The U.S. Geological Survey (USGS) and California State University-Sacramento, in cooperation with the California State Water Resources Control Board and U.S. Bureau of Land Management, compiled and analyzed data for mapping groundwater salinity in selected oil and gas fields in California. The data for the Midway-Sunset (MWSS) oil field includes digitized borehole geophysical data, geochemical analyses of produced water samples from oil and gas wells, core sample analysis from select wells, and groundwater total dissolved solids (TDS) estimations with the related geophysical log data. These data have been compiled from many sources and span several decades. The geochemical data include ion concentrations and TDS which are attributed with the sample date, geological formation, and perforation depths. These data have been in archived scanned pages of historical lab analyses on the Division of Oil, Gas, and Geothermal Resources (DOGGR) website. The borehole geophysical data has been provided by California Division of Oil, Gas, and Geothermal Resources (DOGGR). These data were compiled primarily to create groundwater salinity maps to assist in regional groundwater monitoring as part of the California State Water Resources Control Board's Program of Regional Monitoring of Water Quality in Areas of Oil and Gas Production and the USGS California Oil, Gas, and Groundwater (COGG) program.