Abstract: The compositions of three kinds of mineral oil which were from oil-based mud, oil-based cuttings and recycled mineral oil were studied by GC-MS, and the distribution characteristics of 16 PAHs with controlled priority listed by the US EPA were systematically analyzed. Based on the compositions and distribution characteristics of the organics in the mineral oil, the influences of the drilling and thermal desorption process on the change of mineral oil compositions were explored. The results show that there exist great differences of compositions among three kinds of mineral oil, among which the drilling process exerts significant influence on the mineral oil compositions of the oil-based mud. After drilling, the compositions of aromatics increase from 6 types to 16 types, with the relative content of aromatics increasing by 10%. The chlorinated hydrocarbon species and their relative content also increase obviously. The relative content of halogenated hydrocarbons in the mineral oil further increases after the thermal desorption process. The three kinds mineral oil are mainly with low-ring PAHs, and after the drilling and thermal desorption process, the PAHs in the mineral oil have the trend of changing from low ring to middle or high ring. Finally, by using the recovered mineral oil as the base oil, the experimental oil-based mud was evaluated, and it shows good performance as the recovered mineral oil can satisfy the requirement for composing the oil-based mud.