Study On The Diagenesis And Evolution Of The Middle And Lower Ordovician Reservoirs In The Shunbei Area

In recent years,Sinopec North-West Oilfield Company has encountered industrial oil and gas flow in the lower Ordovician Formation in Shunbei area of tarim basin.However,the exploration degree of Shunbei area is relatively low,especially the study of reservoir diagenesis is not deep enough.This paper takes Yijianfang and Yingshan Formation in Shunbei area as the target strata,through extensive research literature,by using the existing data to study on petrology,diagenesis,element geochemistry and isotopic geochemistry research.The main aim is to summarize the diagenetic evolution of the area,in order to provide theoretical basis for further exploration.Through the core observation of 9 Wells in the study area and the microscopical identification of more than 240 thin sections,rock types of Yijianfang Formation(O2yj)and Yingshan Formation(O1-2y)of Ordovician in Shunbei area are divided.There are four kinds of rock types:granular limestone,micrite limestone,siliceous limestone and dolomite limestone.Granular limestone can be further divided into sparry granular limestone,microsparry granular limestone and micritic granular limestone according to the grain size of the cement between the particles.The main lithology of the study area are micritic granular limestone and micritic granular limestone.There is obvious difference in lithology between Yijianfang Formation and Yingshan Formation,siliceous limestone is commonly developed in the Yijianfang Formation and micritic limestone and dolomite limestone are relatively developed in Yingshan Formation.In the carbonate strata of Yijianfang and Yingshan Formation in Shunbei area,micritization,recrystallization,cementation,compaction and solution,and silicification are destructive diagenesis,while fracturing and dissolution are constructive diagenesis.The relatively small-scale dolomitization has no obvious constructive or destructive effect.The cementation studied in this paper identifies six types of cementing materials in Shunbei area,including radial axial calcite,granular calcite and calcite Cl?C4.Radial axial calcite is filled in the cracks of micritic limestone;the crystalline surface of granular calcite is clean and bright,and it mostly developes between the particles of sparry arenaceous limestone and microsparry granular limestone.Calcite C1 is the residual calcite after being replaced by silica,and its luminescence is orange red under cathode luminescence.Calcite C2 is filled between cracks or broken siliceous breccia,and it is dark red-dim light under cathode luminescence,with bright orange edge.C3 calcite is a fine-coarse-crystal calcite which is filled between the broken particles.Calcite C4 is filled in the crack with high Angle and is characterized by strong red light under cathode luminescence.Silicification is common in the Yijianfang formation,the core is in the form of strip,patch,breccia and so on,and the microstructure of the lower strip is dominated by cryptocrystalline siliceous crystals,the size of which is about 10 ?m.Brecciform siliceous is mainly composed of cryptocrystalline siliceous crystals,chalcedony and a few granular quartz crystals.Patch siliceous metasomatism of the surrounding rock fabric,retaining the surrounding rock particle morphology,mainly composed of cryptocrystalline siliceous crystals.In this paper,the geochemical analysis of calcite C1?C4 in Shunbei area by using isotopic,trace and rare earth analysis methods,and the comparison with the matrix stUcco(?13CVPDB is-1.2‰?0.3‰,?18OVPDB is-9.2‰?-6.9‰)was carried out.Calcite C1 has a negative tendency of-1.2‰?-0.9‰ for ?13CVPDB and a negative tendency of-11.7‰?-11.1‰for ?18OVPDB,with a high content of Fe and Mn,and the REE characteristics of it shows a significant shift fron right to left;The ?13CVPDB span of calcite C2 is large,which is about-3.5‰?-0.6‰,but it is still the normal range of Marine calcite.The negative of ?18OVPDB relative to the matrix is-13.4%o?-9.7‰,the content of Fe and Mn is low,and the REE feature shows a right-leaning pattern;the ?13VPDB of Calcite C3 is-2.8‰?-0.8‰,?18OVPDB is about-12.8‰?-6.0‰,Fe,Mn content is low,and the REE characteristics show a significant right-tilting pattern.?13 CVPDB and ?18OVPDB of Calcite C4 are respectively-2.0%‰?-0.8‰ and-17.1‰?-8.1‰,and rich in Fe and Mn,under different structure backgrounds.The REE distribution patterns show different characteristics:The C4 of Shunbei no.1 fault zone presents the feature of right-leaning to left-leaning excessively,the C4 of Shunbei no.5 fault zone is relatively flat,and the C4 of the Shunping no.3 fault zone presents the feature of right-leaning slightly.According to the geochemical analysis,C1 is slightly modified seawater affected by siliceous fluid,C2-C3 are inherited marine source fluid,and C4 is hydrothermal fluid.Above all,although the diagenesis experienced in Shunbei area is basically similar,the development intensity is certainly differrent.The main differences are fracturing and silicification by lateral,for example,the fracturing and silicification of Shunbei No.1 is stronger than that of Shunbei No.5 fault zone,while vertically the silicification of Yijianfang Formation is relatively common and the dolomitization of Yingshan Formation is relatively high.In addition,radial axial calcite cement formed by cementation is mainly visible in micritic limestone,while granular calcite cement is mostly developed in sparry arenaceous limestone and microsparry granular limestone;Silicification often transforms the granule limestone of Yijianfang Formation.Fracturing is usually manifested in granular limestone and siliceous limestone in the form of cracks,the main strike-slip fault effectively communicates the deep strata,so calcite C4 presents hydrothermal fluid characteristics.