Abstract: The surface soil sampled from Dongpu area is pyrolysed at different temperatures and isotopically analysed by C-isotopic method,and correlated with the underground geochem-ical measurements from an exploratory well. The results show that altered carbonate of soil(△C) does not directly related to vertically upward migration of hydrocardons,but to upward dissemination of mineralized water associated with the formation of hydrocarbon pools. Such a process is the cause for "alteration" of pH value of soil,therefore it may indicate the existence of hydrocarbon accumulation.