Study On The Characteristics And Main Control Factors Of Chang6₃ Reservoirs Of HuaQing Area In Ordos Basin

The Chang6 layer of Yanchang formation in Hua Qing Oilfield in Ordos basin is a tight sandstone reservoir which is abundant in oil and gas resources.The development of this reservoir shows that the quality of reservoir is one of the main factors which restrict the oil production.Some essential differences between the tight sandstone reservoir and conventional reservoir have also been found during the oilfield development.The microscopic characteristic of tight sandstone reservoirs is the key factor to influence the reservoir performance and seepage capacity and these characteristics directly determines the displacement and recovery efficiency of oil and gas.In order to improve the production of this reservoir,some characteristics of tight sandstone reservoirs should be fully studied and quantified such as properties of reservoir and rocks,types and structure of pores and pore-throat connectivity.In order to improve the accuracy of reservoir microscopic characteristics study,this dissertation has used varieties of methods to obtain comprehensive information including physical properties,lithology,pore structure and movable fluid distribution.The tests have been used in this study including the casting thin sections,image analysis,high-pressure mercury injection,constant velocity mercury injection,NMR,SEM,oil-water displacement and X-ray CT scans.The result shows that:The maturity of the mineral is not high and the content of feldspar is slightly higher than that of quartz,and the interstitial material is mainly clay minerals and carbonates and the distribution of interstitial material is not homogeneous.Chang 63 layer of Hua Qing oilfield has been experienced a series of diagenesis processes including compaction,cementation and dissolution.The average porosity after varieties diagenesis is 10.73% and the average permeability is 0.273×10-3?m2.Based on classification of porosity and permeability,Chang 63 layer of Hua Qing oilfield belongs to the ultralow porosity-low porosity and ultra-low permeability reservoir.The statistical correlation coefficient of the porosity and permeability in the study area is only 0.58 and the result show that the diagenesis and complexity of pore structure is the main factor that causes the low correlation coefficient between porosity and permeability.Based on data gained from mercury injection experiment analysis,the reservoir can be divided into four categories.A further analysis have been made combining physical property of each kind and discover that the type ?pore structure has the best physical property and type ? has the worst property.The analysis of pore structure show that type? and ? pore structure are the main types of pore structure in study area.The water displacing experiment has been conducted to verify the classification of pore structure and the results show that the classification is reasonable.Mercury injection and water flooding experiment also show that the study area has a strong microscopic heterogeneity and the pore radius distribution is concentrated.However,the shape of throat in study area is various and the radius of throat is small and varies from sample to sample.Based on analysis above,the throat radius and the ratio of pore to throat are the main factors that lead to the complex pore structure of reservoir in the study area and the micro pore structure determines the efficiency of oil displacement directly.The NMR experiment shows that the reservoir has a low movable fluid saturation in the study area.The difference of pore structure is the main factor that affects the movable fluid saturation.Sedimentation has an influence on reservoir quality by controlling the mineral content,maturity and content of clay minerals;Compaction is the main factor leads to the reduction of pore space,the cementation is the main factor to destroy the internal pore structure of the reservoir and make reservoir heterogeneity stronger;The difference of micro pore structure causes the differences in seepage characteristics of reservoir.The weight coefficient of main factors which influence reservoir quality have been calculated by cluster analysis.Base on the score derived from the results of weight coefficient,the reservoir can be divided into four types and the type ? and ? reservoir are the main target of further exploration.