Abstract: The author thinks that there exists a kind of hydrocarbon pools unconventionally trapped in the deep depression of a sedimentary basin different from any conventional structural traps. Generally, good source rocks are developed in the deep depression of a sedimentary basin, while reservoirs are concentrated in the front of various sand bodies. Because of their low porosity and poor permeability, the sand bodies have quite high expulsive pressure. Large quantity of hydrocarbons entered reservoirs after primary migration when the deeply depressed part in the sedimentary basin had reached the mature threshold, which would expel the original pore water in the reservoirs upward until hydrocarbon pools formed. Because there is no water in the lower part of the pools, or if there is, it would not connect with that on the top, the pools will not be affected by buoyant force. At this time, the sum of static pressure of formation water and expulsive pressure of porosity cappillary in the reservoir itself is the major confining pressure, the trapping results are comparable with those from conventional capping traps.