Sedimentary Characteristics And Evolution Of Microbial Carbonates In The Fourth Member Of Sinian Dengying Formation,Gaoshiti-Moxi Area

Anyue Gas Field is located in the central part of the Sichuan Basin,the accumulated reserves of the fourth member of the Sinian Dengying gas reservoir exceed100 billion cubic meters,indicating the huge oil and gas exploration potential of the microbial carbonate reservoir of the Dengying Formation.However,its petrological characteristics,sedimentary facies distribution and sedimentary model are unknown,which leads to difficulties in the prediction and distribution of favorable reservoir facies.Therefore,this paper studies the sedimentary characteristics and evolution law of the fourth member of Dengying Formation in the study area by using core,thin section,outcrop,drilling,logging and seismic data in Gaomo area,and obtains the following cognition :(1)Three major categories and 10 subcategories of petrological plates are identified.There are three types of lithology in the fourth member of the Dengying Formation,including cohesive rock,granular rock and crystalline rock.The cohesive rock mainly includes algal-bacterial dolomite,algal lamellar dolomite,algal framework dolomite and clotted dolomite.Granular rocks mainly include granular dolomite,micritic granular dolomite and micritic / argillaceous dolomite;crystalline rocks mainly include micritic dolomite,powder crystal dolomite and siliceous strip mud powder crystal dolomite.(2)The combination of sedimentary microfacies of microbial mound and grain beach is clarified,and the sedimentary model of microbial mound and grain beach is established.The microbial mound is mainly affected by tidal flat deposition,including mound base : algal lamina micritic dolomite,algal bacterial dolomite(lime mud mound),subtidal low energy;mound core : algal lamina / framework-condensate dolomite,upper subtidal zone-intertidal zone;mound cap: micrite dolomite,algal laminated dolomite,sand-containing algal laminated dolomite,intertidal-supertidal zone;mound flank: irregular gravel bearing arenaceous dolomite-arenaceous dolomite.The grain beach is mainly affected by wave deposition,including inter-beach sea,storm beach and grain beach core.It is mainly composed of micritic dolomite,micritic calcarenite and calcarenite / algal dolomite.It is vertically manifested as an upward shallowing cycle,which is the product of high-frequency sea level decline.(3)Two long-term transgressive-regressive cycles(SQ1 and SQ2)are developed in the fourth member of the Dingying Formation.The SQ1 bottom interface is characterized by strong amplitude and good continuity,transgressive overlap,large mound reflection in the platform margin,and small mound reflection in the platform.The bottom interface of SQ2 is a medium amplitude medium continuous surface,and the top interface is a strong amplitude good continuous surface,with oblique and lenticular reflections.The thickness of the two sequences shows the characteristics of thinning to the east.(4)During the deposition period of SQ1-TST(lower part of the fourth member of Dengying Formation),granular beach deposits were developed.During the deposition period of SQ2-TST(middle part of the fourth member of Dengying Formation)and SQ2-HST(upper part of the fourth member of Dengying Formation),microbial mound deposits were dominant.(5)The platform margin beach-platform margin mound progradational sedimentary model and platform mound aggradation sedimentary model of Gaoshideng 4th member are established.The sedimentary thickness of the third member of Dengying Formation roughly reflects the paleogeomorphology before the deposition of the fourth member of Dengying Formation.During the SQ1-EHST period,the platform edge beach was formed in the platform margin area,and the depression in the eastern platform was gradually formed.SQ1-LHST,microbial mounds appeared in the platform margin area and grew westward,and microbial mounds in the eastern platform grew.SQ2-TST,experienced a transgression,but did not affect the growth of microbial mounds;SQ2-HST,the microbial mounds in the platform margin area grow westward,and the microbial mounds in the eastern platform grow rapidly and fill up quickly on the basis of the lower depression.In the period of SQ1 sequence and SQ2-TST sequence,the Gaoshi 18 well area was a depression in the platform,and the depression was filled up during the SQ2-HST period.The ancient terrain around the depression is relatively high,and microbial mounds and grain beaches can grow on a large scale,thus becoming favorable exploration targets.