Seismic and Geochemical Evidence for Shallow Gas in Sediment on Navarin Continental Margin, Bering Sea

Abstract

Marine seismic studies coupled with geochemical investigations demonstrate that hydrocarbon gases are ubiquitous in the near-surface (<= 250 m or 820 ft depth) sediment of the Navarin continental margin in the northern Bering Sea. Three types of acoustic anomalies appear to be related to the presence of gas in the sediment. These anomalies are most prevalent in the northern half of the Navarin basin. Acoustic anomalies attributed to gas hydrates and to diagenetic boundaries are present on seismic records of the lower slope between Navarinsky and Zhemchug Canyons. Hydrocarbon gases, methane through butanes, are common in the surface (<= 5 m or 17 ft depth) sediment of the Navarin continental margin. Methane, the most abundant hydrocarbon gas, is present in amounts ranging from 84,000 to 1 µL/L of wet sediment. These concentrations are two to three orders of magnitude greater than the other hydrocarbon gases. The highest concentrations of methane (greater than 1,000 µL/L) were measured in sediment of Navarinsky Canyon and over the central part of the Navarin basin. The source of methane is mainly biogenic, but the hydrocarbon gas compositions in 17 of 141 cores suggest the presence of thermogenic gas. Most of these 17 cores are from the continental slope at water depths greater than 150 m (490 ft). No direct correlation could be found between acoustic anomalies and gas concentrations in the sediment. This lack of correlation is probably due to the limited penetration of the gravity corer and the spotty distribution of hydrocarbon concentrations.