Abstract: Samples of the Mesozoic source rocks, crude oils, and oil-bearing sands in the southern Ordos Basin were collected systematically. Gas chromatography-mass spectrometry (GC-MS) was performed on the saturated hydrocarbon fractions extracted from these samples in order to discuss their biomarker characteristics and the origin of Mesozoic crude oils. The results show that, on the basis of biomarker source parameter distribution, the Mesozoic source rocks can be divided into five types ( type A1, A2, A3, A4 and B), and crude oils can be grouped into three types (I,IIandIII). Type I crude oils should be derived from Chang 7 oil shales (type A1) that were deposited in a reducing, deep lacustrine environment. Type II crude oils should be sourced from Chang 7 dark mudstones (type A2) laid down in a sub-reducing to reducing, deep or semi-deep lacustrine environment. Type III crude oils should be sourced from Chang 7 dark mudstones (type A3) laid down in a sub-reducing to reducing, semi-deep lacustrine environment, but probably also be sourced from Chang 8 or Chang 9 dark mudstones. Apparently, oil shales in Chang 7, as the main source rocks in the southern Ordos Basin, contributed significantly to the crude oils in this area.

Key words: Southern Ordos Basin, geochemical characteristics, oil-source correlation, biomarkers