Variety of Seismic Expression of the Base of Gas Hydrate Stability in the Gulf of Mexico, USA
The base of gas hydrate stability (BGHS) is often manifested on seismic data as a water bottom simulating reflection (“BSR”) or other seismic features that reflect the uppermost common occurrence of free gas. While geophysical indicators of the BGHS have been widely recognized around the globe, they are commonly considered to be relatively rare in the Gulf of Mexico (GOM). A recent subsurface investigation led by the U.S. Minerals Management Service (MMS) using proprietary 3D seismic data has yielded an inventory of over 100 locations where geophysical indicators of BGHS exist in the GOM. This study, which is still underway, covers roughly 200,000 km2 and represents perhaps the most complete regional characterization of “BSRs” and related features anywhere on Earth. The mapping is being done in part to support the MMS’s gas hydrate resource assessment of the GOM, as well as to support the site selection process of the Department of Energy’s gas hydrate Joint Industry Project drilling program. As a result of these investigations, we propose a consistent terminology to describe the three end-member categories of seismic expressions of the BGHS that co-exist in the GOM: Continuous “BSRs”, Segmented “BSRs”, High-relief “BSRs”. Features that combine these elements are also observed. Numerous Continuous “BSRs” are observed in the Gulf, however they are often limited in area, as compared to settings (such as the Blake Ridge) where laterally and vertically homogeneous stratigraphy contributes to better developed “BSRs”. Segmented “BSRs” are not unique reflectors, but instead appear as a series of separated bright spots (often associated with wavelet phase reversals) that, when connected, mimic the geometry of the seafloor. As compared to the Continuous “BSRs”, these localized enhanced amplitudes are interpreted to occur where the BGHS intersects sand units within a heterogeneous stratigraphic section (interbedded sands and shales). The High-relief “BSR” includes both continuous and discontinuous features that rise close to the seafloor in the shape of a cone or plume and is interpreted to mark the BGHS that is not bottom-simulating. The high relief nature of this event is attributed mostly to lateral variations in the thermal regime, which are common in the GOM where shallow diapiric and tabular salt bodies are prevalent. Individual occurrences of these three “BSRs” are on the order of tens to over hundred's of square kilometers.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009