George E. Claypool¹, Leslie B. Magoon², Thomas D. Lorenson², Paul G. Lillis³, Isaac R. Kaplan⁴

(1) Consultant, Lakewood, CO
(2) U.S. Geological Survey, Menlo Park, CA
(3) U.S. Geological Survey, Denver, CO
(4) Global Geochemistry Corp, Canoga Park, CA

ABSTRACT: Natural Gas in the Great Valley of California -- Geochemical Characterization and Petroleum Systems

The east-trending (Stockton) Arch separates the Great Valley of California into the Sacramento basin (north) and the San Joaquin basin (south). Thermally generated natural gas originated from gas-prone Cretaceous source rocks in the Sacramento basin, and from Eocene and Miocene oil-prone source rocks in the southern San Joaquin basin. Microbial processes in shallow reservoir rocks also formed biogenic gas in both basins.

Gas accumulations in the Sacramento basin occur in the Dobbins-Forbes(?) gas system (2.3 tcf) and the overlying Winters-Domengine(?) gas system (6.9 tcf), separated by the Sacramento Shale, a regional seal. Dobbins-Forbes dry gas probably exsolved from formation water expelled from deeper, overpressured shale. Winters-Domengine wet gas migrated as a hydrocarbon phase from mature Cretaceous source rocks in the Delta Depocenter. Some dry gas fields in the Sacramento basin contain high-N2 gas and isotopically heavy CH4 generated from Franciscan metasediments. Tracy gas field in the southeastern Sacramento basin contains microbial methane

Gas in the San Joaquin basin occurs in three petroleum systems. Dry, mixed biogenic-thermogenic gas (175 bcf) of the Winters-Domengine(?) gas system extends southward from the Sacramento basin. Biogenic gas (700 bcf) occurs in shallow Plio-Pleistocene reservoirs. Some of shallow biogenic gas may be secondary due to oil biodegradation. Petroleum-associated wet gas (12 tcf, along with 8 billion barrels of oil) was generated from Eocene and Miocene source rocks in the Tejon depocenter of the southern San Joaquin basin.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado