The Genesis Of Formation Water And Reservoir Response Analysis Of Lower Shihezi Formation In Hangjinqi Area

The lower shihezi formation of Upper Paleozoic in Hangjinqi area,northern Ordos Basin is a typical reservoir with special low porosity and ultra low permeability,which belongs to the tight sandstone reservoir in unconventional oil and gas category.Previous studies on tight sandstone reservoirs in lower shihezi formation of Upper Paleozoic in northern Ordos Basin mainly focused on the study of reservoir petrology,diagenesis and formation water chemistry characteristics,and lacked of study on formation water evolution and reservoir's response,Which caused here is no clear understanding of the relationship between the hydraulic connection between lower shihezi formation and Shanxi Formation and the diagenesis involved in the formation evolution and the secondary pores as well as secondary minerals of the reservoir.This study takes hydrogeological analysis as the main means to analyse the influence of formation water genesis and evolution process on secondary minerals and pores of reservoir,and reveals the hydraulic relationship between strata and the genesis and evolution process of formation water as well as reservoir response.In this study,the oil and gas fields in Hangjinqi area in the northern Ordos Basin are selected as the research area.Based on the analysis of the conventional ion components and trace elements and stable isotopes of the formation water of the tight sandstone reservoirs in the lower shihezi formation of the upper Paleozoic,combined with the structural and reservoir petrological characteristics,through the source,migration power and channel of formation water to analyse the evolution process,and the relationship between formation water and gas migration and accumulation,and the relationship between formation water evolution and reservoir response is analyzed in combination with the rock mineral components and natural gas migration and accumulation characteristics of the upper Paleozoic reservoir,which provides theoretical basis for the prediction of favorable natural gas reservoirs and the evaluation of reservoir quality.The geochemical characteristics of formation water shows that the formation water in the study area is Ca Cl₂ water,which has the characteristics of low sodium chlorine coefficient,low desulfurization coefficient,low carbonate equilibrium coefficient and high metamorphism coefficient;The results show that the reservoir of the upper Paleozoic is well sealed and the oil and gas preservation conditions are favorable.The formation water has experienced a thorough desulfurization and strong water-rock interaction.Based on the comparative study of the major ions,stable isotopes and trace elements in the measured formation water and the related components of rivers,lakes and sea water,the sources of formation water are clarified,including paleometeoric water,pressure released water from underlying Shanxi Formation mudstone and coal seam,and marine formation water;Combined with the characteristics of natural gas migration and distribution in the study area,The source of formation water and the hydraulic connection between the upper Paleozoic Shanxi Lower Shihezi Formation and the Ordovician Marine Strata,as well as the relationship between formation water flow and natural gas migration are clarified,and the model of formation water flow and natural gas migration is established.On the basis of the above analysis,the rainwater evaporation concentration model in the study area is constructed.Combined with the characteristics of carbon,strontium,isotopes and main ions of formation water and the mineral composition data of reservoir,it is determined that calcitization,chloritization,kaolinization and illitization of feldspar,albitization and quartz cementation also occurred in the evolution process of formation water,which caused the formation water formed the unique hydrochemical and isotopic characteristics.The diagenesis related to formation water evolution is mainly attributed to cementation and dissolution,which not only has a direct impact on the porosity of the reservoir,but also has a certain impact on the quality of the reservoir by the spatial distribution of its secondary minerals.Based on the research results and previous quantitative models of porosity evolution,the influence of diagenesis involved in the formation water evolution process on reservoir quality is determined,and the model of formation water evolution and reservoir response is established.