Stratigraphic Response of Depocentral Migration in the Da'anzhai Member of the Jurassic System, Central-Eastern Sichuan Basin, to Lacustrine Basin Evolution

The Da'anzhai Member of the Ziliujing Formation in the Lower Jurassic of the Sichuan Basin represents a pivotal stratigraphic interval for the study of continental Jurassic sequences in China. Its sedimentary records provide a comprehensive understanding of the Early Jurassic lacustrine basin’s response to paleoclimatic variations. This study examines core samples from the Da'anzhai Member obtained from the RA1 Well (central Sichuan) and the YT1 Well (eastern Sichuan), complemented by outcrop profile data. Through geochemical analyses of major and trace elements, we systematically reconstruct the vertical evolution of paleoclimate, paleoredox conditions, and paleosalinity during deposition of the Da'anzhai Member. Furthermore, we explore the influence of paleoenvironmental changes on the migration of the lacustrine depocenter and its stratigraphic implications. The Da'anzhai Member consists of various lacustrine lithologies, including shale, shell-bearing shale, mudstone, silty mudstone, argillaceous siltstone, coquina limestone, and argillaceous coquina limestone. Based on lithological assemblages and geochemical indicators, the Da'anzhai Member is subdivided into three submembers: Da 3, Da 2, and Da 1, in ascending stratigraphic order. Differences in lithological assemblages among these submembers directly reflect changes in the depositional environment. Paleoenvironmental reconstruction indicates that the Da 3 submember was deposited under arid to semi-arid climatic conditions, with freshwater to brackish water, weakly reducing conditions, and a shallow- to semi-deep-lake setting. The Da 2 submember was deposited under a semi-arid to semi-humid climate, characterized by increased precipitation and lower temperatures, with freshwater, relatively stronger reducing conditions, and a semi-deep-lake setting. The Da 1 submember formed under arid to semi-arid climatic conditions, with brackish to saline water, oxic to weakly reducing conditions, and a littoral to shallow-lake setting. Vertically, both the RA1 and YT1 wells show an evolutionary trend from arid to humid and then back to arid conditions, accompanied by changes in precipitation from low to high and then back to low, emperature from high to low and then back to high, redox conditions transitioned from weak to strong and then back to weak, and paleosalinity from high to low and then back to high. Based on the above paleoenvironmental evolution, combined with inter-well sedimentary facies correlation, the migration pattern of the lacustrine depocenter during deposition of the Da'anzhai Member was revealed. From the Da 3 submember to the early stage of the Da 2 submember, the lacustrine depocenter migrated from eastern Sichuan toward central Sichuan, corresponding to climatic humidification and lake-basin expansion. From the late stage of the Da 2 submember to the Da 1 submember, the lacustrine depocenter shifted back from central Sichuan toward eastern Sichuan, corresponding to climatic aridification and lake-basin contraction. This migration process was strongly controlled by paleoclimate. Under humid, low-temperature, and high-precipitation conditions, lake-basin expansion and enhanced reducing conditions favored the deposition of thick shale successions. In contrast, under arid climatic conditions, lake-basin contraction promoted the development of coquina shoal facies. The migration of the lacustrine depocenter directly controlled the stratigraphic thickness, lithological assemblages, and spatial distribution of different submembers, providing an important basis for refined stratigraphic subdivision and correlation of the Da'anzhai Member. As a product of lake-basin expansion, the Da 2 submember has an average total organic carbon (TOC) content of 1.70% and an average porosity of 4.93%, making it the principal oil-generating and reservoir interval within the Da'anzhai Member. Correlation analysis shows that relatively humid climatic conditions, increased precipitation, lower temperatures, enhanced reducing conditions in the water column, and lower salinity were favorable for organic matter enrichment and pore development, further confirming the controlling effect of paleoenvironmental evolution on stratigraphic records. In summary, paleoenvironmental evolution during deposition of the Da'anzhai Member drove the migration of the lacustrine depocenter and thereby controlled the stratigraphic development characteristics of different submembers. This study provides an important theoretical basis for establishing the continental Jurassic stratigraphic framework of the Sichuan Basin and for predicting favorable shale oil exploration areas. It also deepens our understanding of the response mechanisms of Early Jurassic continental lacustrine basins to paleoclimatic changes and enriches the study of Jurassic stratigraphy.