Paleoenvironment and Its Petroleum Geological Significance of the Jurassic Da'anzhai Member in the Central-Eastern Sichuan Basin
[Objective]We attempted to determine the cause of the large difference in paleoenvironment characte-ristics during the Jurassic Da'anzhai period and the unclear distribution law of the shale source and reservoir in the Si-chuan 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 com-parative 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 mud-stone,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 sub-member,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 sedimen-tation 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 enrich-ment,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.
paleoenvironment restorationdepositional center migrationsource storage configurationDa'anzhai memberSichuan Basin
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