Thermogenic gases in near-surface sediments of Norton Sound, Alaska
A plume of hydrocarbon gases, assumed to be of thermogenic origin based on chemical compositions, has been noted by others in the water column of Norton Sound about 40 km south of Nome, Alaska. We used detailed geophysical transects, side-scan sonar, underwater television, and chromatographic analyses of gases in near-surface sediments to define a probable source area of hydrocarbon gases at the southern apex of the water plume epicenter. Geophysical, geotechnical, and geochemical evidence together indicate that hydrocarbon gases of subsurface, thermogenic origin apparently migrate into the near surface sediments along a fault zone. Subbottom reflector terminations on continuous seismic profiles outline a large zone of anomalous acoustic responses about 9 km in diameter and at 100-m depth that may be caused by a subsurface accumulation of gas. Gas migration from the accumulation to the surface sediment is indicated by smaller zones of reflector terminations observed in high-resolution profiles and by seafloor craters seen on underwater television at one station. The presence of gas-charged surface sediment also is suggested by low percent pore water saturation, greater penetration of the penetrometer, and more rapid penetration of the vibracorer in sediment at the station with near-surface acoustic anomalies and seafloor craters. Analyses of hydrocarbon gases in sediments from a 1.6-m vibracore taken at this station showed unusually high concentrations of hydrocarbon gases heavier than methane; the ethane, propane, n-butane, and isobutane were 76, 4, 6, and 52 times greater than in other near-surface sediments in this region, and also significant quantities of gasoline-range hydrocarbons were present. The gas composition and presence in near-surface sediments above a thick underlying section with acoustic anomalies points to the possibility of petroleum at depth in this region. Our work indicates that surface and near-surface studies of the continental shelf may contribute significantly to petroleum-resource evaluations in addition to defining areas of potential hazard from gas-charged sediment.