Abstract

The remnant of a late Mesozoic-early Cenozoic fore-arc basin, the Sacramento Valley is principally a dry gas province. Complex stratigraphic relations, widespread deformation and faulting, and the presence of igneous intrusives testify to an active tectonic history. Sediments are organically immature down to an average of 10,000 ft (3,000 m) and are geopressured throughout much of the basin. Small fields with nitrogen concentrations as high as 88% are found along the shallow eastern margin.

Gases were sampled from 94 producing wells. Methane δ13C and δD values range from -61 to -15 ‰ and -200 to -96 ‰, respectively, indicating a wide range of source rock maturities. Wet gases in the western delta area (∑C2-C5/∑C1-C5 ≤ 12.5%) have ethane δ13C values between -31 and -20 ‰ and propane δ13C values between -27 and -12 ‰. Dry gases elsewhere in the basin (∑C2-C5/∑C1-C5 ≤ 1.6%) have ethane δ13C values as light as -58 ‰.

The chemical and stable isotope data are consistent with four principal sources. Gases in the western delta area were generated from mature source rocks within the oil and condensate window. Gases in the northern and southeastern basin are mixtures of indigenous gas (mostly microbial methane with small quantities of diagenetic ethane), dry thermogenic gas migrated from post-mature source rocks in the basin deep, and nitrogen-rich gases thought to originate in metasedimentary rocks deep beneath the basement.

These results place logical constraints on the location of productive traps with high-Btu gas and suggest caution in the interpretation of stable isotope data from other tectonically active basins due to dynamic migration and mixing processes.

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