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[Objective] We attempted to determine the cause of the large difference in paleoenvironment characteristics during the Jurassic Da'anzhai period and the unclear distribution law of the shale source and reservoir in the Sichuan Basin. [Methods] Based on the geological drilling cores and field profiles of the Jurassic Da'anzhai member in the central and eastern Sichuan Basin, combined with elemental geochemical analysis methods, we performed a comparative analysis of the paleo-climate,paleo-redox conditions,and the paleo-salinity in the Da'anzhai period. [Results] The results show that the Da'anzhai member can be divided into shale, shell shale, mudstone, silty mudstone, argillaceous siltstone, shell limestone, and argillaceous shell limestone facies. The paleoenvironment of different sub-members in the Da'anzhai member is different to a certain extent. During the sedimentation period of the Third sub-member, the climate was dry to semi dry, and the water environment was fresh to semi-saline under sub-reducing conditions, located in a shallow to semi-deep lake environment. During the sedimentation of the Second submember, the climate was semi dry to semi humid, with more precipitation and lower temperatures, and the water environment was fresh water with sub-reducing conditions. It was a semi-deep lake environment. During the sedimentation of the First sub-member, the climate was dry to semi-dry, and the water environment was semi-saline to saline under oxidation sub-reduction conditions, located in a shallow lacustrine environment. Wells RA1 and YT1 both exhibited the following conditions: the climate was dry→wet→dry, the precipitation was less→more→less, the temperature was high→low→high, the reducibility of water was weak→strong→weak, and the paleo-salinity was high→low→high. From the sedimentation period of the Third sub-member to the early sedimentation period of the Second sub-member, the lacustrine basin depositional center migrated from the eastern to central Sichuan Basin. The middle sedimentation period of the Second sub-member was a transitional stage during which the migration direction of the lake basin depositional center changed. From the late sedimentation period of the Second sub-member to the sedimentation period of the First sub-member, the lacustrine basin depositional center migrated from the central to eastern Sichuan Basin. The average total organic carbon (TOC) of the Second sub-member is 1.70%, and the average porosity is 4.93%, significantly higher than the First and Third sub-members. The fluorescence display is strong, and it is the main source and reservoir of shale oil in the Da'anzhai section. The paleoenvironmental factors have a significant influence on the source and reservoir configuration. The more humid climate, more precipitation, lower temperature, stronger reducibility and lower salinity are conducive to shale deposition, organic matter enrichment, and pore development. [Conclusions] The study on the paleoenvironment of the Da'anzhai member in the central-eastern Sichuan Basin is important to the regularity of shale deposition, organic matter enrichment, and pore development degree. This research understanding can provide a theoretical basis for shale oil exploration in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

In view of the imprecise sedimentary facies division, unclear favorable rock assemblage and undefined favorable exploration area of Jurassic Da'anzhai Member in Central Sichuan, a study was conducted on the rock type, sedimentary facies type, sedimentary facies distribution, sedimentary model, rock assemblage and favorable exploration area in the study area by the core description of three fully cored wells and the observation results of six field profiles in combination with thin slice identification, logging interpretation and organic geochemical analysis methods. The study results show that Rock was divided into 10 types. The sedimentary facies was divided into 3 subfacies (shore lake, shallow lake and fairly deep lake) and 10 microfacies (including fairly deep lake mud, organic bank and gravity flow). Gongshanmio-Lianchi-Yingshan line is the sedimentary center developed in a fairly deep lake environment, and three rock assemblage types (C, D and E) were identified. Nanchong-Quxian-Yilong line was circularly developed in a shallow lake environment, and three rock assemblage types (A, B and F) were identified. The study results were applied from conventional tight oil to shale oil exploration. According to the TOC, porosity, permeability, fracturing effect and other evaluation indicators, it was considered that Type A rock assemblage was a favorable combination for tight oil carbonate rock exploration under the matching of fractures and cracks, and Type F was the most favorable combination for shale oil exploration, which was mainly distributed in the shallow lake facies at Sub-member 1 to Sub-member 2a of Da'anzhai Member. The study results are of great significance for shale oil exploration in the future. [ABSTRACT FROM AUTHOR]