Abstract: Ordos,Tarim and Sichuan basins are the import marine basins with abundant paleo oil reservoirs.The reservoir bitumen in paleo oil reservoirs is the intermediate product of buildup and reconstruction of the oil &gas accumulation process.Based on the classification of oil &gas accumulation of marine system in Sichuan Basin,the shape of carbonate reservoir bitumen has been analyzed in detail,and relationship between reservoir bitumen shape and oil & gas accumulation process has been obtained.Shale,tight sand and carbonate are in a wide range,and coupling relationship between gas and paleo oil reservoirs is very complex.In a word,primary and secondary oil-cracking gas in mainshale gas,and tight gas in minor composite accumulation system is the basic characteristic of marine system in Sichuan Basin.The characteristics of bitumen can reveal the paleo oil pools and gas reservoir accumulation and adjustment process.