In recent years, new oil reservoirs have been discovered gradually in Chang 9 member of the upper Trias Yanchang Formation in the Ordos basin. It is important to understand the migration of Chang 9 oil and gas for further exploration and exploitation. By calculation of the fluid overpressure from Chang 7 to Chang 9 members of upper Trias Yanchang Formation and the analysis for the geochemistry of crude oil and resources, migration dynamics of the Chang 9 oil and gas was studied during the formation of Chang 9 oil reservoirs in Gufengzhuang-Hujianshan area. There are similar geochemical characters between Chang 9 crude oils and Chang 7 source rocks in studied area, which reflects that Chang9 crude oils are likely derived from the upper Chang 7 source rocks. Chang 7 member have a very higher overpressure than Chang 9 member so that a significant overpressure difference (>14 MPa) has occurred between them. Such an overpressure difference offers energy of migration from Chang 7 oil and gas to Chang 9 reservoir bed.
