Characteristics of sedimentary environment and evolution of Permian in southern Sichuan Basin: An example from the profile of Gulin Bajiaocun in Sichuan Province

This paper takes the profile of Gulin Bajiaocun in Sichuan Province as the research object to figure out the characteristics of sedimentary environment and evolution of Permian in southern Sichuan Basin. Based on the detailed observation of sedimentary characteristics in the field profile, using the rock slices identification, whole rock X-ray diffraction analysis, element geochemistry, and carbon, oxygen, strontium isotope analysis, characteristics of the profile's formation, sedimentary environment and evolution are studied. It can be figured out that transgression began from the depositional period of the Liangshan Formation to the Qixia Formation, and the water body of the Qixia Formation became deeper and shallower from bottom to top, presenting an anoxic-poor oxygen environment. The bottom of the Maokou Formation was transgressed again, which inherited the sedimentary environment of the Qixia Formation. The water in the first Member of the Maokou Formation was the deepest, and it gradually became shallower upward, presenting the characteristics of anoxic-poor oxygen. The Longtan Formation was with the shallowest sedimentary water, presenting the mainly oxidized environment and partially reduced environment. The bottom of the Changxing Formation was with the deepest sedimentary water, which was characterized by anoxic-poor oxygen. The sedimentary environment of the Permian Qixia and Maokou Formation gradually evolved from carbonate slope to platform from east to west in Sichuan Basin. Large dissolution vugs developed in the third Member of Maokou Formation. It is speculated that karst vuggy reservoirs may be formed in this area. There are platform basin suborder facies deposits of deep water in the bottom of the Changxing Formation of this field profile. The peripheral area around the platform depression or platform basin may be the favorable area for reef and shoal development, which is beneficial to the formation of reservoir. A large amount of asphaltene filling can be seen in the carbonate rocks in this section. Considering the development of organic-rich carbonate rocks, black shale and coal seams in the Permian in southern Sichuan Basin, it is considered that there is a possibility of paleo-oil reservoirs in the region.