The characteristics and genetic mechanism of hydrochemical facies in the Shiwu depression of the Songliao Basin

摘要

The hydrochemical characteristics and genetic mechanism ofthe Shiwu rifted stratum in Songliao basin are discussed based on data of the 96 hydrochemical samples and 3 measurements ofD and 18O. Spatial and temporal evolution of the hydrogeochemical field was controlled by pressure, temperature, fluid and rock properties. Stratification of formation water can be subdivided into 3 zones, which correspond to the normal pressure zone (＜1,250 m), the pressure transition zone (1,250-1,650 m) and the underpressure zone (＞1,650 m).In the normal pressure zone, formation water is NaHCO3-type and salinity averages about 2.9 g/L. The formation water in the pressure transition zone is Na2SO4-type and salinity averages about 3 g/L. The formation water in the underpressure zone is CaCl2-type, but the salinity decreases sharply at a depth of 1,650 m and then increases vertically downward to 10 g/L. The relationship of Cl- and HCO3- as well as Na++K+and Ca2+ in the stratum show that the stratum water is a low-salinity liquid rich in HCO3- in early times, and this chemical discrimination in the vertical direction is due to different hydrogeological conditions. The analysis of D and 18O indicates that the albitization of anorthite causes Ca2+ increased in the underpressure zone, whereupon, CaCl2-type fluids dominate the deep underpressured system.