California Oil, Gas, and Groundwater Program
Publication: Conference and Public Meeting Abstracts or Presentations
As part of the California State Water Resources Control Boards Regional Monitoring Program of Water Quality in Areas of Oil and Gas Production (RMP), formation and clay layer mapping, sequential pressure tests, geophysical log analysis, and groundwater sample lab analyses were used to determine: 1) the presence and extent of confining layers, 2) groundwater salinity trends with depth, 3) depth to the base of protected groundwater (TDS <10,000 mg/l) and 4) the impact of oilfield activities on groundwater salinity within the Tulare Formation in the Lost Hills and Belridge oil fields and adjacent areas.
The Tulare Formation is erosionally thinned to about 250 feet where it crops out on the northern Lost Hills and North Belridge anticlines. It thickens to as much as 4,000 feet to the south and east. Sequential pressure tests indicate the presence of contrasts in fluid pressure locally across confining layers within the formation, suggesting these clay layers restrict groundwater flow, but these patterns are not observed at all sites.
Calculation of formation water salinity using borehole geophysical logs indicates that the base of protected groundwater usually occurs within the Tulare Formation except for a small portion of north Lost Hills where the Etchegoin Formation produces water less than 10,000 mg/l along with oil at very shallow depths. The depth to the base of protected groundwater is approximately 500 feet in the northwest part of the area. It increases to 1,600 feet to the south and drops off sharply east of the Lost Hills anticline to a depth of about 1,700 feet within one mile of the administrative boundary of the field. Where log analysis is not feasible (i.e. in oil-bearing zones within the field boundaries), chemical lab analysis of produced water was used to help determine salinity trends with depth.
Several generations of produced water disposal wells were drilled in the area. Geophysical logs collected at the time of drilling of older wells (pre-1985) show a linear increase in salinity with depth. Logs in newer wells (primarily 2012-2015) have irregular salinity patterns with depth and indicate saline waters at shallower depths than in adjacent older wellslikely due to injection of produced water over time. The presence of produced water disposed of in surface ponds can be seen in logs from newer wells where saline water (>10,000 mg/L TDS) has seeped into the alluvium above the Tulare Formation. These data are consistent with water sample data collected from monitoring wells downgradient of the surface ponds. These shallow saline plumes are also visible on airborne electromagnetic surveys of the study area.