• Oils in the Xihu Sag have diverse sources with both terrigenous and marine organic matter. • Biomarker and carbon isotope signatures elucidated multiple source kitchens in the Xihu Sag. • Ternary diagrams of phenanthrenes provide new tools for oil-source correlation in the study area. The Xihu Sag is a significant petroleum-bearing area in the East China Sea. Its petroleum exhibits unclear origins owing to the presence of both terrigenous and marine organic precursors and the limited availabil