Geochemical Characteristics Of Potassium-rich Brine Reservoir In The Lower-middle Trias Series Of Sichuan Basin

This paper mainly rely on the project of China geological survey “The enrichmentregularity and survey and assessment of favorable area of potassium-rich brine inTriassic,Sichuan” and the project of the Hongfeng potassium fertilizer limited liabilitycompany “The Geological characteristics and comprehensive evaluation research to brinestrata in Pingluo structure”Potassium-Rich Brine is mainly concentrated in the final stages of seawaterconcentrated in Sichuan basin, a large number of double salt （as polyhalite） is an importantmaterial base to form the Potassium-Rich Brine. It is the important reservoir formation fromthe fourth member of Jialing river formation to the Leikoupo formation, carbonate as themain lithology, and holes, seams, salt dissolve structure and tectonic fault zone as the mainstorage space for the brine. The Study to the geochemical characteristics of the reservoirs, itis the certain reference value to understand the mechanism of the formation and determinethe prospecting direction.Through the field exploration, the sample collection of the strata rock and brine,laboratory analysis and simulation, with the following main conclusions:1．The Characteristics of Major elementsThe contents of Mg、Ca、Sr、K、Na and SO₄^2-enrich, The contents of CO₃^2-reduceobviously in the stratum from the fourth member of Jialing river formation. There are thehigher contents of dolomite and lazuli in the fourth and fifth member of Jialing riverformation, then the green-bean rock within Mg and K form between the Jialing river andLeikoupo formation, the gypsum and dolomite appear in the early Leikoupo formation, andit is the only gypsum in the later. 2．The Characteristics of Trace elementsIn the eastern and western Sichuan, the contents of trace elements in the brine reservoiris the difference largely, the contents of Cu Pb and Zn in the brine and reservoir rock is themore higher in the eastern than western Sichuan, it is the main reason for the different to thedeep hydrothermal activity later.3．The Rare earth elements （REE） CharacteristicsThe rare earth elements patterns incline to the right, belonging to LREE-rich type, it isthe same with the patterns of Emeishan Basalt, there is the different on the δEu, it is thereason of the different sedimentary environment, it appear that the two thing is thecorrelation.4．The Isotopic CharacteristicsThe oxygen isotope: The δ¹⁸O of carbonate is low appreciably, and the δ¹⁸O of brineis high obviously, it is the reason of the role between water and rock.The strontium isotope: The value of⁸⁷Sr/⁸⁶Sr is lower than the the same period globalseawater（0.70775-0.70830）, show that it is the correlation with mantle derived material（Emeishan Basalt）.The sulfur isotope: the value of δ³⁴S is the highly enrichment （31.3‰） in the fourthand fifth member of Jialing river formation, the sedimentary environment belong to thelagoon facies. It is the main reason of the Thiobacteria which make32S into H2S easily, andthe residual seawater is more34S. The value of δ³⁴S get the nomal value with the changedsedimentary environment and the afflux of tetys seawater.5．The Relation between Material with Potassium in the K-Rich Brine ReservoirWith the drought conditions in the Early and Middle Triassic, potassium ion increase inthe residual ancient seawater, and enter into solid phase with the formation of solid material（polyhalite and green-bean rock）. And with later structure function and the changedconditions of physical and chemistry, fluid filter the soluble material with potassium andmake it to get back into liquid phase, and form the potassium-rich brine in certain structureenvironment. Therefore, the role of " absorption and release Potassium" affect the content ofpotassium in the brine, and the geochemical characteristics of reservoir will also happen theaccording change under the influence of "water-rock role".