Distinguishing between Natural Crude Oil Seepage and Anthropogenic Petroleum Hydrocarbons in Soils at a Crude Oil Processing Facility, Coastal California

Abstract

Crude oil from offshore deposits in the Miocene Monterey Formation is commonly processed at facilities along the California coast. This formation is known for natural crude oil seepage (NCS), manifested at a California oil and gas processing facility (the site) as small pools on the ground surface, discharge from an adjacent bluff, and as free product in a hand-dug well. In addition, hydrocarbons are also present in site soils due to historical leaks at the facility, referred to here as anthropogenic petroleum hydrocarbons (APH). An investigation using gas chromatography/mass spectrometry (GC/MS) and isotopic techniques compared the chemical signatures of NCS and APH in soil to determine if it was possible to separate the populations. The NCS samples have a well-developed unresolved complex mixture (UCM) compared to the APH samples, yet lighter polycyclic aromatic hydrocarbons (PAHs) in the NCS are more numerous and in higher concentrations than in the APH samples, possibly indicating a different degradation trajectory. Other useful discriminatory parameters appear to be the greater abundance of: tricyclic terpanes (in NCS), C₂₇-diasteranes (in NCS), C₂₈-ergosteranes (in APH), C₂₇- and C₂₉-monoaromatic steranes (in NCS), and C₂₀- and C₂₁-triaromatic steranes (in NCS). Other parameters, e.g., δ¹³C and δ D, had no apparent discriminatory power. This study demonstrates that NCS appears chemically distinct from APH at this site. However, it would be necessary to obtain empirical evidence for other sites with similar concerns due to vagaries in facility-specific sources, degradation rates, etc.

Keywords:

• hydrocarbon
• fingerprinting
• oil seeps
• gas chromatography
• mass spectrometry

Acknowledgments

The authors are grateful to Mike Mohr of Chevron for providing nomenclature for the hydrocarbon product train used at the facility over the period of interest. This study was sponsored by Chevron Environmental Management Company. The authors appreciate the comments of three anonymous reviewers that resulted in significant improvements in the manuscript, and Ioana Petrisor for her editorial handling.