirism beneath the structural high, pore water salinities are close to sea-water values in the minibasin (36.6 ppt) and increase up to 50 ppt within the structural high.

The thermal gradients and geochemical measurements can be used to constrain the heterogeneity of the hydrologic system and fluid fluxes near KC195. The thermal gradient magnitudes and the base of the zone of sulfate depletion are spatially correlated. Within the minibasin and the undisturbed sediments of the structural high, thermal gradients are lower (30-40 mK/m) and the estimated depths to the base of the zone of sulfate depletion are greater (6.4-7.5 m) than within disturbed sediments of the structural high. The most highly disturbed sediments have the highest thermal gradients (40-50 mK/m) and shallowest depths to the base of the zone of sulfate depletion (3.5-4.5 m). Less disturbed sediments have intermediate gradients (31-38 mK/m) and inferred depths to the base of the zone of sulfate depletion (4.7-5.2 m). These data suggest that fluid, methane, and thermal flux is lowest in the minibasin and in undisturbed sediments and greater and more variable within the structural high, as might be expected if warm saline fluids are migrating up faults above the salt diapirs. Details about the geometry of pathways and flux magnitudes are not well understood. Preliminary modeling shows that gas hydrate formation is favored in the minibasin, and may not occur at all or only very near the seafloor in parts of the structural high because of higher temperatures and greater salinities.

This initial interpretation about basic differences between the minibasin and the structural high has complications. For example, thermal gradients in and near the minibasin, if projected to the depth of the BSR, give temperature estimates that would place the base of the gas hydrate stability zone below the BSR, i.e., they are generally several degrees too low for a methane hydrate system. Also, the location with the lowest thermal gradient (28 mK/m) has an unusually shallow depth to the zone of sulfate depletion (3.1 m). The models and interpretations developed in this multidisciplinary project are due to be tested when industry-coordinated drilling occurs, in 2004 or 2005.