Abstract: Qikou sag is one of the rich-oil areas in Bohai Bay basin, in which three sets of lacustrine source rocks developed in Tertiary and Paleozoic reservoirs. The geochemical analyses of 59 crude oil and 102 source rock samples from Qikou sag show that the crude oils in Qikou sag belong to mature oil, combined with the biomarkers of n-alkanes, steroid and terpenoid as well as light hydrocarbons index, the biodegraded oil only locates in shallow layers in Tangjiahe region. SPSS cluster analysis based on four variable parameters of gammacerane index, m(Pr)/m(Ph), m(4-methyl-sterane)/m(C₂₉ regular sterane) and oil carbon isotope indicates that the crude oils can be classified as fresh water lacustrine oil from Qikou sag and western uplift of Banqiao sag and salt water lacustrine oil from Beitang sag and northwestern Banqiao sag.

Key words: Bohai Bay basin, Qikou sag, geochemistry, crude oil group, maturity, cluster analysis