Classification Evaluation And Evolution Characteristic Of Tight Reservoirs In The Tongbomiao-Nantun Formation Of Wuerxun-Beier Depression,Hailaer Basin

Hailaer basin is the largest oil-bearing basin in the peripheral basin of Songliao Basin in Daqing exploration area,among which The remaining resources of Wuerxun depression and Beier depression have great potential.With the increasing difficulty of exploration,it is more difficult to find traps with shallow burial,large scale,good shape and good reservoir forming conditions,which leads to the transition of exploration targets to unconventional reservoirs.This paper focuses on the reservoir characteristics and hydrocarbon accumulation law of Wuerxun-Beier depression,with the help of micro-nano CT scanning,nuclear magnetic resonance analysis,petroleum injection simulation experiment,argon ion polishing and field emission scanning electron microscopy analysis,the pore structure characteristics of the reservoir were finely characterized,and then the dynamic evolution process of pore diagenesis was restored.Based on the main controlling factors of reservoir forming elements,analysis of experimental results,diagenetic facies division and new reservoir forming model,the distribution rules of high-quality reservoirs are clarified and favorable exploration target areas are pointed out.The results show that the reservoir in the study area is characterized by rich volcanic rock cuttings,a large number of argillaceous complex groups are filled between grains,and the reservoir physical properties are characterized by"high porosity and low permeability".Under the influence of source input,the porosity and permeability of the reservoir gradually decrease along the source to the center of the lake basin,and the reservoir space is dominated by dissolution pores,followed by primary intergranular pores.Intergranular pores are mainly developed in the collection body of clay minerals,but the presence of clay minerals reduces the primary residual intergranular pores.The dissolution of acidic fluid is the main factor for the formation of secondary pores in this area,which provides a large amount of space for oil and gas accumulation.The continuous diagenetic transformation further complicates and enhances the diversity of reservoirs whose original fabric is already quite different.The diagenetic facies belts of the reservoirs in Wuerxun-Beier Depression can be divided into five types:strong dissolution facies,strong cementation weak dissolution facies,strong compaction weak dissolution facies,strong cementation tight facies and strong compaction tight facies.Due to the differences in the original sedimentary fabric and diagenetic transformation,the five types of rocks have different densification processes.Strongly compacted tight facies sandstone undergoes plastic deformation and particle rearrangement under the overlying load and confining pressure in the early stage of diagenesis,and becomes an ineffective reservoir before the critical accumulation period;strong cemented tight facies sandstone increases with burial depth And some unstable components continue to dissolve,and the Ca²⁺content in the pore fluid gradually increases and reaches supersaturation.As the temperature and pressure conditions enter the window period,Ca²⁺combines with HCO₃^-to form carbonate cements,which continue to precipitate and even the critical accumulation period Previously completely cemented pores made this part of the sandstone an invalid reservoir;the strong cementation facies,the strong cementation weak dissolution facies and the strong compaction weak dissolution facies sandstones alternately occur during the densification process,and still have relatively high levels during the critical accumulation period.The good physical properties are only due to the selectivity of oil and gas charging caused by the seepage barrier formed by the heterogeneity of the reservoir structure.Some of them have been charged and some have not yet been charged.Based on the classification of lithofacies,different typical samples of different lithofacies were selected to carry out supporting experiments such as CT scan,high pressure mercury intrusion,constant velocity mercury intrusion,and nuclear magnetic resonance.It was found that the Tongbomiao and Nantun reservoirs can be divided into five types.The corresponding reservoir pore throat structure types are coarse pore throat-roughness-poor sorting,medium coarse pore throat-medium distortion-poor sorting,medium pore throat-medium fine distortion-good sorting,fine pores Throat-fine skewness-good sorting,very fine pore throat-fine skewness-good sorting.From the first to fifth types of reservoirs,the displacement pressure and the median saturation pressure of the rock samples gradually increase,and the pore throats gradually become smaller,indicating that the irreducible water saturation of the reservoir rocks gradually increases,the percolation capacity decreases,and the mining difficulty increases.Using empirical statistical method,the lower limit of physical property in the study area was determined as follows:porosity of 4%,permeability of 0.02md;On this basis,combined with the fluid flow characteristics,the reservoir types can be comprehensively classified into conventional reservoirs,tight reservoirs I,tight reservoirs II and invalid reservoirs according to the diagenetic facies types and characteristics,the lower limit of reservoir physical property,rock stress sensitivity and characteristics of oil seepage.