USING BOREHOLE GEOPHYSICAL LOGS TO MAP GROUNDWATER TOTAL DISSOLVED SOLID PATTERNS IN THE CAT CANYON OIL FIELD, SANTA BARBARA COUNTY, CALIFORNIA

Water and steam injection in the Cat Canyon Oil Field, located in northern Santa Barbara County, California, has led to an interest in the potential effects of oil and gas production on nearby groundwater resources. However, the distribution of potentially useable groundwater in and around the Cat Canyon Oil Field, defined as having less than 10,000 milligrams per liter (mg/L) total dissolved solids (TDS), is poorly characterized. To address this knowledge gap, groundwater TDS is mapped from near the ground surface to the top of the oil-producing reservoir (~600 meters) using geophysical and geochemical methods, then compared to an existing structural and newly constructed stratigraphic model to determine possible geologic controls on TDS. TDS was mapped in three dimensions (3D) using produced-water sample geochemistry data and Archie-based methods for analyzing resistivity, porosity, and temperature data from borehole geophysical logs. Borehole resistivity was read from geophysical logs in clean, water-saturated sands. Depth-dependent, linear porosity and temperature models were constructed to make predictions at the location of collected resistivity when not available directly from geophysical logs. Calculated TDS and produced-water sample data were then interpolated into a 3D volume using a Gaussian process. The results of the 3D TDS volume were then compared to existing structure data and a new stratigraphic model that we constructed from geophysical logs and driller's notes submitted to regulatory agencies. Specifically, we mapped the main oil-producing reservoirs, the Monterey and Sisquoc Formations, and the overlying aquifers, the Foxen, Careaga, and Paso Robles Formations. Preliminary results show TDS ranges from <1,000 to ~30,000 mg/L throughout the area with lateral and vertical variations. TDS varies vertically within the oil field, increasing with depth to the Sisquoc Formation, then decreasing with depth in the underlying Monterey Formation, outlining a TDS reversal. At most depths, TDS generally increases laterally within the field from the southeast to the northwest, the general direction of regional structural dip. Initial results suggest that the distribution of usable groundwater within the field generally extends to the Foxen Formation.