Abstract

Highlights

• 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.

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), NH_3, 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.