Abstract

The Oxnard oil field is classified as high priority by the State Water Resources Control Boards Regional Monitoring Program (RMP) based on vertical proximity of aquifers and hydrocarbon reserves and the high volume of injection for water disposal. The Oxnard oil field is centrally located in the Oxnard coastal plain in Ventura County, CA, and contains two oil-bearing formationsthe Monterey Formation found at depths greater than 2000 feet (ft) below land surface (bls) and the Sespe Formation found at depths greater than 6000 ft bls. From 2017 to 2019, the United States Geological Survey (USGS) compiled information from the Oxnard oil field and surrounding area including historical groundwater quality data, water and oil well construction, oil field development history and injection data, and hydrogeologic features including formation depth surfaces and faults. This information was used to meet the two objectives of the RMP: 1) to determine the hydrogeologic relations between oil and gas activities and protected groundwaters, and 2) to determine whether there is evidence of fluid migration and water quality changes. In addition to the compiled information, samples from 14 groundwater wells were collected in 2017 within and surrounding the Oxnard oil field administrative boundary and analyzed for a full suite of RMP constituents. Sites were selected to characterize groundwater quality upgradient (northwest) and downgradient (southeast) of the oil field. Sampling sites included wells screened in the lower aquifer system, the upper aquifer system, and across the aquifers. Sampled well depths ranged from 155 to 1,500 ft bls and included monitoring, municipal supply, and irrigation wells.

The collected groundwater samples had no detections of petroleum hydrocarbons, inorganic constituents, isotopes, or other dissolved constituents that indicate mixing of oil field water in groundwater overlying the field has occurred. However, thermogenic methane, in some cases mixed with microbial methane, was detected in 3 wells with concentrations ranging from 0.25 mg/L to 9.1 mg/L. We used multiple lines of evidence including the analysis of methane isotopes of carbon (?13C-CH4) and hydrogen (?2H-CH4) and ratios of methane to heavier hydrocarbon gases to discern the source of methane as naturally occurring microbial methane, thermogenic methane associated with hydrocarbon reserves, or mixtures of these sources. The detected thermogenic gases occurred in deep groundwater wells, with the highest concentration associated with relatively high density of oil wells, large injection volumes of water disposal, and shallow oil development. Thermogenic gases detected in the absence of co-occurring oil field water may result from stray gas movement along preferential pathways such as through wells or boreholes or vertical migration through the formation. Further sampling of groundwater wells and oil well casing gases within the area of high-density oil development could help better distinguish pathways of thermogenic gases.

Deep formation brine signatures were detected in wells east of the oil field and had concentrations of minor ions, dissolved organic carbon, and hydrogen and carbon isotopic composition similar to historical data. Results of this study are consistent with findings in previous studies that detections of these constituents are related to groundwater supply wells being placed close to naturally occurring hydrocarbon-bearing formations east of the oil field rather than oil field activities within the administrative boundary. Other factors that may affect groundwater quality surrounding the Oxnard oil field include managed aquifer recharge to the north, seawater intrusion from the southwest, agricultural return flow, and upward movement of deep formation brines in response to groundwater development.