Sedimentary microfacies and environmental evolution of the Middle Permian Maokou Formation in the eastern Sichuan Basin: A case study of the Yangjiao section in Wulong District, Chongqing, China

In recent years, industrial gas flow has been obtained in the Permian Maokou Formation in the eastern Sichuan Basin, underscoring the great significance of exploring the sedimentary microfacies and environmental evolution of the formation for oil and gas exploration. This study focuses on the Yangjiao section in Wulong District, Chongqing, China. Based on the field observations of sedimentary characteristics, coupled with thin section observation and geochemical analysis, we delve into the sedimentary microfacies and sedimentary environmental evolution characteristics of the Maokou Formation. The results reveal the predominant distribution of five sedimentary microfacies. Bioclastbearing micrites and micrites, deposits in a deep-water, low-energy environment, prevail in the lower formation. The middle formation exhibits the primary distribution of sparry bioclastic limestones, micritic bioclastic limestones, bioclastic micrites, and bioclastic micrites, and the presence of sparry bioclastic limestones suggests a shallow-water, high-energy sedimentary environment. The upper formation predominantly contains micritic bioclastic limestones, bioclastic micrites, and bioclast-bearing micrites, and the water becomes deeper again. Geochemical analysis reveals that the lower Maokou Formation exhibits low δ¹³C_carb values with an average of 3.00 ‰, while its middle and upper portions show δ¹³C_carb values mostly exceeding 4.00 ‰. These findings suggest that the formation’s first member was deposited in an anoxic environment, which converted into a dysoxic environment with an increase in oxidation and then recovered to an anoxic environment in the late depositional stage. The sedimentary microfacies characteristics and the sea-level changes in the Maokou Formation in the eastern Sichuan Basin were affected by glacial periods, with the global sea-level drop serving as the dominant factor in the sedimentary environment transitions. The shallow-water sedimentary environment created favorable geological conditions for the emergence of high-energy grainstone shoals, which is, in turn, the main cause of the formation of high-quality reservoirs in the middle and upper Maokou Formation in the basin.