Abstract: Hydrocarbon migration and accumulation is an irreversible process and a dissipative structure with order in time and space. Sedimentary water is the carrier and major driving force of hydrocarbon migration and accumulation. It controls fluid motion in basins and flows from basin center to its edge in centrifugal flow. While infiltration water generated from filtration of meteoric water and surface water flows from basin edge to its center in gravity -induced centripetal flow. The interaction of sedimentary water and infiltration water results in a pressure equilibrium zone, where hydrocarbons accumulate. Oil and gas traps should have both an equilibrium structure and a dissipative structure. The upper part of a trap has equilibrium structure which ensure the preservation of oil and gas, while the lower part has dissipative structure which ensure the continuous charging of oil and gas, so as to form large oil and gas fields. Oil and gas reservoirs in Biyang depression distribute in circularity in the pressure equilibrium zone on the frontal surface between sedimentary water and infiltration water.