Main factors controlling the formation of excellent marine source rocks in Permian Maokou Formation of northwest Sichuan, China

The Permian Maokou Formation in southern China is taken as regional source rocks. Both the bottom and the top layers of Maokou Formation at the Shangsi section in Guangyuan, Northwest Sichuan developed siliceous rocks, however, only the top layer is taken as excellent source rocks with a thickness of 16 m and the maxium residual organic carbon of 10.90%. Based on the analysis of petrology, carbon isotopes and trace elements, it suggests the existence of upwelling in both the Wordian and Capitanian periods. Nutrient elements brought by upwelling are beneficial to form high primary productivity. The paleo-oceanic phytoplankton growth rate became from high to low and finally to high which coincided with the changes of atmospheric pCO2. The lower Maokou Formation with high primary productivity is poor source rock. The reason is that normoxic environment during the deposition and earlier diagenesis stages of the lower Maokou Formation went against the conservation of organic matter. While the upper Maokou Formation with high primary productivity is taken as excellent source rocks, as a great transgression event during Late Maokou period result in the occurrence of anaerobic environment which led to the formation of excellent source rock layer. The study on the controlling factors in the formation of excellent source rocks in the Maokou Formation is the base of distribution prediction of marine excellent source rocks and hydrocarbon reservoirs prediction in southern China.