ANALYSIS ON THE MIGRATION CHARACTERISTICS OF THE MESOZOIC PETROLEUM IN THE ORDOS BASIN

The Mesozoic structures in the Ordos Basin are simple and smooth,with reservoirs mainly of low porosity and low permeability and distinctive hydrocarbon migration.The conditions of porosity and permeability are better in the Jurassic and the upper reservoirs of the Yanchang Formation,and the driving forces of hydrocarbon migration and accumulation are dominated by buoyancy.The surfaces of unconfor-mity on the top of the Triassic and the paleochannel sandbodies of the Jurassic constituted the major migration pathways of hydrocarbon in the Jurassic oil pools,and the oil reservoirs were distributed mainly in the Jurassic paleochannels and the places near the surfaces of unconformity.During the period of a large amount of petroleum creation,the middle and lower reservoirs of the Yanchang Formation were very dense,and the formations basically losed water alternating ability.Abnormal high pressure then exceeded static buoyancy and became the main dynamic of secondary petroleum migration.Permeable sandstone was the main petroleum migration pathway to northeast and southwest,and oil reservoirs were distributed chiefly in hydrocarbon-generating centers and regions around them.