Water-, mud-, gas-, and petroleum-bearing seeps are part of the Salton Sea geothermal system (SSGS) in Southern California. Seeps in the Davis-Schrimpf seep field (∼14,000 m2) show considerable variations in water temperature, pH, density, and solute content. Water-rich springs have low densities (<1.4 g/cm3), Cl contents as high as 45,000 ppm, and temperatures between 15 and 34 °C. Gryphons expel denser water-mud mixtures (to 1.7 g/cm3), have low salinities (3600–5200 ppm Cl), and have temperatures between 23 and 63 °C. The main driver for the seep system is CO2 (>98 vol%). Halogen geochemistry of the waters indicates that mixing of deep and shallow waters occurs and that near-surface dissolution of halite may overprint the original fluid compositions. Carbon isotopic analyses suggest that hydrocarbon seep gases have a thermogenic origin. This hypothesis is supported by the presence of petroleum in a water-dominated spring, composed of 53% saturated compounds, 35% aromatics, and 12% polar compounds. The abundance of polyaromatic hydrocarbons and immature biomarkers suggests a hydrothermal formation of the petroleum, making the SSGS a relevant analogue to less accessible hydrothermal seep systems, e.g., the Guaymas Basin in the Gulf of California.