Abstract: Based on the results studied from 100 large and medium oil-gas fields in 15 petroliferous basins of China, it is suggested that the large and medium oil fields of China are mainly distributed in rift basins, while the large and medium gas fields distributed in cratonic basins and piedmont basins. Continental source rocks as the main ones in the oil and gas fields in China are found from the Early Paleozoic and the Cenozoic. The gas source rocks are mainly distributed in the Tertiary in the South China Sea and the East China Sea, Jurassic, Carboniferous and Permian in northern China, and the Tertiary, Cretaceous, Jurassic, Triassic and Permian are the main oil source beds. The reservoirs in the large and medium gas fields are characterized by terrestrial rocks such as medium sandstone, fine sandstone and glutenite, with genetic types of fan delta and delta system, of which carbonate reservoirs are dominated by fractured and weathering crust types and cap rocks by homogenous shale. The hydrocarbon accumulation periods are generally late, most of the fields are formed in the Cenozoic. Therefore, many oil-gas fields with a quite lot of reserves are still not found in Lower Paleozoic strata. It is concluded that China is rich in petroleum resources, most of basins are in the early-middle stages for petroleum development, and findingout large and medium oil-gas fields in China is big in probability in the future.

Key words: China, large and medium field, structure, distribution rule, basin type, source rock, reservoir, cap rock