Abstract: Eogene System in East Depression of the Liaohe basin is at the diagenesis stage. The correlation between the Extractable Organic Matter (EOM)，kerogen and the Bio-thermocatalytic Transition Zone Gases (BTZG) in the source rocks has been discussed m this paper. It is discovered that there exist different organic source materials in the EOM and kerogen of the same sample. A higher carbon -number preference of the saturated hydrocarbons, and a significant distribution of land plant-derived biomarkers, i. e.，C₂₉ sterane , pimarane , abietane , r-lupane and oleanane in the EOM show that the EOM is characterized by organic matter of type Ⅱ_b-Ⅲ.However，the carbon isotopic composition of the kerogen indicate that it mainly consists of organic matter of type I-Ⅱ_a.The difference in organic compositions between the EOM and kerogen at the diagenesis stage can be explained by the selective accumulation of sedimentary organic matter. Specific 1R absorption spectra and corresponding parameters of bitumen A, non-hydrocarbons and asphaltene show the same pattern of variation in the depth of 1850^-3060m. Three evolution stages have been determined in this study. The EOM are rapidly degradated in about 2200~2770m，i. e.，the second stage，and highest at about 2500m，which results in forming BTZG. Although kerogens are characteristic of deoxygen and delipidic groups at the stage, they have no direct relations to the formation of BTZG because they mainly produce depolymeric betumen at the stage. Therefore the EOM，especially non-hycrocarbons and asphaltene, could be considered as the most important source materials of BTZG in East Depression of the Liaohe basin.