Biogenic Origin of Pliocene Dry Gas, Southern San Joaquin Basin, California

Marc Kamerling, R. M. Lewy, Lee Lundell

Known ultimate reserves of 780 billion ft³ of dry gas occur in the Pliocene Etchegoin Formation and the overlying San Joaquin Formation of the southern San Joaquin basin. These formations represent the filling in of the San Joaquin basin. The Etchegoin is marine to shallow marine, and the San Joaquin is shallow marine to brackish water to nonmarine. The San Joaquin Formation accounts for 693 billion ft³ or 88% of the total dry gas ultimate reserves. Some of the Pliocene dry gas zones occur in association with oil fields, and others do not.

The source of the dry gas could be the result of either biogenic or thermogenic processes. Biogenic gas is generated by bacteria in the sediment and is characterized by greater than 97% methane and light carbon isotopic ratios. Thermogenically generated natural gas typically has higher contents of other hydrocarbon gases and heavier carbon isotopic ratios. Composition of the Pliocene dry gas is typically greater than 98% of the total hydrocarbon fraction. Carbon isotopic analysis of samples of Pliocene dry gas yield ^dgr ¹³C ratios in the range of -60 to -70 ^pmil. Biogenic methane gas has ^dgr ¹³C ratios in the range of -55 to -80 ^pmil. These data indicate a biogenic origin of the Pliocene dry gas. The biogenic origin of the Pliocene dry gas is consistent with he paleoenvironment of the Pliocene section and suggests that gas zones may occur in areas not associated with oil fields.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.