| The lower member of Yanchang Formation in Ordos Basin is a research hotspot.Chang8 and Chang 9 in Baibao area,southwest of Ordos Basin,are important strategic succeeding pay zones.At present,there are still a series of problems to be solved in the study of deep tight sandstone reservoir,including the micro pore throat characteristics of the tight reservoir and its control on oil enrichment,and the lack of deep understanding of hydrocarbon generation potential and accumulation power of source rocks in the study area,which leads to the unclear oil enrichment law in the lower member of Yanchang Formation in the study area.In order to clarify the oil enrichment law of Chang 8 and Chang 9 formations in the study area,the petrological characteristics,diagenetic characteristics,lower limit of reservoir physical properties and physical property evolution characteristics of Chang 8 and Chang 9reservoirs in this area were deeply explored by using scanning electron microscope,casting thin section observation experiment,X-ray diffraction experiment and physical property test.The pore throat distribution characteristics of Chang 8 and Chang 9 reservoirs in the study area are compared and analyzed by image pore,high pressure mercury injection and constant velocity mercury injection experiments.Four types of reservoir types with pore throat quality are classified according to quantitative parameters.At the same time,the full-scale pore throat distribution characteristics of different types of reservoirs are obtained by combining mercury injection experiments with nuclear magnetic resonance T2 spectrum distribution characteristics.The percolation characteristics of Chang 8 and Chang 9 reservoirs are quantitatively analyzed by oil-water relative permeability experiment,nuclear magnetic resonance experiment and nuclear magnetic imaging technology.In view of the physical properties,pore throat characteristics and seepage capacity of reservoirs,seven effective parameters are selected and four types of reservoirs are divided by cluster analysis to realize the vertical and horizontal continuous classification of reservoirs.The distribution characteristics of hydrocarbon generation capacity and accumulation power of main source rocks in the study area are calculated by using?log R method and equilibrium depth method respectively.Finally,combining the quality of source rock,reservoir forming power and high quality reservoir,the vertical and horizontal distribution law of oil enrichment area in the study area is clarified.The results of this study are as follows:(1)The study interval is a typical dense fine-grained arkose or lithic arkose reservoir.The reservoir physical properties of Chang 8 and Chang 9 are poor,of which the samples with permeability less than 0.1×10-3?m2 account for 21.4%.The lower limits of porosity and permeability of Chang 8 and Chang 9 reservoirs in the study area are obtained by empirical statistics and cross plot of porosity and permeability,which are 5.78%and 0.05×10-3?m2,respectively.(2)The tight reservoirs of Chang 8 and Chang 9 in the study area are mainly in the B stage of middle diagenesis.After the comprehensive influence of diagenesis in different stages,the reservoir pore types are mainly residual intergranular pores protected by membrane clay minerals and laumontite dissolved pores and feldspar dissolved pores produced by dissolution.(3)Based on the quantitative parameters obtained from image pore experiment and non wetting phase injection experiment,Chang 8 and Chang 9 reservoirs are divided into four types of pore throat structure;the full-scale distribution characteristics of reservoir are obtained by mercury injection experiment combined with nuclear magnetic resonance T2spectrum,and high-quality reservoirs are mainly"bimodal",while type III and IV reservoirs are mainly"left single peak".The reservoir with high quality pore throat structure usually has stronger recovery ability.(4)According to the comprehensive utilization of"three aspects and seven parameters"of reservoirs,the production performance of different types of reservoirs is obviously different.The high-quality reservoirs with good pore throat structure(type I and type II)are the important controlling factors of oil accumulation.(5)The Chang 7 source rock in Baibao area of Ordos Basin has a large thickness,and the average value of total organic carbon content and hydrocarbon generation potential(Pg)of shale is high,which is an important source rock for Chang 8 and Chang 9 reservoirs in the study area;the thickness of effective source rock of Chang 9 in study area is relatively thin,mainly distributed in the central and western regions of the study area,which is an auxiliary hydrocarbon generation layer.(6)The overpressure produced by hydrocarbon generation pressurization and undercompaction of large mudstone is the main driving force of oil accumulation in Chang 8and Chang 9 tight reservoirs.The overpressure produced by Chang 7 source rock is calculated by equilibrium depth method,and its average value is 11.8 MPa.(7)The reservoir forming rules of Chang 8 and Chang 9 in the study area are slightly different.The combination relationship between the upper reservoir of Chang 8 member and the source rock of Chang 7 is good.Therefore,it is necessary to predict the oil-gas accumulation area of this interval by combining the distribution characteristics of high-quality reservoir and effective source rock.The Chang 9 reservoir is far away from the main hydrocarbon generating layer of Chang 7,so it is necessary to combine the quality of source rock,accumulation power and reservoir quality distribution characteristics,and comprehensively predict the oil accumulation area of Chang 9 reservoir by using production and test data. |
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