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

Study region: Santa Barbara County, California, USA.

Study focus: 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.

New hydrological insights for the region: 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.