Abstract
Water co-produced with petroleum—termed produced water—is often disposed of through subsurface injection. Regulations require that the injected fluid stays in the target zone and does not migrate into usable groundwater aquifers. While impermeable rock provides the primary barrier to fluid migration, faults have also been asserted to act as barriers to fluid flow, often without evidence of their degree of seal. In previous studies, discontinuities in salinity, measured in ppm TDS (total dissolved solids) were used as a method to determine if a fault seals. Here, discontinuities in formation resistivity (Rt) were used as proxy for TDS discontinuities to examine the sealing ability of the Kern Front and West Premier Faults on the east side of Kern County, California. A publicly-available online database of borehole geophysical logs and historical well records from oil and gas wells, maintained by the California Geologic Energy Management Division (CalGEM), was utilized to determine fault throw patterns and investigate the sealing ability of the Kern Front and West Premier Faults. Historical well records from wells in Kern Front and Kern River and geophysical logs from wells within the Poso Creek field were used to map variations in fault throw. After fault throw patterns were determined, Rt data was collected from borehole geophysical logs from wells on either side of the fault. When a Rt measurement is taken in a 100% water saturated sand unit, free of any clay or hydrocarbon, it is denoted as Ro. A multiple linear regression model was used to test the statistical significance of the observed trends. Results show apparent Ro discontinuities across the Kern Front Fault, which vary in magnitude along-strike of the fault. A multiple linear regression analyses indicate the side of the fault in which the Ro measurement was taken was statistically significant and heavily impacted the prediction of log Ro (coefficient= 0.386, p=<0.001). Prediction of log Ro was also strongly dependent on an interaction term that measured the difference in Ro moving along strike of the fault (coefficient= -0.563, p=<0.001). The R-squared of this model was 0.38. Discontinuities in Ro suggest the Kern Front Fault creates a hydrologic seal in the Kern River Formation. Results from Poso Creek show only slight Ro trends near the West Premier Fault. These apparent trends suggest the West Premier fault does not create a lateral seal above the Macoma Claystone and groundwater can migrate across the fault plane. These observations were supported by a multiple linear regression model that showed no statistical difference in Ro from one side of the fault to the other Ro (coefficient= 0.0015, p=0.997). The R-squared of this model was 0.210.