The authors regard the entire process within a continuous period from oil and gas being formed to them being lost as a unified system, which can be divided into pre-reservoir, reservoir and post-reservoir stages. The concept of unsteady-state oil reservoirs is defined and the theoretical framework of unsteady-state hydrocarbon accumulation is established. An unsteady-state oil reservoir is defined as the oil and gas enrichment unit with a dynamically balanced gas-oil-water state instead of a unified oil-water contact, which is formed in the process of hydrocarbon fulfilling or adjustment. It is mainly characterized by multiple or significantly tilted gas-oil-water contacts within interconnected reservoirs or laterally inversed gas-oil-water reations. More targets for oil and gas exploration and development are emerged by physical modeling for Donghe Sandstone reservoir in Hadexun Oilfield, Tarim Basin, and research to the types of hydrocarbon enrichment duirng the post-reservoir stage. The theory of unsteady-state hydrocarbon accumulation that was applied to the rolling exploration and development in Hadexun Oilfield and Tazhong-4 Oilfield has preliminarily confirmed the concept of post-reservoir and improved exploration effect.