Sequence Stratigraphy And Reservoir Characteristics Of The Ma5₆ Sub-Member Of The Daniudi Gas Field,Ordos Basin

In the late period of the Caledonian tectonic movement,the northern part of the Ordos platform was uplifted,and the Lower Paleozoic carbonate rock suffered a weathering and dissolution of up to 140 Ma,and formed a unique karst landform.The Daniudi gas field is located at the junction of the paleo-weathering crust highland and slope,which already has reservoir development conditions.The existing exploration results also show that the Lower Paleozoic strata of Daniudi gas field have good natural gas exploration potential,but the low current approved resources,indicating that the exploration potential of natural gas is huge,and the Ma5₆ sub-member,as a potential gas layer,has not been studied in sequence stratigraphy,sedimentary facies and karst reservoirs.What's more the reservoir characteristics and development laws of the Ma5₆ sub-member are still Not sure.On the basis of fully absorbing the previous research results,this paper analyzed the sequence stratigraphy,sedimentary facies,karst facies and reservoirs of the Ma5₆sub-member through core,thin-film observation,cathodoluminescence,CT scanning,and high-pressure mercury intrusion.The physical properties and diagenesis of the stratum are studied.As a result,the main controlling factors of effective reservoir in Ma5₆ sub-member are clarified,and the distribution of effective reservoirs in the study area are predicted.The paper has achieved the following results:?1?The Ma5₆ sub-member is located in the forced retreat system terrain?FRST?of the Ordovician third-order sequence SQ2,which is formed in the continuous decline stage of the third-order sea level.In this sedimentary period,the evaporation is strong,the climate is dry and hot,it can be identified that the research area is a prograding quasi-sequence group consists of two quasi-sequences.?2?The Ma5₆ sub-member is belongs to sedimentary environment of restricted platformin epeiric sea,which mainly controlled by the subtidal-tidal zone.The rocks are mainly gypsum contained dolomite,dolomicrite,argillaceous dolomite,and micrite.In addition,The gypsum contained dolomite related to the reservoir mainly develops in the intertidal zone.?3?The Ma5₆ sub-member is severely brecciated and can be divided into four karst facies.They are crackle-mosaic breccia facies,clast supporting chaotic breccia facies,matrix-rich chaotic breccia facies,and cave filling facies.The crackle-mosaic breccia facies has large thickness and good continuity.It is mainly developed in the Ma5₆² small stratum and the Ma 5₆⁴ stratum,with good physical properties.While the clast supporting chaotic breccia faciesand matrix-rich chaotic breccia facies have thin thickness and poor continuity,mainly developed in Ma5₆¹ stratum and Ma5₆³ stratum.As for cave filling facies,the thickness is very thin and the continuity is very poor,it is mainly developed in the Ma5₆¹ stratum.?4?The reservoir space types of the Ma5₆ sub-member are mainly not completely filled gypsum moldic pores,intercrystalline pores,intergranular dissolution pore and fractures.By analyzing the correlation of different physical properties,it can be concluded that the overall physical properties are relative poor,with an average porosity of 2.39%,and an average permeability of 0.06×10^-3?m².The crackle-mosaic breccia facies gypsum-containing dolomite have relative better physical properties.?5?The diagenesis of the Ma5₆ sub-member is complex and various.Particularly,dolomite,dissolution and fracturing have obviously constructive effect on reservoir property,Among them,the dissolution is the most important.while filling and cementation cause significant loss of pores in the study area and have a destroy effect..?6?The reservoir development of the Ma5₆ sub-member is mainly controlled by four factors:sequences,sedimentary facies,paleo-karst facies and diagenesis.The sequences and sedimentary facies control the lithology,while the lithology and fresh water properties control the distribution of karst facies.The distribution of the paleo-karst facies and the difference in diagenesis processes then control the development of the reservoir.The favorable zone of study area is related to the distribution of crackle-mosaic breccia facies.In this paper,favorable area for reservoir development meet the condition that crackle-mosaic breccia facies more than 5m thick,and porosity more than 3%.On this basis,the favorable area prediction of Ma5₆² stratum and Ma5₆⁴ stratum can be carried out.The research results provide geological basis for futher exploration and development of Daniudi gas field.