The Study Of Geological And Geochemical Characteristics Of The Brines In Sichuan Basin

The paper focuses on the brine of major halogen-containing layer on the south of Sichuan Basin supplemented by other areas of the basin,some of the samples were collected continuously.Combined with previous studies data and related isotopic data,the paper firstly discusses the deuterium excess evolution of brines,which provides a new thought and forms the basis for further study of the brine accumulation mechanism.By studying of the geological structural characteristics of brine-bearing stratum,evolution of brines fluid source of Upper-Middle Triassic and the forming of potassium rich brines,we draw some conclutions as follow:?1?Comparing with other brine-bearing strata,Triassic Leikoupo-Jialingjiang Formations which brine reservoir is widely dispersed over the Sichuan basin,have high quality brine and long developmental history.The study indicates that the brine reservoir is composed of compact rocks with a low porosity and a low permeability,being unfavourable for brine migration.Brine migration and accumulation are cont rolled by f racture systems.?2?The hydrogeochemical characteristics of the brines in Sichuan basin assume obviously layered and partitioned.The major hydrogeochemical compositons,in keep with salinity,are consistent in the same Formation and variant between different Formation.The brine-bearing strata of Upper-Middle Triassic and Cambrian which were marine sedimentary have high formation water salinity,mainly Cl-Na style with high SO₄^2-.This kind of strata is characteristic by high K⁺?B₂O³ without Ba²⁺,particularly such as T₂l¹?T₂l³ and T₁j⁵.?3?On the?D-?¹⁸O plot,the oxygen and hydrogen isotope compositions of brines sampled in marine sedimentary brine-bearing strata fall in the right part of the plot,and the hydrogen isotope compositions are lower than SMOW,while oxygen isotope compositions are higher than SMOW.From Sinian stratum,up to Cambrian,Permian Maokou Formation Triassic Jialingjiang Formation,the?D value of brines gets relatively positive with the strata becoming younger.Brines of terrestrial deposit are considered to be derived from precipitation and the isotope compositions fall in the neighborhood of the globe meteoric water line?GMWL?.Brines of transition between marine and terrestrial deposit?Upper Triassic Xujiahe Formation?have?D and?¹⁸O values falling between the two end members of marine deposit brines and precipitation,implying that the brine is a mixture of precipitation and vaporing seawater.?4?In the basin,water samples from the brine-bearing strata of different ages,show various deuterium excess?d?with evident decreasing tendency as the age of strata gets older.The brine-bearing strata of Triassic Leikoupo-Jialingjiang Formations.Permian Maokou Formation,Cambrian and Sinian are all carbonate rocks which have intensive action with water.Having the same lithology,the strata present brines whose deuterium excess essentially change with time.All brine-bearing strata surrounding the basin or locating at t he faults,as well as those brine wells exploited for resources,have been obviously influenced by precipitation supply.Therefore,the deuterium excesses of their brines have increased to different extents,depending on the amount of mixing precipitation.The change and distribution of d values of the brines f rom different strata of Triassic are related to the burial of stratum depth.The deuterium excesses of brine become lower with the increase of burial depth of the stratum.?5?The deep formation brines,very rich in potassium,found in the Upper-Middle,are suggested that originated from marine,almost not affected by precipitate water since formed.The sources of K,Sr,Mg are suggested that related to Emeishan basalts eruption in late Permian.It's about Longmenshan deep fault activities which made more Cu?Zn?Pb?Sn?Cr?Ga and K⁺?B and less Mg²⁺?SO₄^2-of the western brines comparing with the eastern of Sichuan basin.?6?According to the characteristics of geology and geochemistry,the potassium rich brines can be grouped into homozygous depositional brine,leaching mixed brine and deposition deformation brine.On that basis,key target areas were given for prospecting potassium rich brines.