Abstract: The first major breakthrough of marine oil-gas reservoir of Paleozoic discovered from Shacan-2 Well in northern Tarim basin was made since 1984 followed by that a number of large-middle scale fields has been found in Tarim, North China, Yangtze and Junggar blocks. In-depth researches of the conditions for forming large scale oil-gas fields are presented in view of source rocks, reservoir body, multiple reservoir-caprock assemblages, palaeohigh, palaeoslope-controlled oils, multistage hydrocarbon accumulation, multistage charging and sealing condition, etc. and the exploration directions are pointed out. These are of great significance for seeking more domestic large scale oil-gas fields of Paleozoic with very complex geologic structures.

Key words: China, Paleozoic, source rock, hydrocarbon accumulation, condition, large scale oil-gas field, exploration direction