It is important to determine the physical threshold of effective reservoirs for hydrocarbon exploration. The physical property of reservoir sandstones and surrounding rocks is used to study in this paper. It is found that the criterion of effective reservoirs is not constant and unconditional, while controlled by the relative physical property between reservoir sandstones and surrounding rocks, having a relationship with burial depths. A new evaluation method is proposed in this paper, in which the ratios of capillary pressure of sandstone reservoirs to surrounding rocks are used to determine the threshold of effective reservoirs. The ratios on the critical conditions become larger with increasing depth, which means the threshold of effective reservoirs has a certain relationship with burial depth. The physical property of effective reservoir with certain depth must exceed the threshold with the same depth, otherwise it cannot accumulate oil or gas. The new method is applied to evaluate the Paleogene reservoirs of Zhu 1 Depression, and the effective reservoirs recognized by the new method coincide well with the well logging results. The new method would be significant for petroleum accumulation and distribution prediction.