| The Tianhuan Depression of Ordos Basin is close to the northwest of ordos basin and the Sulige gas field in the east,which has good exploration potential.In this paper,the diagenetic evolution and pore throat evolution of reservoirs were analyzed by logging,casting thin slice,constant velocity mercury injection,nuclear magnetic resonance and scanning electron microscope,and the favorable reservoir was predicted,which provided a theoretical basis for the future development of this area.Combined with the analysis of slice and element geochemistry,it is inferred that the source area is arshan area.The sedimentary environment is a semi-oxidizing and semi-reducing environment of shallow sea-bathyal water,fresh water-brackish water.According to the data of core,well logging and casting section,the study area is shallow water delta deposition,and the main sedimentary microfacies include distributary channel,interchannel depression,underwater distributary channel and tributary bay.The sand body is distributed from northwest to southeast,and the channel controls the distribution of the sand body.The main lithology of Shan 1 and He 8 member in the study area is lithic quartz sandstone,and the main reservoir space is intergranular pore,intergranular dissolved pore and intercrystalline pores.The average porosity is 6.4% and the average permeability is 0.1md,which are typical tight sandstone reservoirs.The reservoir has undergone complex diagenetic transformation,mainly through rigid particle compaction fracture,plastic particle compaction deformation,secondary quartz enlargement,kaolinite and calcite cementation and metasomatism,lithic dissolution and other diagenetic processes.The main source of diagenetic materials is the transformation between clay minerals and the dissolution of lithic particles.According to reservoir lithology,pore structure,diagenetic characteristics and physical characteristics,diagenetic facies in the study area are divided into 6 types and 9diagenetic facies types,among which weak kaolinite cementation diagenetic facies and lithic dissolution diagenetic facies are favorable reservoir facies.Combined with a variety of pore throat analysis techniques,it is concluded that the pore throat structure in the study area can be divided into two categories: one is pore-dominated,whose size determines reservoir quality;the other is throat dominated,whose connectivity determines reservoir quality.At present,the diagenetic stage of the reservoir in the study area is mainly located in the middle diagenetic phase B.After the compaction and the alteration of the complex base in the early diagenetic stage,the secondary enlargement of quartz and the cementation and metasomatism of calcite in the middle diagenetic phase A and the cementation and metasomatism of kaolinite in the middle diagenetic phase B,the porosity of the reservoir decreased from 38.61% to 7.47% at present,and the throat structure became more curved and complex,of which the compaction had the greatest impact on the porosity,resulting in a decrease of 26.64% Therefore,compaction is the main reason of reservoir densification.In the study area,the reservoirs can be divided into four types,and the type III reservoirs are the main types in each interval,but the type I and type II reservoirs in the upper part of He 8 are relatively large,followed by the lower part of He 8.Type I and Type II reservoirs with good reservoir quality are mainly distributed in the northern part of the study area.Therefore,the reservoir quality in the upper part of He 8 is the best,and that Shan 1 is the worst. |
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