Fluid levels in the Oxnard Oil Field, Ventura County, California

The U.S. Geological Survey (USGS), in cooperation with the California State Water Resources Control Board (SWRCB), compiled Fall 2017 fluid level elevation data from idle oil and gas wells in the Oxnard Oil Field to estimate vertical hydraulic head difference between oil production and overlying groundwater aquifer zones. Fluid elevations came from two sources, measurements in idle oil and gas wells and groundwater elevations in water wells in the overlying aquifer estimated at the points of idle well measurements using geographic information system (GIS) procedures. The fluid elevations from idle oil and gas wells were compiled by the California Geologic Energy Management Division (CalGEM) as part of their Idle Well Program; oil producers take the measurements and submit the data to CalGEM. These oil wells are perforated in the oil producing zones which includes the Vaca Tar Sands. Fluid elevations from the shallower groundwater system were extracted using GIS procedures at the locations of these idle oil and gas wells from a groundwater elevation contour map for Fall 2017 provided by the United Water Conservation District (UWCD). Groundwater elevation contours were calculated by UWCD from water-level measurements in groundwater wells monitored seasonally in the Oxnard Plain groundwater sub-basin and adjacent sub-basins. The groundwater elevation contours represent the lower aquifer system in the Oxnard Plain groundwater sub-basin and overlie the oil zone including Vaca Tar Sands. The fluid elevations in idle oil wells and calculated groundwater level elevations at the same location were compared to estimate vertical differences in groundwater head to assess potential fluid flow direction. Of the 65 idle well locations where vertical head differences were calculated, 43 had head differences indicating upward fluid gradients (head higher in oil wells than groundwater), 21 had head differences indicating downward fluid gradients (head lower in oil wells than groundwater), and 1 had head differences too small to discern vertical differences (within plus or minus 2 m). These data were analyzed in an accompanying manuscript as part of the SWRCB oil and gas Regional Monitoring Program and the USGS California Oil, Gas, and Groundwater (COGG) Program to assess regional groundwater quality overlying and adjacent to the Oxnard Oil Field.