Characteristics and hydrocarbon seepage at the Challenger Knoll in the Sigsbee Basin, Gulf of Mexico

Active seafloor hydrocarbon seepage from three distinct, positive seafloor features, termed knolls, in the Sigsbee Knolls area, Gulf of Mexico (GOM), was investigated in March 2015. The study sites included the Challenger Knoll, which was drilled by the Deep Sea Drilling Program (DSDP) in 1969 and showed the influence of salt tectonics on its evolution and the presence of subsurface petroleum. This study used gas flare mapping in the water column, detailed seafloor bathymetry, backscatter mapping, sub-bottom profiling, and remote sensing of surface water to locate seafloor areas of hydrocarbon emissions. Flares of gas bubbles were detected in the water column above all three knolls, making them the deepest reported gas emission sites in the GOM. Although surface oil slicks above the Sigsbee Knolls have been detected by satellite imaging in the past; in this study, neither satellite imagery nor ship-based observations detected floating oil, this suggests that oil seepage in the region is intermittent. High-resolution video surveys of the seafloor at Challenger Knoll, using a camera system mounted on a lowered tow-vehicle, documented the presence of seep-related chemosynthetic fauna including clam fields and microbial mats at the western part of the knoll. These findings are consistent with previous investigations that link hydrocarbon seepage at Challenger Knoll to salt tectonics. During visual seafloor inspections, no asphalt deposits, such as those known from sites in the Campeche Knoll area in the southern GOM, were observed. These findings indicate that hydrocarbon seepage is less pronounced in the Sigsbee Knolls region, central GOM, than is the case in hydrocarbon systems in the northern or southern GOM.