Oil and natural gas are initially formed in a dispersed state and must undergo accumulation to form industrial reservoirs that are exploitable. The accumulation of oil and gas, as well as the formation of reservoirs, requires specific geological conditions. Once formed, these reservoirs may still face geological processes such as adjustment and destruction, which influence the final distribution of oil and gas. Therefore, understanding the accumulation of oil and gas and the formation and preservation of reservoirs is essential. This chapter mainly introduces the concepts and measurements of traps and oil and gas reservoirs, the principles of oil and gas accumulation, the conditions for the formation and preservation of reservoirs, the destruction and reformation of reservoirs, the timing of reservoir formation, and the formation mechanisms of condensate gas reservoirs and natural gas hydrates.