Abstract: A concept of flow field with new meanings is introduced into study of secondary petroleum migration, and systematically clas-sified and characterized. Flow field refers to the space occupied by fluid, comprising reservoir, transport layer or carrying layer. It is divided from large to small scale into basin-region flow field, oilfield -reservoir flow field, layer flow field and porous flow field. This paper discusses the flow field type, scale, difference and capacity for collecting hydrocarbon in source area and their ffcts on contolling petroleum migra-tion/ accumulation, reservoir scale and location. It is definitely pointed out that the unconformity flow field of bottom of Karamay formation in northwestern margin of Junggar basin is major pathway for accumulating, transporting and distributing hydrocarbon from source rocks up and down in study area.

Key words: Junggar basin, northwest, flow field, secondary migration, reservoir, unconformity, fault