Study On The Relationship Between Formation Water And Reservoir In Chang 8 And Chang 9 Low Permeability Sandstone Reservoirs In Jiyuan Area,Ordos Basin

In recent years,great achievements have been made in the exploration and development of low permeability sandstone reservoirs of the Yanchang Formation in Ordos Basin.With the accumulation of the technical means and experience,systematic understanding has been achieved in structure,sediment,reservoir,diagenesis and reservoir formation,but relatively little research has been done on the formation water.Formation water is closely related to reservoir migration,accumulation and preservation.How to use formation water to guide exploration and development of low permeability sandstone reservoirs in Yanchang Formation is a problem to be solved.Based on a large number of formation water analysis data of Chang 8 and Chang 9 formation in Jiyuan area,using hydrogeology,petroleum geology and geochemistry analysis methods,this paper systematically studies the formation water characteristics,genesis and its relationship with reservoirs in Chang 8 and Chang 9 low permeability sandstone reservoirs,and obtains the following main understandings:(1)The salinity of the formation water in Chang 8 and Chang 9 reservoirs in Jiyuan area is relatively high.The formation water is mainly calcium chloride type,the average sodium chloride coefficient is low,and the average metamorphic coefficient is high.All these reflect that the whole formation of Chang 8 Chang 9 reservoir group is well sealed and conducive to the preservation of oil.(2)The formation water of Chang 8 and Chang 9 reservoirs is mainly sedimentary-burial water,and there is mixed water of leaching-infiltration water and sedimentary burial water locally.The original sedimentary water of Chang 8 and Chang 9 reservoirs is mainly fresh water,which are concentrated and desalinated at different burial depths in the burial process,making the formation water of Chang 8 and Chang 9 reservoirs have obvious vertical zoning characteristics.According to the vertical distribution characteristics of formation water chemical parameters,they can be divided into four zones in turn: mudstone compaction drainage desalination and mudstone pressure filtration concentration zone,clay mineral desalination zone,clay mineral percolation concentration zone and clay mineral desalination zone.(3)Formation hydrodynamic field controls the plane distribution of hydrochemical field in Chang 8 and Chang 9 formations.There exists downward seepage and centripetal flow of exotic fluid in the Western thrust zone of the study area.The axis of Tianhuan depression and the central part of Yishan slope in the study area are the center of compacted centrifugal flow.The centrifugal flow superimposed overflow drainage zone is formed between the two zones in the transition zone between Tianhuan depression and Yishan slope.The formation water in the centrifugal flow area is desalinated by surface water or shallow water,the centrifugal flow central formation water is desalinated by mudstone compaction and drainage,the salinity and ion concentration of formation water increase along the direction of centrifugal flow,and reach the maximum in the overflow drainage area.This is mainly due to the seepage concentration in the process of formation water flow.(4)The formation water ions of Chang 8 and Chang 9 reservoirs are greatly influenced by water-rock interaction.Sodium ion depletion and calcium ion enrichment in formation water are mainly controlled by plagioclase albitization.The slight enrichment of local sodium ions may be related to feldspar dissolution.The low enrichment of local calcium ions is controlled by carbonate cementation and precipitation.Magnesium ion depletion is mainly related to chloritization and sulfate radical dissociation.The content of hydrogen carbonate is mainly related to desulfurization and mudstone compacted water entering formation water.The low content of hydrogen carbonate ion is mainly related to carbonate cementation formed in late stage.(5)The difference of water characteristics between Chang 8 and Chang 9 in Tianhuan depression is mainly related to the compaction and drainage of mudstone at the top of Chang 9.The axis of Tianhuan depression is the center of compaction centrifugal flow,and the degree of mudstone compaction is large.In the process of mudstone compaction and drainage at the top of Chang 9,due to the difference of physical properties between Chang 8 and chang 9 reservoirs,it is easier for mudstone compacted water to enter chang-9 reservoir with good permeability.At the same time,the good sand body connectivity of chang-9 reservoir group expands the desalination range of mudstone compacted water to formation water,which leads to the difference between Chang 9 formation water and Chang 8 formation water in Tianhuan depression.(6)Based on the comparative analysis of the formation water type and the dominant migration channel of oil and gas such as fault fracture,in Yanchang Formation,it is considered that under the background of calcium chloride type formation water as the main body,local non calcium chloride type formation water is the response characteristics of formation water with dominant migration channels such as faults and fractures nearby.If there is non calcium chloride type formation water in the same area of Chang 8 and Chang 9,it indicates that there is a dominant channel for vertical migration of oil and gas in this area.(7)Based on the variation of formation hydrodynamic field and geochemical parameters of crude oil,the secondary migration direction of oil and gas in Chang 8 and Chang 9 reservoirs is judged respectively.It is found that the secondary migration direction of oil and gas coincides well with the variation of formation water salinity,especially in Chang 9 reservoir group.Therefore,Chang 9 reservoir group in the study area can use the changing trend of formation water salinity to indicate the direction of secondary migration of local oil and gas together with other methods.