Three systems of the oil and gas formation in the world

Crude oil and natural gas are generated by organic matters in rocks in sedimentary basins. After incisive and systematic research on global petroliferous basins, it is realized that the distribution of oil and gas fields is highly heterogeneous, and most of oil and gas are enriched in a few sedimentary strata. Source rocks are the most significant factor that governs the distribution of crude oil and natural gas in sedimentary basins. The prerequisite to finding out a petroliferous basin is to search for source rocks. The abundance of organic matters determines the quality of source rocks and the quantity of generated hydrocarbons. In a sedimentary basin, the biological nutrients come from rivers. The biological nutrition is the key factor that controls the degree of reproduction and organic matter abundance in source rocks, which is a governing factor for the amount of oil and gas generated and the degree of petroleum resource enrichment in the sedimentary basin. Oil and gas are mainly distributed in three systems in the world: river-lake system, river-gulf system and river-delta system. The river-lake system is the main location of continental oil distribution. Lacustrine oil is mainly produced by organic matters originating from dead algae in lakes preserved in sedimentary rocks. Algae growth mainly depends on the nutrients that come from rivers, especially those with a long history, flowing through a wide area. The nutrients have a large amount of phosphorus, potassium and other minerals dissolved in water, providing a prerequisite to the growth of algae and a guarantee of the formation of high-quality source rocks. The river-gulf system is the main location of marine oil distribution. The gulf is the estuary of the river, which brings abundant minerals to promote the growth and proliferation of aquatic organisms such as algae. Gulfs are relatively closed and their exchange with the ocean is restricted, therefore the gulf is also conducive to the preservation of organic matters. The coaliferous gas has the world's most widely distributed and biggest reserves; many giant coaliferous gas fields are located in the river-delta system. The sediments brought by the river are fertile soil for the growth of higher plants, and the native higher plants on the river-delta plain are the solid material basis for the formation of coal-measure gas source rocks. Well-developed delta reservoirs with good reservoir-caprock configuration are beneficial for natural gas enrichment and accumulation.