Abstract: Biodegraded oils have been discovered in the Qingdong Sag, Jiyang Depression, Bohai Bay Basin. On the basis of the geochemical characteristics of source rocks and crude oils, the less affected C₂₉ hopane, is chosen to calculate biomarker parameters, including β-carotane/(2×C₂₉hopanes), gammacerane/(2×C₂₉hopanes). The tricyclic terpane parameters (2×C₂₄tetracyclic terpane)/(C₂₆ tricyclic terpanes), (2×C₁₉)/(C₂₈+C₂₉) tricyclic terpanes and chrysene/phenanthrene, combined with 4-methylsteranes and carbon isotopes. Crude oils in the Qingdong Sag were divided into 4 types (Ⅰ, Ⅱ, Ⅲ, Ⅳ), while source rocks were divided into 3 types (A, B₁, B₂). Crude oils of type I derived from mature source rocks (type A) in the lower part of the third member of Shahejie Formation in the northern part of the sag. Crude oils of type Ⅱ were sourced from rocks (type B₂) in the upper part of the fourth member of Shahejie Formation in the southern part of the sag. Crude oils of type Ⅲ were derived from type B₁ source rocks in the upper part of the fourth member of Shahejie Formation in the central and northern parts of the sag. Crude oils of type Ⅳ were sourced from rocks in the upper part of the fourth member of Shahejie Formation in the Qingnan Sag. Oils in well QD-8 partially originated from source rocks in the southern part of the Qingdong Sag.