Reservoir Characteristics,Formation Mechanism And Main Controlling Factors Of Majiagou Formation In Northern Ordos Basin

The deep marine carbonate layers onshore are the focus of oil and gas exploration nowadays.The karst and dolomite reservoirs are the two main reservoir types in the Ordovician Majiagou Formation of northern Ordos Basin,which is constantly considered as the hotspot for oil and gas exploration and development.This study emphasizes on the Lower Paleozoic Majiagou Formation in Daniudi and Hangjinqi area.In Daniudi,the targeted stratum ranges from M₅⁶ to M₄,while in Hangjinqi,the study focuses on the M₄ layer.We are aiming to improve the classification of reservoirs with strong heterogeneity and establish an interpretation model for pore analysis by accurately characterizing the Lower Paleozoic carbonate reservoir in the northern basin.We are also aiming to build A comprehensive and systematic classification scheme,clarify formation mechanism and main controlling factors of reservoirs,predict the sweet spot for reservoirs.The study results can provide direction and insights for further exploration and development of the Lower Paleozoic carbonate reservoir in Ordos basin.Lithological properties and characteristics of wireline logs was used for determining the lithological association and microfacies of the reservoir.After characterization of the reservoir space from multiple scales on different level,we build the compute model of porosity and the standard of reservoir classification.Through pore type recognition,diagenesis and geochemical analysis of different reservoirs,the formation mechanisms of different reservoirs were generalized.The main controlling factor of the reservoirs are extracted by considering diagenesis and sedimentary environment,and the distribution of different types of reservoirs are predicted.Limestone,dolomite,evaporites,and transition rocks are sediment lamelleted in Majiagou Formation.Dolomites are mainly microlite dolomite,micritic dolomite dolomite,gypsum-bearing dolomite and gypsum dolomite.The rock assemblage of intertidal zone and subtidal zone develops vertically.The sedimentary characteristics and geochemical characteristics prove that lithology is controlled by the salinity of seawater.In Majiagou Formation,dolomite was produced by seepage refluxion dolomitization in the seawater environment with high salinity near the surface.Considering the reservoir heterogeneity,a pore calculation model applied to Majiagou Formation is established by analyzing the loggings and pore characterization results.Based on the clasification and fomation mechanism of pore space in reservoir,the reservoir types are classified into dolomite porouos reservoir,limestone paleocave reservoir,gypsum bearing dolomite fracture-vuggy and vuggy reservoirs.Dolomite reservoirs are all porous type reservoir and developed in micritic dolomite in the second member of M4 and the fifth member of M5 in Daniudi formation.Pore space of dolomite reservoirs is mainly consist of intercrystalline pores.The openness of the sedimentary environment causes the changing in salinity and the constituent of dolomite.The content of dolomite decreases gradually,because the strength of dolomitization is gradually weakened with the fluid migration.Only when appropriate dolomitization occurs can dolomitization contribute to the formation of pore space.Thus,the degree of dolomitization are the dominating factors of the dolomite pore reservoirs.Breccia reservoir include fracture-vuggy and vuggy reservoirs,pore space is dominated by incompletely filled gypsum mold pores and vugs,and intercrystalline dissolved pores.Karstification conducted long the strata.Different rock composition resulted in the difference of dissolution effect.Breccia assemblages are superimposed on each other,and are summarized and summarized as six types,which are crackle-mosaic breccia+grain-supported chaotic breccia,crackle-mosaic breccia+matrix-supported chaotic breccia,grain-supported chaotic breccia+matrix-supported chaotic breccia,Matrix-supported chaotic breccia+cave fillings conglomerate,Matrix-supported chaotic breccia+Cave fillings conglomerate,Crackle-mosaic breccia+Cave fillings conglomerate,respectively.In the north,west and south of Daniudi,M₅⁶ is dominated by crack-mosaic breccia+grain-supported chaotic breccia.The content of undisturbed carbonateis increases from the north to the south.The degree of brecciation increases,and the content of crack-mosaic breccia decreases gradually from the west to the east.In the east,M₅⁶ is dominated by crack-mosaic breccia+matrix-supported chaotic breccia.Among all type of breccias assemblages,crack-mosaic breccia+grain-supported chaotic breccia assemblage can always present as good quality reservoir,and crack-mosaic breccia have the best petrophysical property of all breccias.Where crack-mosaic breccia occupied more than 50%,and the accumulated thickness is greater than10m in M₅⁶ is most likely to become good quality reservoir.Reservoir quality in the south and the west of Daniudi is generally better than the other counterparts.The gypsum-bearing dolomite is developed in the intertidal dolomite flat facies of early regressive,which is the basis of breccia reservoir formation.The solubility of gypsum,limestone and dolomite decrease in turn.When the fluid is saturated with limestone and dolomite,it can still dissolve the gypsum within the layers.What's more,the micro-cracks produced by gypsum expansion are conducive to further dissolution.According to reservoir formation mechanism,breccia distribution and petrophysical properties,the distribution of breccia reservoir is controlled by sedimentary microfacies and karst facies.Paleocave carbonate reservoir is located in thick micritic limestone of Hangjinqi M4 fomation.The reservoir spaces are caves that filled incompletely in cave system.The paleocave system is formed by different types of breccias by a cretain way.From bottom to top,the paleocave system successively developed superimposed cave collapse facies and sediment filled from underground river,unfilled cave facies,cave roof facies and cave wall facies.Among them,cave collapse facies dominant by grain-supported chaotic breccia,sediment filled from underground river is consist of matrix-supported chaotic breccias and cave fillings,cave roof facies and cave wall facies are consist of crackle-mosaic breccias.The unfilled cave facies only exists beneath the crackle-mosaic breccias.The formation of paleocave reservoir is related to the distance from the base level.As the limestone is naturally dense,when fresh water migrates down to the phreatic zone through the surface,a large scale dissolution of the surrounding limestone happened during the lateral migration,forming layered caves.The position of the water table changes by tectonic uplift.Three layers of caves can be recognized in the M4formation of Hangjinqi.Caves located in vally are easily eroded by river cutting,are not easy to be preserved.The distance of the base level and paleogeomorphology are the main controlling factors for the development of the limestone cave.In this dissertation,reservoirs are characterized by different levels and multi-scales in detail,the distribution and reservoir physical properties of breccia formed by karst action are summarized.The study fills in the shorts of previous studies,and set the basis for discussing the classification,origin and prediction of carbonate reservoirs.The results have extended the exploration scope of lower Paleozoic carbonate reservoirs in Ordos Basin,which is of great significance.