Thermogenic hydrocarbons in unconsolidated sediment of Eel river basin, offshore northern California

Thermally produced hydrocarbons were recovered from unconsolidated sediment ponded within a bathymetric depression on the surface of a shale diapir in the offshore Eel River Basin of northern California. Evidence that the hydrocarbons are thermogenic consists of the following: (1) very high concentrations of hydrocarbon gases, particularly ethane through butanes (C₂-C₄); (2) methane having a carbon isotopic composition (relative to the PDB standard) of −43 and −44 per mil; (3) presence of gasoline-range (C₅₊) hydrocarbons; and (4) presence of a complex mixture of heavy hydrocarbons (C₁₅₊) with n-alkanes having a petroleumlike distribution. This mixture of gaseous and liquid hydrocarbons likely originated deep within the basin and migrated to the surface through fractures and faults developed during the emplacement of the diapir. The presence of thermogenic hydrocarbons in unconsolidated surface sediment indicates that conditions for petroleum generation have existed within this offshore basin.