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Petroleum was generated from sedimentary rocks. The world’s oldest oil source rock so far was found in Proterozoic rocks. Since then, 73% to 81% of the earth’s surface has been covered with sedimentary rocks. However, only quite a limited area is rich in oil and gas. It is found that source rocks have controlled oil and gas distribution, and they are mainly formed in two systems: (1) river-lake systems and (2) river-gulf systems. Phytoplankton is an important source of kerogen, the blooming of which is strongly dependent on nutrients. Rivers are the major nutrient provider for basins. Rivers around lakes and an undercompensation (where the sedimentation rate is less than the rate of basin subsidence) environment provide favorable conditions for phytoplankton blooming in lakes. Gulfs are usually located at the estuary of big rivers, characterized by restricted current circulation and exchange with the open sea, which benefit maintaining the nutrient density, phytoplankton levels and organic matter preservation. The river-gulf system is the most favorable place for marine source rock development. Most of the world famous marine petroleum-rich provinces are developed from river-gulf systems in geological history, such as the Persian Gulf Basin, Siberian Basin, Caspian Basin, North Sea, Sirte Basin, Nigerian Basin, Kwanza Basin, Gulf of Mexico, Maracaibo Basin and the Eastern Venezuelan Basin.