The Secondary Logging Interpretation And Productivity Evaluation Of Reservoirs Above Chang 3 In Ansai Area

The Ansai area is located in the central part of the Ordos Basin,and is the earliest developed onshore oil field with very low permeability.The geological conditions are complex and the prominent feature is"low permeability,low pressure and low production".In recent years,the lower part of the Jurassic Yan’an Formation,Yan 8 and Yan 9 reservoirs have been discovered in the Ansai-Zhidan area,and the reservoirs above Chang 3(Yan 8,Yan 9,Chang 2 and Chang 3)are characterized by shallow depth of burial and high production compared with the lower oil formations,which have good prospects for exploration and development.However,the development effect contradicts and conflicts with the initial interpretation results,so it is necessary to conduct a second interpretation of the logs in conjunction with the current production dynamics to provide a basis for the next development and production of the study area.The study is based on logging interpretation,rock sample analysis and other techniques,and makes full use of logging,oil testing and test production data.By analysing the characteristics and relationships of the reservoir"four-property relationship of reservoirs",studying the method of establishing the interpretation model of reservoir porosity and permeability,selecting the suitable interpretation model in conjunction with the production situation in the study area,and determining the lower limit of effective reservoir and the relationship between production capacity and electrical parameters.The main insights gained from this study are as follows:(1)Yan 8 and Yan 9 are mainly developed as rock chip feldspathic sandstones,followed by feldspathic rock chip sandstones;Chang 2 and Chang 3 are mainly developed as feldspathic sandstones followed by rock chip feldspathic sandstones.The reservoirs are all dominated by intergranular pores,followed by feldspathic pores,with poor porosity and permeability,and are low-porosity-low-permeability extra-low-permeability reservoirs.Overall,the pore-permeability conditions of Yan 8 and Yan 9 are slightly better than those of the Chang 2 and Chang 3 formations.(2)A study of the correlation between depth and AC,Rt,S_O and Por shows that there are some differences in physical and electrical properties between the eastern and western parts of the Yan’an and Yanchang Formations reservoirs,with the AC and Rt being higher in the eastern part of the Yan8-Yan9 Formation above 800 m above sea level,while the Por is lower;in contrast,the AC and Rt are lower in the western part below 750 m above sea level,but the Por is higher and may be influenced by depth of burial.AC,Rt and Por increase with depth in the Chang 2-Chang 3 below 450 m and above 500 m,and change at 450-500 m,possibly due to stratigraphic denudation.Therefore,the pore seepage interpretation model,rock electrical parameter model,saturation quantitative interpretation model and mud content calculation model were established separately for each sub-layer in the shallow zone.(3)The oil-bearing index method and oil test method were also applied to discriminate between oil and water layers,determine the electrical and physical characteristics of the lower oil limit,and carry out secondary interpretation of logs in the whole area by region and by stratum,with a total of 4,599 secondary interpretations,646layers improved and 509 layers reduced.(4)Combined with the results of secondary interpretation of logs,the initial water content method was applied to establish the relationship between electrical properties and production capacity,and capacity analysis was carried out in conjunction with typical wells to clarify the factors influencing the difference in oil well production capacity.It was concluded that the reason for high production was that the reservoir developed main river channel massive sandstone with large sandstone thickness,coring showed oil immersion,high oil saturation and blocky terrace-shaped high resistance,while the reason for low production was that the reservoir developed branching river channel sandstone with thin thickness and The reason for the low production is that the reservoir is developing branching river sandstone,thin in thickness,small in size,more developed in laminar structure,and the coring shows oil spotting and dentate high resistance.Based on the results of the secondary interpretation of the sediment and logging,recommendations are made to fill the hole in the study area and provide a basis for the next step in the development and production of the study area.