Dr. Jan Gillespie, USGS will be presenting:
Use of geophysical logs is common in the oil industry, but these tools can also be used in groundwater resource and environmental applications. For example, resistivity and porosity logs can be useful in determining salinity variations within aquifers including determining the depth at which salinity exceeds 10,000 milligrams per liter (mg/L) and, with less precision, where salinity exceeds 3000 mg/L. These two values of salinity typically define accepted values for saline vs. brackish and brackish vs. fresh water respectively and are often used to make regulatory decisions regarding potential effects of practices such as surface or underground disposal of waste fluids into aquifers. In addition, brackish water aquifers may become important sources of water in the future as the fresh water portions of the aquifers become depleted. We are currently mapping the extent of these aquifers so that their water can be treated and used.
Geophysical logs can also be used to assess the effects of saline water disposal in areas near oilfields where high salinity produced water is often disposed via water disposal wells and surface disposal ponds into shallow brackish aquifers overlying oil reservoirs. These assessments can be especially informative where well densities are high, and the wells are drilled and logged in the same areas over decades.
Finally, porosity and, sometimes, resistivity logs can be used to determine the slope of the water table in areas where groundwater monitoring wells are scarce. Areas with brackish water aquifers often lack information on water table elevation. A more complete picture of groundwater flow in a basin can be obtained by integrating water table elevations from monitoring well data with oil-well geophysical logs.
Jan Gillespie received her BS degree in geology from Bemidji State University, her M.S. from South Dakota School of Mines and Technology and her Ph.D. from the University of Wyoming. After several years in the oil industry she joined the faculty at California State University Bakersfield where she taught courses in petroleum and groundwater geology as well as GIS applications. She currently works as a geologist for the US Geological Survey on projects such as California’s Sustainable Groundwater Management Act and the California Oil Gas and Groundwater assessment.
