Modern unconsolidated sediment overlying a diapir-like structure in the Eel River Basin, offshore northern California, contains an unusual mixture of gas and gasoline-range hydrocarbons. Although concentrations of methane in a 2-m gravity core at a single sampling locality (water depth of 500 m) are in the same range as found in modern anoxic sediments, the concentrations of higher molecular weight hydrocarbon gases are anomalously high relative to background. For example, ethane, propane, isobutane, and n-butane are about 18, 5, 10, and 2 times, respectively, more abundant than the highest concentrations of these same hydrocarbons observed elsewhere in the same region. Associated with these high gas concentrations are anomalous contents of gasoline-range hydrocarbons. Mu h of the methane and part of the ethane appear to be derived from modern microbial processes. Most of the ethane and the higher hydrocarbons, however, probably have a thermogenic origin deep within the sediment of the Eel River Basin. These hydrocarbons may reach the surface through fractures in the diapir-like structure and seep into overlying unconsolidated sediments that are ponded locally within structural and bathymetric depressions on the surface of this structure. Thus, the hydrocarbons in these near-surface sediments may be derived from petroleum that has formed and accumulated in Tertiary sedimentary rocks of the Eel River Basin.

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