Diagenetic Facies And Main Controlling Factors Of Upper Paleozoic Reservoirs In The Southern Ordos Basin

The Upper Paleozoic reservoir sandstone in southern Ordos Basin is a typical low-permeability reservoir,which is mainly affected by its late diagenesis.The Benxi Formation in the study area mainly developed the barrier island-semi-enclosed lagoon-tidal flat system,Shanxi Formation and He 8 Section mainly developed the shallow-water delta system,including meandering river delta and braided river delta.Due to the low level of exploration,the diagenesis and pore evolution of the target strata and the relationship between diagenetic facies distribution and sedimentary facies and high-quality reservoirs still need further discussion.The research of He 8 Section,Shanxi Formation and Benxi Formation of tight sandstone reservoir petrology,diagenesis,pore types based on field survey,core observation,conventional thin section and cast thin section observation,scanning electron microscope,X ray diffraction analysis,which is in-depth exploration,to restore the pore evolution and divide diagenetic facies.The distribution of diagenetic facies is related to sedimentary facies,the distribution of the sand bodies and logging identification marks.The results are as follows:(1)The main types of reservoir sandstone in the study area include lithic quartz sandstone,lithic sandstone and quartz sandstone.Among them,He 8 section and Shanxi Formation are mainly lithic quartz sandstones,the sandstone of Benxi formation is mainly quartz sandstone.Different lithology have a certain influence on diagenesis,combined with the physical data and thin sections,it was found that sandstones with coarse granularity,better sorting and grinding,and higher compositional maturity are more conducive to the preservation of pores,crude-medium quartz sandstone and medium-crude lithic quartz sandstone have better physical properties than the medium fine lithic quartz sandstone and the medium lithic sandstone.(2)Erosion is the main controlling factor for the development of effective reservoir,and the formation of dissolution pores are the main reservoir space.Acidic water from early coal-bearing formations dissolved feldspar and provided siliceous materials as a source of siliceous cements.However,the content of feldspar in the study area is less,the secondary pores are limited and the cementing material has a large loss in filling.From the Triassic to the Cretaceous,organic acid entered,eroded rock debris,clay miscellaneous,cement and some quartz edges,forming intragranular or intragranular dissolution holes,transforming and expanding the residual primary intergranular pores.The surface clean quartz particles are less heterozygous,which is beneficial to the hole roaring connectivity,and the dissolution pores are easier to be preserved.The late microfracture transformation made some micropores connected and formed the present pore structure,but the whole pore size was small and the connectivity was poor.(3)The favorable diagenetic facies of He 8 section of the study area and the Shanxi Formation are medium compacted silica cemented dissolution phase and medium compacted muddy cemented dissolution phase.It is mainly developed in the braided channel sandstone,followed by the underwater branched channel sandstone of the tribe of the meandering river delta.Strong compaction siliceous cemented clay filling microporous facies is a neutral diagenetic facies,which is widespread in braided delta branched channel sandstone,subaqueous distributary channel sandstone and meandering delta subaqueous channel sandstone.The favorable diagenetic facies of Benxi formation are siliceous cementation dissolution facies,calcareous cementation mud filling micropore facies,siliceous cementation mud filling micropore facies,which develops in the barrier island sandstone.