Analyzing Geophysical Electric Logs and Historical Geochemical Data for Regressions from Deep Resistivity to Total Dissolved Solids to Characterize the Shallow Freshwater Aquifer in the Fruitvale Oil Field, Kern County, California, USA
Neil O. Seitz
Advisors: Dr. Gary Girty and Jan Gillespie
December 11, 2020
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A method is presented for analyzing the effectiveness of deriving groundwater total dissolved solids (TDS) from borehole geophysical resistivity as an aid to mapping TDS where historical groundwater geochemical data is sparse, but borehole geophysical logs are plentiful. Borehole resistivity (RO) data from petroleum wells are compared by linear and logarithmic regression with groundwater well TDS from the same zone to determine whether any empirical relationship exists between the two. While this method of TDS mapping has proved successful in other areas, complex geological settings such as the Fruitvale oil field in the San Joaquin Valley of California present a number of challenges. Comparing borehole resistivity data from petroleum wells with TDS from groundwater wells in the same zone results in a linear regression coefficient of determination (R2) of 0.05 and a logarithmic linear regression R2 of 0.12. To account for the relatively low coefficient of determination, groundwater geochemistry and controls on fluid migration within the study were analyzed. As bicarbonate waters dominate the study area, no relationship appears to exist between TDS and the ratio of bicarbonate and TDS. This results in the inability to correct the bicarbonate water type present to the Na-Cl water type assumed by the RO method of salinity estimation. The shallow freshwater aquifer in the study area was found to lack the broad hydraulically separated sand formations and fault planes that act as controls on fluid migration. Lack of these controls results in unhindered recharge from the Kern River in the shallow freshwater aquifer causing salinity values below the magnitude needed to achieve the strong correlation between RO and groundwater TDS. Complexities within the geological structure, stratigraphy, and geochemistry suggest that the RO method of salinity estimation is ill-suited for use in the Fruitvale oil.