[Objective]The carbonate rocks from the Sinian Dengying Formation in the northern Sichuan Basin expe-rienced multi-stage fluid alteration,which has exerted a significant influence on the quality of the reservoir.[Methods]Laser in-situ carbon and oxygen isotopes,micro-area strontium isotopes,petrography,and cathodolumi-nescence were used to systematically analyze the petrology and diagenetic evolution of dolomite in the Dengying For-mation.Based on this research,we clarified the diagenesis process of different diagenetic fluids and its influence on reservoir formation.[Results]The results indicate that:(1)The dolomite can be divided into three different lithofa-cies:microbial,granular,and crystalline.Growth-framework porosity of carbonates is the fundamental factor for de-veloping high-quality reservoirs;therefore,laminated,stromatolite,and clotted dolomites with a large number of pri-mary pores can be regarded as high-quality reservoir rocks.(2)Four diagenetic environments associated with the de-velopment of high-quality reservoirs were identified in the 4th member of the Dengying Formation,including evapo-rated seawater,shallow buried"trapped"seawater,medium-deep buried organic acid migration,and hydrothermal environments.Isopachous fibrous and blade cements precipitate in the evaporated seawater environment,and the ce-ments have no cathodoluminescence.Their δ13C,δ18O,and 87Sr/86Sr values agree with the Sinian seawater.Fine-crystalline cements with moderate orange-red cathodoluminescence are precipitated in shallow buried"trapped"sea-water environments,the δ13C,δ18O,and 87Sr/86Sr compositions of the cements are similar to the Sinian seawater,and samples have slightly lower δ18O than that of Sinian seawater.The medium-crystalline cements precipitate in the medium-deep buried organic acid migration environment,while the cements feature strong orange-red cathodolumi-nescence and a relatively more negatively biased δ18O.The 87Sr/86Sr value of the cements is sufficiently close to the Cambrian,indicating that the cement provenance may come from the Cambrian.Furthermore,the dissolution of buried organic acid exerts a positive effect on the reservoir evolution.Hydrothermal minerals such as saddle dolomite and quartz precipitate in the hydrothermal environment.Saddle dolomite is characterized by bright red cathodolumi-nescence,significant negative δ18O anomalies,and significantly high values of 87Sr/86Sr.With the influence of hydro-thermal fluid,a large number of dissolution pores are formed,which greatly improve the property of the reservoir.(3)The development of high-quality reservoirs in the 4th member of the Dengying Formation in northern Sichuan was controlled and restricted by sedimentary environment,diagenetic evolution,and tectonic movement.Various sedi-mentary facies have laid a solid foundation for the reservoir development.The platform margin and interior mound-shoals are the predominant facies for the development of the reservoir.During their deposition,they feature powerful hydrodynamics and develop various combinations of microbial dolomite,forming in a significant number of primary pores.Despite being filled by various cements,they still serve as relatively favorable parts for pore penetration in the stratum,which has furnished a foundation for the modification of diagenesis in the later stage.Complicated diagenesis serves as a critical factor for the formation of high-quality reservoirs.Multi-stage cementation has caused the primary pores of the reservoir to basically disappear,whereas the secondary pores formed by buried organic acid dissolution and hydrothermal dissolution has turned into the primary spaces of the reservoir.Sichuan Basin has experienced multi-cycle tectonic movements,and multi-stage fractures have formed in the Dengying Formation,which have provided favorable conditions for hydrothermal activities.Among these,the favorable sedimentary facies belt is the fundamental character,multi-stage fluid dissolution is the key factor,and the faults formed by tectonic movement provide favorable the deep fluid fluxion.[Conclusions]The study of the identification and quantitative reformation degree of high-quality reservoir in the 4th member of the Dengying Formation provides a theoretical basis for sedimentary-diagenesis for predicting oil and gas exploration targets in this area.
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