Study On Sequence Division And Lithofacies Paleogeography Of Ordovician In The Western Margin Of Ordos Basin

The Ordovician in the western margin of the Ordos Basin was affected by the Helan Aulacogen of the Qin Qi He trigeminal rift system.The sedimentary water is deep,the sequence cycle is not obvious,and there is a lack of intraformational marker beds.Stratigraphic division and correlation are controversial.On the other hand,due to the weak exploration level in the western edge of the basin,the research on lithofacies and paleogeography is relatively lagging,which seriously restricts the natural gas exploration process of the Ordovician in the western edge of the Ordos Basin.Based on this,this paper uses single factor analysis and multi factor comprehensive mapping method to finely depict the distribution characteristics of lithofacies and paleogeography in the key layers of the Ordovician system,based on the division and comparison of small layers in the Ordovician system.On this basis,the development types and distribution patterns of favorable reservoirs are revealed,providing support for the exploration and development of Ordovician natural gas in the western edge of the Ordos Basin.Based on the current difficulties in dividing and comparing the Ordovician strata in the western edge of the Ordos Basin,on the basis of conventional logging,elemental logging,and imaging logging,targeted Milankovitch high-frequency cycles are used to divide and compare the key Ordovician strata in the western edge.The results indicate that the method of identifying sequence boundaries based on Milankovitch high-frequency cycles,using natural gamma logging data,has a good correspondence between the inflection points of the cumulative cycle thickness deviation curve and the third to fourth order sequence boundaries,and can establish an isochronous stratigraphic framework that is connected to the Ordovician strata in the central and eastern Ordos Basin;On this basis,by analyzing the morphological characteristics of the cumulative cycle thickness deviation curve,the Ordovician stratigraphic cycles in the western margin are divided into four types: "central eastern" type,"relative transgression" type,"relative regression" type,and "inverted S" type.The identification of sequence boundaries by Milankovi (?) high-frequency cycles can better classify strata with relatively single lithology and less significant changes in logging curves compared to conventional well logging.Moreover,due to the use of multiple methods to jointly classify small layers,the division of small layers is more accurate.Based on the division and comparison of key strata and sub layers,the Ordovician key strata in the western edge of the Ordos Basin were divided into 11 types of lithofacies using single factor analysis and multi factor comprehensive mapping method;There are differences in rock composition and sedimentary microenvironment in different facies regions,among which platform margin,ridge shaped underwater uplift and circumcontinental cloud flat are favorable facies types for the development of favorable reservoirs;At the same time,based on the analysis of micro reservoir characteristics,the effective reservoir types of key strata in the western edge of the Ordos Basin are divided into 8 types,namely,biological reef beach limestone,karst(pore and fissure)limestone,bioclastic microcrystalline limestone,mud microcrystalline dolomite,residual sandstone dolomite,grain dolomite,dissolved pore(fissure)grain dolomite,and shale micro nano pore reservoir;Effective reservoirs are mainly controlled by favorable facies belt,diagenesis,paleokarst and tectonic hydrothermal activity.Based on this,the diagenetic sequence and pore evolution model of key reservoir series in the western edge of the basin are established,and the distribution law of favorable reservoirs is revealed.The research results provide geological basis for the deployment of natural gas exploration in the western edge of the Ordos Basin.