Geophysical and Geochemical Evidence for Subsurface Hydrocarbon Leakage in Southern Bering Sea, Alaska

Michael A. Abrams

Geophysical and geochemical surveys were conducted in the southern Bering Sea to investigate whether hydrocarbons found in the surficial sediments, particularly in association with certain geophysical events, indicate the presence and nature of more deeply reservoired oil and gas deposits. Surface sediment samples were obtained at 1,000 sites where high-resolution seismic data indicated the presence of gas, active seepage, and/or surface faulting. In addition, sites for background measurements were chosen away from areas of gas seepage. Sediment samples were frozen to inhibit bacterial action and sent to Houston for analysis. Each sample was examined for free light hydrocarbons, adsorbed light hydrocarbons, liquid hydrocarbons, nonhydrocarbon gas (carbon dioxide, nitrogen and oxygen), and the isotopic composition of methane and carbon dioxide.

Results of the geophysical survey showed seismic wipeout zones present throughout the southern Bering Sea. Sediment samples obtained within the shallow wipeout zones confirmed these features are caused by gas. In addition, compositional and isotopic analyses indicated the gas is of both biogenic and thermogenic origin. The thermogenic leakage appears to be related to localized faulting and shallow structuring. Therefore, the high-resolution seismic data provided an excellent tool for locating leakage points, thereby enhancing the opportunity for finding migrated thermogenic hydrocarbons in surficial sediments.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.