Abstract

A component of the California State Water Resources Control Board’s Regional Monitoring Program (RMP) of Water Quality in Areas of Oil and Gas Production is to characterize and build a database of oil-field source fluid chemistry for comparison with groundwater chemistry. The chemistry of oil field waters is highly variable, with natural differences between field, pool, geologic formation, and in response to field management practices such as enhanced oil recovery (EOR). Characterizing variability in oil-field fluid chemistry is necessary to better understand chemical differences and/or similarities between produced water and regional groundwater in complex crustal environments.

The Lost Hills and Fruitvale oil fields were identified as high-priority study areas for the RMP due to their proximity to protected groundwater resources, density of oil wells, and volumes of injection.During 2016-17, the United States Geological Survey (USGS) compiled and analyzed data from the Lost Hills and Fruitvale study areas, including available produced water quality data, oil well construction and geophysical records, oil field development history and injection data, and hydrogeologic features including formation depths and fault locations.

A campaign of oil- field fluid sampling was designed to characterize the predominant oil-producing area and formation combinations, as well as fluid injected into the oil reservoirs. USGS staff coordinated with operators to identify active sites meeting the selection criteria and to obtain permission for sampling.Samples were collected for an extensive array of analytes dissolved in produced water. Some analyses are not yet complete and will be described in future presentations. The sampling also included collection and analysis of the noble gas isotope and relative abundance signature of natural gas that accumulates in the casing of oil wells.Casing gas samples were collected from eight oil wells in the Lost Hills field and two injectate sites in the Fruitvale oil field.

Lost Hills casing gas data show a noble gas signature diagnostic of injected fluids is present in oil wells completed in zones where extensive EOR is used. In contrast, wells completed in zones where EOR is not used retain more pristine noble gas signatures. Published studies in some natural gas regions have identified that noble gases are useful tracers of fugitive natural gases detected in groundwater.However, use of noble gas signatures as tracers in oil dominated systems is not yet well developed and requires precise characterization and comparison with the full suite of produced water and groundwater chemistry data. The ability to differentiate noble gas signatures from oil zones affected by EOR practices compared with conventional extraction may be useful in understanding the origin of fugitive gases, if any, in adjacent aquifers in the future.