SUBSURFACE GAS HYDRATES IN THE NORTHERN GULF OF MEXICO

The northern Gulf of Mexico (GOM) is known to have both abundant thermogenic gas from depth and numerous gas hydrate mounds on the sea floor. These two phenomena suggest favorable conditions for the presence of gas hydrate. However, little geophysical or drilling evidence exists for its presence deeper in the sedimentary section. To investigate this apparent enigma, USGS collected approximately 1000 km of 2D high-resolution multichannel seismic reflection profiles in two sites in the GOM in spring, 2003, near lease blocks Keathley Canyon 195 (KC) and Atwater Valley 14 (AV). Both are in about 1300-m water depth and are potential targets for an industry-government-academia collaborative drilling effort in 2004. Results from the KC site have a Bottom Simulating Reflection (BSR) of limited areal extent on the structural high flanking a salt withdrawal basin. The BSR is identified both as a single reflection that cuts across stratigraphic units and as a series of high amplitude but discontinuous, isolated events within stratigraphic units that may image gas trapped beneath the hydrate stability zone. The region is structurally complex and is dissected by numerous faults, making BSR identification difficult. On the structural high, both high- and low-amplitude reflections are common, suggesting that either free gas or hydrate could be present. Data from the AV site, on the floor of the Mississippi Canyon, show no indication of a BSR, but sea-floor mounds and presumed gas vents are present. Preliminary interpretation indicates that reverse amplitude reflections exist in the shallow subsurface near the vents, although they are not bottom simulating. Localized zones of enhanced and reduced amplitudes also occur there. One of the biggest challenges in interpreting the seismic-reflection data will be separating the amplitude anomalies caused by the presence of free gas or of hydrate from lithologic changes caused by discontinuous and faulted units. For both sites, the areal extent of the amplitude anomalies in relation to the geologic framework offers the greatest promise for detecting the presence of hydrate. Integration with other geophysical (for example, thermal gradient) and chemical (e.g., salinity) data is essential for full characterization of both sites.