The distribution of oil and gas in the Earth's crust is controlled by regional geological structures and geological conditions such as lithology and facies, and appears in clusters, belts, and zones. Dividing oil and gas accumulation units at different scales can effectively reveal the distribution and enrichment patterns of oil and gas, providing guidance for oil and gas exploration. This chapter introduces oil and gas reservoirs, oil and gas fields, oil and gas accumulation belts, petroleum systems, and petroleum basins as different levels of oil and gas accumulation units. It compares and analyzes the petroleum geological characteristics of different types of petroleum basins and introduces the distribution characteristics of global and Chinese oil and gas resources and the main geological factors affecting the distribution and enrichment of oil and gas.