Control Factors On Favorable Tight Reservoirs Of The Upper Paleozoic In The Hangjinqi Area,northern Ordos Basin

Hangjinqi area is an important position for tight gas exploration in Ordos Basin.Large-scale tight sandstone gas reservoirs are developed in the Upper Paleozoic strata.However,the development mechanism of the reservoir is not clear in the area,the transformation process is complex,and the spatial heterogeneity is strong.The controlling factors and the plane distribution characteristics of favorable reservoirs are the difficulties of current research.In this paper,the Upper Paleozoic tight sandstone reservoir in Hangjinqi area of Ordos Basin is taken as the research object,and the sedimentary characteristics and reservoir characteristics of Shanxi and Lower Shihezi Formations of Upper Paleozoic are emphasized by comprehensive utilization of the analysis and laboratory data,the observation of casting slices,cathodoluminescence and scanning electron microscopy,combined with carbon and oxygen isotope analysis and fluid inclusion analysis and testing.The pore evolution process of favorable reservoirs is defined by inversion and stripping method,and the pore evolution model is established.Combining with the analysis of reservoir control factors,the reservoir evaluation criteria and the distribution model of favorable reservoirs are established.The study shows that the sedimentary facies types of the Upper Paleozoic tight sandstone reservoirs in Hangjinqi area are mainly braided river delta front subfacies and braided river delta plain subfacies.The lithology of the reservoir in Shan 1 is mainly sublithic sandstone,Shan 2 is mainly sublithic sandstone and lithic sandstone,and the reservoir lithology in Box 1is mainly lithic sandstone.The whole reservoir is characterized by high quartz,low feldspar,high lithic,low component maturity and low structural maturity.Lithofacies types mainly include gravel sandstone,coarse sandstone,medium-coarse sandstone,medium sandstone and fine sandstone,of which the content of coarse sandstone and medium-coarse sandstone is higher.Reservoir space types of tight sandstone reservoirs in the study area are various,mainly developing secondary pores such as intergranular dissolution pore,intragranular dissolution pore,casting pore,kaolinite intergranular pore and some fractures.Diagenesis is complex.Destructive diagenesis such as mechanical compaction,cementation,metasomatic filling and constructive diagenesis such as atmospheric water leaching and formation fluid dissolution are developed.It mainly undergoes three stages of diagenetic fluid evolution,which is characterized by acid-base alternation,including weak alkaline fluid controlled by original lake sedimentary water,acid fluid controlled by atmospheric fresh water leaching,and alkaline fluid controlled by sedimentary formation water in burial environment.The sequence of diagenesis evolution is: Mechanical compaction?Chlorite-coated rim?Pyrite cementation?Early Carbonate Cementation?Quartz secondary enlargement?Feldspar dissolution /Debris dissolution?Authigenic kaolinite / montmorillonite illitization?Authigenic quartz particles?Quartz dissolution / Quartzite dissolution?Late Calcite Cementation / Iron calcite cementation ? Kaolinite illitization.The quantitative model of pore diagenetic evolution is established by inversion and stripping method,and the quantitative process of pore diagenetic evolution is clarified to restore the diagenetic evolution history,thus providing theoretical parameters for the study of reservoir genesis mechanism.The influence of debris development on reservoir physical properties is multifaceted.Taking the Lower Shihezi Formation reservoir in the study area as an example,according to the influence of debris types on reservoir physical properties,all debris types are divided into three categories: A(pore-reducing debris),B(pore-increasing debris)and C(pore-preserving debris).The pore-reducing debris include mudstone debris,schist debris,slate debris and phyllite debris;the pore-increasing debris include metamorphic quartz sandstone debris,sandstone debris,metamorphic quartz debris,carbonate debris,granite debris and acid eruptive debris;and the pore-preserving debris mainly include vein quartz debris and flint debris.Based on the study of the basic characteristics of reservoirs,diagenetic environment evolution and diagenetic sequence,it is clear that the prerequisite for the formation of favorable reservoirs is favorable original sedimentary conditions,which mainly include dominant sedimentary microfacies and coarse clastic original sediments.The key to the improvement of reservoir quality depends on the original grain composition.In Lower Shihezi Formation reservoirs,quartz in skeleton grains is the key factor.The physical properties of reservoirs are controlled by debris B and C,while the positive correlation between quartz particles and reservoir properties is good in Shanxi Formation reservoirs.The development of extensive dissolution pore during burial diagenesis is the direct reason for the formation of favorable reservoirs.Through the above systematic research,the Upper Paleozoic reservoirs in the study area are divided into type ?—Favorable reservoirs,type ?—Subfavorable reservoir,type ?—General reservoirs and type ? —Non-reservoirs.The distribution of favorable reservoirs is mainly controlled by the dominant sedimentary microfacies and the relative coarse-grained lithofacies,The total content of quartz,B-type debris and C-type debris in skeleton grains jointly control the physical characteristics of reservoirs.Late diagenesis is a necessary condition for reservoir development,and the relative strength of compaction,cementation and dissolution can also be used as the basis for reservoir classification.