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River-Gulf System: The Most Important Place for Global Marine Petroleum Accumulations


Gulfs in different geologic times are the most important locations for marine petroleum accumulation. Studies revealed that in main hydrocarbon-bearing marine basins, including Persian Gulf basin, West Siberian basin, Gulf of Mexico, North Sea basin, Atlantic margins, Sirte basin, Maracaibo basin, Paleo-Tethys basin etc., all the marine source rocks were developed in paleogulfs surrounded by continents in three sides and opened to ocean in one side. In Gulfs, nutrients for aquatic organisms such as algae are mainly provided by rivers and not by ocean upwelling which are too far away from the gulfs. In modern China, algae blooming forms red tides and blue tides during the summer in the estuaries of Yangzte river, Pearl river, Yellow river, Haihe river, Suizhong river and so on. Industrial and domestic waste water supply abundant minerals to stimulate explosive growth of red algae and blue algae. Besides, modern biological study also indicates that the materials derived from river promote the great growth of algae in modern gulfs in other places in the world. There are many Mesozoic-Cenozoic lacustrine hydrocarbon-bearing basins in China, Indonesia, Africa continent and Atlantic margins. The excellent source rocks in these basins were from rivers flowing into the lakes and suppling abundant minerals for algae growth to produce large amount of organic matter. Lacustrine basins and marine gulf basins have no essential differences in sources of organic matter but their sizes. Sichuan Basin in southwest China is a large Paleozic-Mesozoic basin, where exploration activity started in 1953. However, the largest gas field, Anyue gas field, wasn't discovered until 2013. We learned an important lesson that the source rock distribution in Sichuan Basin was controlled by the location of paleogulfs (rifting troughs). In the past, it was believed that source rock is widely spread, covering the whole Paleozoic platform. The recent giant gas discoveries (Puguang, Yuanba, Longgang, Anyue, etc.) in the basin are apparently all related to two Early Paleozoic gulfs. Moreover, China's three hydrocarbon-rich marine basins (Sichuan, Tarim and Ordos basins) share a common feature, that is, the hydrocarbon distribution is controlled by excellent source rocks developed in paleogulfs. In conclusion, Gulfs formed in various geologic periods are the most important places for marine petroleum accumulation, while the open marine basins are less favorable for source rocks development.