Complex Origin of Natural Gas in Sacramento Basin
Peter D. Jenden, Isaac R. Kaplan
Models for natural gas geochemistry allow us to distinguish biogenic and thermogenic sources, to estimate source rock maturity, and to evaluate mixing, migration, and possible association of gases with oil or condensate. These simple models appear adequate for old, tectonically stable basins such as the Permian and Anadarko and for younger basins bordering passive continental margins such as the Gulf Coast. However, study of 95 natural gases from the Sacramento basin suggests that the origin of gases in relatively young, tectonically active basins may be much more complicated.
The Sacramento basin produces dry gas from Upper Cretaceous-early Tertiary clastic sediments deposited in an active forearc environment. Virtually all fields produce from immature sediments (Ro <= 0.5%) with methane ^dgr13C values ranging from -62 to -15^pmil and C2 + concentrations averaging less than 2%. Most of the gas has probably migrated eastward from deeply buried shales and mudstones along the western margin of the basin. However, a few fields contain methane with ^dgr13C values suggesting an indigenous biogenic or early thermogenic origin. Calculations indicate that localized pyrolysis of shallow sediments by igneous intrusives could have produced large amounts of methane in the Colusa and Sutter Buttes areas. Gases with heavy me hane ^dgr13C values (> -25^pmil) are generally rich in nitrogen (up to 88%) and may be derived from metasedimentary rocks subducted beneath the basin. Complex geological history and multiple hydrocarbon sources combine to make the application of geochemistry a challenge in Sacramento basin exploration.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.