Abstract: Hydrocarbon generation,migration,accumulation and diffusion loss are the products of sediments after a series of physical and chemical changes during basin formation,development and withering.The first migration of oil and gas is caused by physical field of unloading and decompression.Source rock experiences cracking and hydrocarbon expulsion at first,and then hydrocarbon expulsion and compaction,and finally compaction and hydrocarbon expulsion.Oil accumulation quantity can be comprehended as oil expulsion quantity territorially.Basin withering stage is the period of reservoir consummating and definitizing.