Abstract
With the recent drought in California, a greater demand has been placed on groundwater resources. Publicly-available data indicate that produced water from oil and gas extraction in California is commonly being injected into the subsurface for disposal at relatively shallow depths in some oil fields. This may conflict with the U.S. Environmental Protection Agency (EPA) Safe Drinking Water Act and California Senate Bill 4. These measures protect groundwater containing < 10,000 ppm total dissolved solids (TDS), also defined as underground sources of drinking water (USDW). Groundwater TDS maps are needed to identify and protect the USDW because there is no current survey of the USDW boundary. Direct measurements of groundwater TDS are not common, which necessitates using alternative methods to quantify TDS. This study uses the resistivity-porosity (RP) method to calculate TDS from resistivity, porosity, and temperature measurements recorded in borehole geophysical data. We use existing measured geochemical data to guide proper parameterization of the petrophysical equations by examining comparisons with the ground truth data. The RP method is only accurate within clean, wet sand intervals. Hydrocarbons and shale, if present, distort the TDS calculations, rendering inaccurate results. Due to the large number of wells needing to be analyzed, an algorithm was coded in Python to process digitized geophysical well data to identify clean sand intervals where TDS calculations are more accurate. The RP-derived TDS data were fed into a 3D geostatistical kriging model to create an interpolated volume model of TDS. The model is validated by comparing the predicted TDS values to the measured TDS data. The average error is 11%, with a correlation of R2 = 0.97. The model was used to visualize a cross section throughout the Fruitvale and Rosedale Ranch oil fields in the San Joaquin Basin of California. Groundwater TDS in the area varies significantly. The 10,000 ppm TDS boundary is at ~3,500 ft in Rosedale Ranch and deepens to the southeast in Fruitvale to ~4,500 ft. According to the data, several factors including depth, stratigraphy, faults, and fresh water recharge control the TDS distribution. Within the Rosedale Ranch area, Miocene normal faults and a low-permeable clay layer isolates aquifers from fresh water recharge from the Kern River. Stratigraphic control seems to dominate the TDS structure in the area between the fields. The groundwater in the Fruitvale area is flushed by recharge from the Kern River. This study provides a realistic and effective approach to quantify formation water TDS with available data as well as a better understanding of the controls on groundwater TDS, which can assist future mapping efforts and lead to better information for water resources managers.