Abstract

The California State Water Resources Control Board is collaborating with the U.S. Geological Survey (USGS) to determine the hydrogeologic relationships between oil and gas activities and protected groundwaters, and to determine whether there is evidence of fluid migration and water quality changes. This work is part of the Regional Monitoring Program (RMP) authorized by California Senate Bill 4 of 2013. The Orcutt oil field, one of several large oil fields in Santa Barbara County, was identified as a high priority study area for initial sampling because relatively large volumes of fluid have been injected into the ground for enhanced oil recovery or water disposal. From 2017 to 2019, the USGS compiled information in 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 geometry, groundwater flow directions, and faults. This information and new data from sampling sites adjacent to and overlying the Orcutt oil field were used to characterize the different physical and chemical properties in the Paso Robles and Careaga Formations that make up the primary groundwater aquifers in the area and the shallower unconfined to semi-confined alluvial aquifers. A total of 16 groundwater samples were collected from seven domestic, six irrigation, and three monitoring wells which covered a broad range of depths (total well depths 185-980 feet) between September 2017 and July 2018. The 16 groundwater samples, along with produced-water samples collected from five oil wells and one injection site, were analyzed for the full suite of RMP constituents including organic and inorganic chemicals associated with oil and gas production activities and isotopic tracers that are used for determining the origin of dissolved organic chemicals, salts, and gases detected in protected groundwater. Results of the sampling were compared with produced-water chemistry to investigate multiple lines of geochemical evidence for the potential migration of oil-field fluids to groundwater aquifers.

Mixing between oil-field fluids and groundwater was evident in four of 16 wells sampled by the RMP. One well adjacent to the Orcutt oil field appeared to contain groundwater mixed with produced water based on concentrations of total dissolved solids (TDS), chloride, and dissolved organic carbon, minor-ion ratios, and the detection of 5 dissolved petroleum hydrocarbons.The same site, however, also appears to be affected by an industrial source of chemicals based on detection of 28 manufactured volatile organic compounds (VOCs) not associated with oil field sources.More conclusive interpretations of sources and pathways at this well cannot be performed at this time because not all laboratory analytical data for this sample have been received.A second well adjacent to the oil field appeared to contain groundwater mixed with produced water from historically documented surface disposal ponds based on concentrations of TDS, chloride, ammonia, and dissolved organic carbon, minor-ion ratios, and enriched carbon isotopic values of dissolved inorganic carbon. At two wells overlying the Orcutt oil field, methane concentrations and isotopic values in combination with trace amounts of light hydrocarbon gases (ethane, propane, and isobutane) indicate the presence of thermogenic or mixed microbial/ thermogenic methane gas and elevated concentrations of TDS and chloride, and minor- ion ratios indicate the presence of produced water. Potential pathways explaining the detections at these two wells could include upward movement of oil- field fluids and gas through formations, along leaky wells/wellbores, or faults.

Other sites and historic data showed no evidence of oil- field fluids present in groundwater but do exhibit patterns consistent with recharge from widespread irrigated agricultural land and/ or natural rock/ water interactions. One factor limiting this study was the lack of wells available for sampling groundwater overlying, and in some areas adjacent to, the Orcutt oil field; new wells and additional monitoring of these areas in the future may yield additional insight.