Abstract

Two objectives of the State Water Resources Control Boards Regional Monitoring Program (RMP) are to determine the hydrogeologic relationships between oil and gas activities and protected groundwaters, and to determine whether or not there is evidence of fluid migration and water quality changes. From 2014 to 2017, the United States Geological Survey (USGS) compiled information on the study area, including available groundwater and produced water quality data, water and oil well construction and geophysical records, oil field development history and injection data, and hydrogeologic features including formation depth surfaces, faults, and vertical pressure gradient data. Using this information and the conceptual model for sampling site selection (https://ca.water.usgs.gov/projects/oil-gas-groundwater/science/design), samples from 27 existing groundwater wells were collected and analyzed for the full suite of RMP constituents quality conditions near the Lost Hills, North Belridge, and South Belridge oil and gas fields. About half of the sampled sites are monitoring wells and water-source wells that overlie active oil and gas production activities, and include wells also sampled for the SB4 local-area monitoring. Irrigation, monitoring, and industrial wells near the fields were sampled to characterize groundwater hydrologically upgradient (west) and downgradient (east) of the fields. Selected sites covered a broad range of depths (total well depths 220-1,170 feet) to characterize the different physical and chemical properties in the confined Tulare aquifer that makes up the primary groundwater aquifer in the area and the shallower unconfined to semi-confined alluvial aquifer. Results of the sampling were compared with produced-water chemistry (see abstract of McMahon et al.) to investigate multiple lines of geochemical evidence for the potential migration of oil-field fluids to groundwater aquifers.

Mixing between oil field fluids and overlying groundwater may be expected considering the proximity (both vertical and lateral) of sampled sites to hydrocarbon-bearing formations and oil and gas production activities, although the specific mechanisms explaining the presence of oil-field fluids in groundwater are generally difficult to discern given the complexity of the system. Geochemical characteristics often associated with oil fields including higher salinity, thermogenic methane and heavier hydrocarbon gases, and greater concentrations of dissolved soluble hydrocarbons and ammonium were observed in groundwater overlying the fields in comparison to adjacent to the fields. Other evidence of mixing that may indicate the migration of oil-field fluids from underlying formations included elevated chloride, bromide, and boron and heavy carbon isotopic values of dissolved inorganic carbon.

Some groundwater wells sampled adjacent to the South Belridge oil field had chemical characteristics consistent with the presence of fluids derived from historic disposal of produced waters in surface ponds. These areas are monitored through ongoing programs under the Central Valley Regional Water Quality Control Board. The following relative characteristics were identified for groundwater affected by surface disposal: heavy oxygen and hydrogen isotopic values of water and carbon isotopic values of dissolved inorganic carbon; high concentrations of chloride, bromide, boron, and dissolved organic carbon; increased nitrate or excess dissolved nitrogen gas, possibly associated with elevated ammonium in produced water in this area; distinctive methane isotopic values suggesting degradation of dissolved hydrocarbons; radium isotopic values suggesting disequilibrium with aquifer sediments; and noble gas values suggesting mixing with air-saturated water, which is consistent with atmospheric exposure in ponds. Groundwater upgradient of the oil fields and groundwater at some sites overlying and downgradient of the fields showed no evidence of the presence of oil-field fluids. As the density of sampling downgradient of the fields was relatively sparse due to limited existing well availability, additional monitoring of these areas in the future may yield additional insight. In particular, two multiple well monitoring sites are being installed as part of the RMP to fill gaps in information on vertical variations in ground-water quality and flow conditions through the full thickness of the aquifer system immediately east of the oilfield areas.