Formation-Mechanism And Controlling Factors Of The Microbialite-Dominated Reservoir In The Ediacaran Dengying Formation Of The Sichuan Basin

The successful exploration of natural gas in the microbial reservoirs of the Edia-caran Dengying Formation reveals the great resource potential in the central part of the Sichuan Basin.Moreover,the discovery of hydrocarbon in microbialites-rich carbonate successions in all over the world indicate that the microbialites can be an ideal target for the hydrocarbon exploration.However,the depositional characteristics and its distribu-tion of microbialites can be further delineated at multiple scales with the progress of ex-ploration.The diagenetic parasequence and the evolution of porosity are still in debate.In addition,classification and comparison for different types of microbial reservoirs could help understand their formation mechanisms and main controlling factors.Based on numerous outcrop and core observations with petrological descriptions of polished slabs and thin sections,this study established the depositional model of micro-bialites at different scales.The paragenetic sequence and evolution of porosity were il-lustrated by geochemical and petrographic techniques.Based on the above sedimentary and diagenetic achievements,as well as porosity and permeability data,a reservoir de-velopment model has been established.The petrophysical data can be used to character-ize the microbial reservoirs and established the model for the formation of favorable reservoirs.Two types of microbialites,stromatolites and thrombolites,were identified in the Ediacaran Dengying Formation in the Sichuan Basin.Despite that,non-skeletal grains such as intraclasts,oncoids,ooids,and peloids have also been observed in the Dengying Formation.These grains are classified as grainstone,packstone,wackestone and mud-stone.Based on these petrological characteristics,eleven lithofacies and three sedimen-tary cycles have been identified,which correspond to three depositional environments,including peritidal,shallow subtidal,and lagoon environments.In the peritidal envi-ronment,the multi-stage microbial build-ups are observed with the frequently occur-rence of the fenestral and teepee structures.Intraclastic grainstone-packstone deposited in the shallow subtidal environment,which could develop lens-shaped bioherms as macroscopic build-ups.The lagoon cycle is dominated by medium-thick bedded dolo-mudstone in the lower part.The thickness of microbialites evolves to thin lay upwards and consequently decreases the development of microbial build-ups.The second mem-ber of the Dengying Formation is dominated by peritidal and shallow-subtidal cycles,more than 80%in thickness,in the central Sichuan Basin.However,in the northern area of the Sichuan Basin,the development of a mixed siliciclastic-carbonate system pro-duces a decrease in the stratigraphic thickness of microbialites,about 70%in thickness,at the top part of the second member of the Dengying Formation.In the fourth member of the Dengying Formation,the microbialites in the platform margin account for nearly70%of the stratigraphic thickness.In contrast,the lagoonal dolo-mudstone dominates the inner platform,with microbialites accounting for only about 40%in thickness.Microbial reservoirs of the Dengying Formation have experienced marine,meteor-ic,and hydrocarbon diagenetic fluid alterations.The dolomite cements consist of fi-brous-bladed,medium-coarse granular,and saddle cements.Laminated and clotted mi-crobial fabrics are well-preserved and are composed of micritic dolomites with less than100μm.The carbon(δ¹³C),oxygen(δ¹⁸O),and strontium(⁸⁷Sr/⁸⁶Sr)isotopes in the mi-crobial dolomite are similar to the Ediacaran seawater,indicating the marine origin of the dolomitizing fluid.Theδ¹³C,δ¹⁸O,and ⁸⁷Sr/⁸⁶Sr values of the fibrous and bladed dolomite cements close to the microbial matrix,which indicates that the diagenetic fluid is originated from the coeval seawater.The medium-coarse crystalline granular dolo-mites filled later than the fibrous-bladed cement.Theδ¹⁸O values of this cement was affected by the recrystallization caused by the increasing temperature in the burial stage.Numerous vugs and dissolved channels filled by breccias,transported sediments,and meniscus cements suggest the dissolution and cementation by freshwater.Fibrous ce-ments generally occur as earliest-generation cements within vugs.In contrast,the fi-brous dolomite crusts can be observed as breccias produced by the dissolution and pre-cipitation of meteoric diagenesis.These two phenomena reveal that marine and meteoric fluids alternately influenced the microbialites in the near-surface realm.Finally,the saddle dolomite cement filled in fractures and veins.Theδ¹³C value of the saddle dolo-mite is similar to the matrix and early cement,while theδ¹⁸O value is significantly neg-ative.The high Mn,Fe concentrations,homogenization temperature derived from the fluid inclusion and concomitant silicification suggest its formation from Fe-rich high-temperature burial fluid.The porosity of the peritidal microbialites is higher than that of shallow subtidal microbialites.Peritidal microbialites experience intense dissolution in the near-surface domain.Therefore,it has higher initial porosity and still retains higher porosity after deep burial.Although the porosity of the shallow subtidal microbialites is slightly lower than the peritidal environment,it has greater thickness.For the microbialite-dominated reservoir,its initial porosity is high.The porosity and permeability characteristics show the selective dissolution,suggesting the key factor of meteoric diagenesis in the en-hancement of porosity.Marine cements mainly play a role in the reduction of the poros-ity.The distribution of microbialite and dolo-grainstone to-packstone controls the dis-tribution of reservoirs in the Dengying Formation.In the second member of the Dengy-ing Formation,the porosity is higher in the central than in the northern areas.Moreover,the quality of reservoirs in the platform margin are better than that in the platform inte-rior in the fourth member of the Dengying Formation.