Occurrence of water and thermogenic gas from oil-bearing formations in groundwater near the Orcutt Oil Field, California, USA

• Published in: Journal of hydrology. Regional studies 2022-06, Vol.41, p.101065, Article 101065
• Main Authors: Anders, Robert, Landon, Matthew K., McMahon, Peter B., Kulongoski, Justin T., Hunt, Andrew G., Davis, Tracy A.
• Format: Article
• Language: English
• Subjects: anthropogenic activities; California; groundwater; Groundwater quality near oil fields; infrastructure; Multiple lines of evidence; oil fields; oils; salinity; Salinity tracers; Thermogenic hydrocarbon gas
• ISSN: 2214-5818; 2214-5818
• DOI: 10.1016/j.ejrh.2022.101065

Santa Barbara County, California, USA. To analyze a wide array of newly collected chemical, isotopic, dissolved gas, and age dating tracers in conjunction with historical data from groundwater and oil wells to determine if water and/or thermogenic gas from oil-bearing formations have mixed with groundwater in the Orcutt Oil Field and surrounding area. Three of 15 groundwater samples had compositions indicating potential mixing with water and/or thermogenic gas from oil-bearing formations. Relevant indicators included salinity tracers (TDS, Cl, Br), NH3, DOC, enriched δ13C-DIC, δ2H-CH4, δ13C-CH4, and δ13C-C2H6 values, and trace amounts of C3-C5 gas. The potential sources/pathways for oil-bearing formation water and/or thermogenic gas in groundwater overlying and adjacent to the Orcutt Oil Field include: (1) upward movement from formations developed for oil production due to: (a) natural migration; or (b) anthropogenic activity such as injection and/or movement along wellbores; and (2) oil and gas shows in overlying non-producing oil-bearing formations. Groundwater age tracers, elevated 4He concentrations, and isotopic compositions of noble gases indicated legacy produced water ponds were not a source. This phase of the study relied on samples and data from existing infrastructure. Additional data on potential end-member compositions from new and existing wells and assessments of potential vertical head gradients and pathways between oil and groundwater zones may yield additional insight. •Water and gas from oil-bearing formations mixed with groundwater in some samples.•Mixing identified using analytes with little or no historical data.•Sources/pathways could include oil-development and natural geologic processes.•Microbial methane is common in groundwater near the oil field.