Petroleum Systems of the Sacramento Basin and Adjacent Area, California

Magoon, Leslie B.¹, Allegra Hosford Scheirer², Paul G. Lillis³ (1) U.S. Geological Survey, Menlo Park, CA (2) U.S. Geological Survey, Menlo Park, (3) U.S. Geological Survey, Denver, CO

The Sacramento basin and adjacent area contains oil and gas fields and surface seeps that represent four natural gas systems and two oil systems. The four gas systems were identified and mapped on the basis of geology and stable carbon isotopic composition of methane. Two of the gas systems are commercially insignificant. These include a one-field microbial gas system, the Tracy field (17 BCFG) whose composition averages -61.3 per mil. The Franciscan Complex gas system includes four small gas fields – Freeport (3 BCFG), Poppy Ridge (0.02 BCFG), Robbins (31 BCFG), and Nicolaus – in which the gas has a range of isotopic compositions from -22.3 to -15.2 per mil and is presumed to be derived from Franciscan metasediments. The remainder of the gas seeps and fields in the Sacramento Valley constitute the two commercially significant thermogenic gas systems which are stratigraphically separated by the Sacramento shale. The Dobbins-Forbes(?) gas system (2,169 BCFG) below the Sacramento shale includes nitrogen and methane with ?13C methane values of -28 +/-5 per mil, and the Winters-Domengine(?) gas system (6,890 BCFG) above the Sacramento shale contains nitrogen and ?13C methane of -40 +/- 5 per mil and condensate with a ?13C of saturated hydrocarbons of -26.3 +/-0.5 per mil.

Both oil systems were identified on the basis of stable carbon isotope geochemistry of oil samples. One system includes the black oil in the Brentwood, Livermore and other gas fields which is derived from the Eocene Nortonville shale in the vicinity of Mount Diablo. Numerous black oil samples from seeps and accumulations from the Bunker gas field on the south to the Culver Ranch seep on the north are all derived from an Upper Cretaceous source rock in the Coast Range. Though small, these oil systems represent exploration potential.