Abstract: The Lower Ordovician weathering crust in north slope of Tazhong low-swell is an important channel for petroleum migration and reservoir together with underlying Cambrian as a cap rock. Following the muddy limestone deposit of Lianglitag formation of the Upper Ordovician, the hydrocarbon generated in the Middle-Late Ordovician as peak stage for oil generation in the Cambrian shale source rocks in Manjiaer sag of northern Tazhong area could be trapped effectively. The reservoir-caprock assemblage of the Lower Ordovician formed earlier may be connected with that of underlying Cambrian through faults and fractures, which is a significant natural gas transport layer during Himalayan movement. The Lower Ordovician weathered crust reservoirs linking to the underlying Cambrian reservoir -cap combination by faults and fractures is important gas migration layer during the Himalayan. In the places of suitable sealing conditions such as the fault blocking, compact layer sealing saturated with water and asphaltene sealing formed by early biological degradation, it is believed that natural gas accumulation in continuous distribution could be formed.

Key words: Tazhong low-swell, Lower Ordovician, oil-gas migration and accumulation