渤海莱州湾南岸第四纪地下卤水演化的地球化学模拟

GEOCHEMISTRY MODELING OF QUATERNARY SUBSURFACE BRINES IN SOUTH COAST OF THE LAIZHOU BAY,THE BOHAI SEA-TAKING BRINES FROM CORE-AOLI501 IN CHANGYI AREA AS AN EXAMPLE

渤海莱州湾南岸地区赋存有大量的地下卤水,许多研究者认为这些卤水是第四纪地质历史时期古海水经强烈蒸发浓缩后埋藏而成的。运用反向地球化学模拟理论,选用美国地质调查局(USGS)提供的PHREEQC(1997)地球化学模拟软件,对莱州湾南岸昌邑廒里501孔各层卤水的水化学演化过程进行模拟。研究结果表明,该地区的地下卤水是由古海水经强烈蒸发浓缩后被沉积物埋藏,并在赋存过程中发生水 岩相互作用,使地下水出现方解石、石膏及CO2气体的过饱和析出,并伴随与沉积物发生K+、Na+、Mg2+、Ca2+之间的阳离子交换反应。其中的浅层地下卤水还受到地表盐场析盐后排放卤水的侵染。

In the south coast area of the Laizhou Bay, the Bohai Sea, a lot of subsurface brine is buried. Some scholars believe that these brines came from buried ancient seawater, which was intensively evaporated and concentrated in the Quaternary. In this paper, geochemistry evolution process of subsurface brines in Aoli, Changyi County, is simulated, based on the equilibrium-constant method, which is commonly used in hydrogeochemistry simulating research, and also on the PHREEQC(1997), a geochemistry modeling software provided by the U.S. Geological Survey. It is assumed that the Bohai Sea water and the subsurface brine in Aoli area is respectively the starting water and the ending water in the research. The input data for this modeling research are from the chemical analysis data of the Bohai Sea water and brines in different strata of CoreAoli501. As a result, the distribution of species of major ions in the seawater and brines, the SaturationIndex (SI) of solution minerals, and the hydrogeochemistry patterns of seawater evolving into brines in different strata were revealed. Although the mineralization degree and formation time of the brines are different in each stratum, the tendency of their geochemistry evolution is consistent. The modeling results suggest that the subsurface brines in this area originated from ancient seawater. They were intensively evaporated in geological time, but these processes are not simply seawater concentrating, because there were some complex waterrock interactions between brines and sediments after they were buried. These processes show that calcite, gypsum and CO2 separated out from saline groundwater after they were oversaturated, and ionexchange reactions among K+, Na+, Mg2+ and Ca2+ occurred between brine and sediment. In addition, although halite (NaCl) in brines in shallow phreatic aquifers in this area are unsaturated (logSI=-155<0), the modeling results show that precipitation of the halite has occurred in this brine. This result has revealed that components of the shallow subsurface brine are affected by some bittern infiltration from salt fields. 