| With the rapid development of China’s economy, the demand of potash resources is increasing. The exploration work of potash resources were carried out in some sedimentary basin of China. Sichuan basin is China’s important sedimentary basin which has abundant brine resources, especially in Triassic formation. The utilization value of potassium-rich brine is extremely high.This paper studies the geochemical characteristics of the lower-middle Triassic brine, mainly analyzes the characteristics and distribution of water chemical, isotope and gas in the brine. This paper studies summarizes the evolution history of the Sichuan Basin and the Triassic strata lithology, geological structure, hydrogeological conditions, and analyses water chemical composition and distribution feature of the brine, and compares with water evaporation sequence. The isotopes characteristics of D, 18 O, 34 S and 11 B was also analyzed in this paper. Finally, this paper discusses the process of different hydorgeochemical effects to trace the evolution of the brine. The mainly achieved understandings as follows:1. Sichuan basin is a multi-layered rock parallel superposed artesian basin and can be divided into several aquifers from top to bottom. The lower-middle Triassic aquifer which is deep in the hinterland is one of the main aquifers. The boundary of the aquifer is under the control of regional facture. The aquifer lithology is carbonated and there are several stable reservoir bine in this aquifer.2. The salinity of brines differ in the study areas. Anion is given priority to chloride ion and the content of sulfate and bicart root is low. Cation is given priority to sodium ions. The content of trace elements is high. The hydrochemical type of brine in Midwest Sichuan Basin mainly is Cl-Na. The content of Cl and Na is higher in the north and western Sichuan Basin, so it is salinity.The content of SO42- is higher in eastern Sichuan Basin. The content of Mg2+ and HCO3- is low, and there is no obvious difference in the Sichuan Basin. The content of Ca2+ is higher in the western and middle Sichuan basin. The content of K+ is higher in the western and north Sichuan basin. The hydrogeochemical ratio varies in different area of study area. The brine has the characteristics of leached water, and a strong metamorphism in the late. Base on condition hydrogeochemical ratios, Q type cluster analysis also shows that potassium-rich brine has a similar origin with brine in the west and middle of Sichuan Basin.3. Through the analysis of the correlation with water chemical composition in the study area, the salinity has a good correlation of Cl- and Na+, and a poor correlation with HCO3-, and even negative related to SO42-.The R cluster analysis of different district of Sichuan Basin shows that the brine in the north, southwest and west of Sichuan Basin has a similar origin while has a different origins in the east. The result of factor analysis showed that the leaches of the halite, sylvite, calcite and dolomite has a great influence on water chemical composition.4. Compared with evaporation series of seawater, the Na+, Cl- and K+ change trend with salinity in the study area is similar with change of seawater, which means that the origin of the brine associates with sedimentary rocks. There are some sample with abnormally high K+ content. The content of Mg2+ is lower than those of seawater evaporation while the content of Ca2+ is higher than that. It happened in the process of bine dolomiaztion and cation exchange. The bacteria decompose SO42- into H2 S led to the low content of SO42-.5. The D value of brine in the study area is 7 ~ 46‰, 18 O value is + 1 ~ + 8‰, and the d value is 31 ~ 66‰. Hydrogen and oxygen isotopic composition of brine is close to the original seawater and influence by surface water and the water of gypsum dehydration. The hydrogen isotopic composition of brine is slightly lower than the water of the sea, and water rock isotope exchange cause high oxygen isotopic compositions in the brine. The 34 S value is 15 ~ 37‰, Sulfur isotope of brine has a similar with the anhydrite in the same formation.6. In the brine of study area, calcite and dolomite is in the saturated state, gypsum, halite and potassium is in the unsaturated state. Inverse geochemistry simulation shows that the formation of potassium-rich brine need to leached potassium rich mineral. The formation of the brine in the study area, can be summarized into three stages, the first is the original sedimentary water from sea water. Evolution of the brine has influenced by the adjacent strata gypsum dehydration and dolomization cationic modification effects in second stage. In the last stage, the brine in some part of tectonic zone leached adjacent salt mineral and formed potassium-rich brine. |
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