Abstract
Underground water injection, whether for enhanced oil recovery or disposal of produced water, is a common practice in California oil fields. In regulatory filings, it is often asserted that faults prevent these injected fluids from causing fluid migration into nearby protected water (<10,000 ppm total dissolved solids). Fault zones are often thought to be continuous planar features; however, offset along a fault can vary based on the stage of evolution of the fault, distance from the fault tip, and fault segmentation, leading to variations in the sealing nature of the fault. How this inherent variability in faults influences communication between zones where oil and gas development is taking place and zones where groundwater resources occur is poorly understood. We explore the influence of along-strike variations in fault offset and geometry and the impacts on fluid movement near the Premier Fault in the Poso Creek oil field on the east side of California's San Joaquin Valley. This fault, which has been previously mapped as a single-stranded normal fault, plays a key role in trapping petroleum; however, its role in controlling the migration of groundwater is poorly understood. Stratigraphic data from over 1,300 borehole geophysical logs from within 200 m of the previously-mapped fault zone were used to create a high-resolution structural contour map. Several different interpretations of fault geometry were then compared to borehole resistivity measurements across the fault to assess the sealing nature of the fault with respect to groundwater chemistry. We found that the offset along the Premier Fault varies along strike, decreasing from north to south in the study area. It is not yet clear if this is due to a decrease in fault displacement toward the fault tip, step-over from one fault segment to another, or a broken relay ramp. Borehole resistivity measurements across the fault will be presented to help test the proposed fault geometries, and the nature of fault offset on the sealing nature of the fault to groundwater migration.