Abstract
In California, a vast population and agricultural industry rely on groundwater for their main water supply source. Recent drought conditions have placed a higher demand on groundwater resources, and in order to preserve this resource for future use, groundwater must be managed carefully. In areas where oil fields are located, there is concern that petroleum extraction operations and produced wastewater disposal are negatively impacting groundwater that may qualify as protected. To protect these waters, it is essential to know their location and extent. To accomplish this, we develop maps that locate the distribution of groundwater containing less than 10,000 parts per million (ppm) total dissolved solids (TDS) since these waters are more likely to qualify as protected. To map groundwater TDS at the Midway-Sunset oil field, we use the Stephens et al., [2018] method which relies on produced water geochemical measurements, TDS calculated using the RP method, and ordinary kriging. However, unlike the areas mapped by Stephens et al., [2018], the Midway-Sunset oil field presents unique challenges to mapping groundwater TDS. At the Midway-Sunset oil field, decades of enhanced oil recovery operations have led to significant variations in the geothermal gradient, while detrital diatomite in the sediments causes geophysical porosity logs to overestimate total porosity. This is a problem because TDS calculations require resistivity, temperature, and porosity data. The following thesis addresses these challenges to mapping groundwater TDS at Midway-Sunset in several key ways. Thermal effects from enhanced oil recovery were removed by only considering a subset of wells that were unaffected by thermal processes while three different porosity models were developed and tested for different regions of the field in order to remedy the diatomite-porosity-problem. The resulting calculated TDS values were then combined with existing geochemical measurements of TDS and kriged to generate salinity maps and cross sections for the Midway-Sunset oil field. In the northern portion of the field, the TDS maps and cross sections reveal that the depth to the 10,000 ppm TDS boundary extends to greater than ~-1500 feet elevation, while in the southern region the boundary is much shallower, with a maximum depth of ~150 feet elevation. In the study area, stratigraphy, faults, and freshwater recharge appear to control the distribution of groundwater TDS.