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Abstract: Natural Gas in the Great Valley of California--Characterization, Origin and Petroleum Systems

CLAYPOOL, GEORGE, Consultant, Lakewood, CA; LESLIE MAGOON, U.S. Geological Survey, Menlo Park, CA; THOMAS LORENSON, U.S. Geological Survey, Menlo Park, CA; ZENON VALIN, U.S. Geological Survey, Menlo Park, CA; AUGUSTA WARDEN, U.S. Geological Survey, Denver, CA; PAUL LILLIS, U.S. Geological Survey, Denver, CO; ISAAC KAPLAN, Global Geochemistry Corp, Canoga Park, CA

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. Some dry gas fields in the Sacramento basin contain admixed, high-N2 gas with isotopically heavy CH4 generated from underlying Franciscan metasediments. Tracy gas field in the southeastern Sacramento basin contains microbial methane.

Gas accumulations in the San Joaquin basin occur in three petroleum systems. Dry thermogenic gas (175 bcf) of the Winters-Domengine(?) gas system extends southward from the Sacramento basin. Dry biogenic gas (700bcf) occurs in shallow Plio-Pleistocene reservoirs, some of which overlie deeper oil and wet gas accumulations. 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 #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California