Based on systematic measurement of the distribution of acidolysis hydrocarbon in well log with unconformities of different epochs and different depths, the authors found that under the unconformity surface there is an abnormal distribution zone of hydrocarbon which has low total abundance of hydrocarbon, low concentration of methane and relative enrichment of hydrocarbon macromolecules. It is considered that this anomalous zone is a degassing zone formed by weathering and degassing, and its thickness is related to the diagenetic degree of the lower strata. This discovery has great significance for evaluating the relationship between the basin erosion and the gas reservoir preservation or destruction as well as for studying the effect of the unconformity on oil and gas migration, and also provides a new scientific clues for solving the problem of restoration of denuded strata.