Abstract: The vitrinite reflectance analysis of the samples from the Well X indicates that the threshold depth for oil generation is deeper than 2 600 meters, and the Eocene Liushagang mudstone with higher abundance of organic matter ranging in the depth of 2 100~2 285 m is above the threshold, while the organic matter of Liushagang mudstone below the threshold is too low to provide sufficient hydrocarbon supply for oil accumulation. As the result, the exploration of the Well X is failed. However, the Well X is located in southern edge of the East-3 Depression, where the Liushagang Formation is 568.5 m in thickness, of which 62.3% are mudstone and the maximum thickness of a single lager of mudstone is over 115 m. It means that water was rather deep during the deposition of the Liushagang Formation and the accommodation space was large. It is obviously different with the conclusion from previous studies, which believed that the Mai 5 growth faulting was weak then, that resulted in small accommodation space, so the East-3 Depression is lack of organic matters for oil and gas generation. Furthermore, as the Well X is located in the southern edge of the East-3 Depression, denudation of the Liushagang Formation might be quite intensive, so we speculated that the thickness of the Liushagang Formation must be larger and might exceed the threshold for hydrocarbon generation. The exploration potential in East-3 Depression remains lange and should not be ignored.