Head-space gas of well logging is a fast and cheap technique to identify the depth of oil and gas reservoir, and the hydrocarbon indexes are usually used to distinguish the characteristics of oil or gas in the reservoir. Due to the unavoidable weight loss of gas during the sampling process of head-space gas, the problem as to whether the property of the hydrocarbons in the can is the same as the reservoir gas is an argumentative issue. Therefore, it is questionable to use the indexes of hydrocarbon of head-space gas to distinguish the characteristics of reservoir petroleum. In this study, a model for identifying characteristics of the reservoir gas or oil by head-space gas of drill core was established by a comparative study between the drill core head-space gas and the reservoir gas from the same depth. The result shows that the drill core head-space hydrocarbons are different from the reservoir hydrocarbons in concentrations, which further results in the difference of hydrocarbon indexes of the two gases in identifying the characteristics of petroleum reservoir. However, the isotopic composition (δ¹³C, δD) of the hydrocarbons of the head-space gas and that of the reservoir gas are identical, suggesting that the δ¹³C and δD values could be used to identify the characteristics of petroleum reservoir.