Distribution and Evolution of Sedimentary Facies in the Intersection Area of Kaijiang Liangping Trough and Pengxi Wusheng intra-platform depression in Northwest Sichuan Basin, from Changxing Formation to Feixianguan Formation

Key words: 
• Intersection area /
• Northwest Sichuan Basin /
• Changxing Formation /
• Feixianguan Formation /
• sedimentary facies

Abstract: [Objective] To clarify whether the western edge of the Kaijiang-Liangping Trough and the eastern edge of the Pengxi-Wusheng intra-platform depression intersect in the north during the Late Permian Changxing Early Triassic Feixianguan period in northwest Sichuan, the sedimentary facies distribution of each layer in the region was constructed, and the sedimentary filling evolution process was analyzed. [Methods] Based on the latest drilling, field outcrop, and seismic data, this article systematically analyzes the distribution of lithofacies and paleogeography during the Changxing-Feixianguan period, as well as the evolution process of sedimentary filling, on the basis of stratigraphic and sedimentary facies division and comparison. [Results and Discussions] The intersection area of Changxing Formation and Feixianguan Formation can identify six types of sedimentary facies: shelf basin, slope, platform edge, open platform, restricted platform, and evaporative platform; Based on the sedimentary records of the front slope of the platform, it is clear that the western edge of the Kaijiang-Liangping trough and the eastern edge of the Pengxi-Wusheng intra-platform depression are merged to form a unified platform edge belt in the northwest Sichuan Majiaoba Shangsi area. The platform edge belt and the front slope show a northwest pointing interdistribution, rather than a smooth belt distribution; The intersection area of the two troughs in the Changxing Formation presents a sedimentary pattern of two troughs sandwiched by a platform from southwest to northeast. In the northwest direction of the platform edge zone, there are finger shaped distribution of platform edge biological reefs and shoals, which have the characteristics of developing bioclastic shoals in the first section of the platform edge zone and developing biological reefs and shoals in the second section of the platform edge zone; The sedimentary period of the first and second members of the Feixianguan Formation inherited the overall sedimentary pattern of the Changxing period. However, in the early stage of the Feixianguan Formation, only local platform edge oolitic beaches were formed due to sea flooding. In the late stage of the Feixianguan Formation to the Feixianguan Formation, platform edge oolitic beaches gradually developed and expanded in scope, and overall had the characteristic of eastward migration. The slope in front of the platform was prone to sliding and sedimentation; During the sedimentation period of the Feisan section, the sea trough gradually filled up, and the finger shaped platform edge zone disappeared, leaving only a small amount of lagoons and only developing ooid shoals in the Majiaoba area; The Feisi section deposited its entire area and filled it up, forming an evaporative platform facies. The platform margin zone in this region did not migrate northwestward along the finger shaped platform margin zone, but rather exhibited eastward migration characteristics. [Conclusions] The determination of Changxing-Feixianguan connectivity and the definition of five sedimentary filling evolution stages in Kaijiang-Liangping Trough and Pengxi-Wusheng intra-platform depression supplemented the understanding of sedimentary pattern and sedimentary filling evolution process in the intersection area of Changxing-Feixianguan trough, and formed a unique example of sedimentary filling evolution sequence in the transition area connected by two rows of cross-platform edge belts in the same period. It also provides ideas and references for oil and gas exploration in this area.