摘要
地层水化学特征往往反映出盆地流体的成因和演化信息。莺歌海盆地地层水样品分析表明地层水矿化度可划分3个带,即非底辟带、底辟带上方常压段和超压段。非底辟带地层水矿化度接近于正常海水,而超压带为具有较低矿化度的NaHCO_3型水,底辟带上方常压段地层水矿化度变化极大,与垂向断裂距离有关,越靠近垂向断裂的井往往具有更低的矿化度和更高的 HCO_3^-和 CO_3^(2-)离子浓度。在远离垂向断裂的底辟带侧翼或较浅地层(<1200m)孔隙水接近正常海水,以 MgCl_2型地层水为主:而在毗邻于垂向断裂的浅部储层具有明显低的矿化度,与超压带流体相似,也具有较高的 HCO_3^-和 CO_3^(2-)离子浓度这些观察结果反映垂向断裂为深部超压热流体突破的主通道,底辟带上方成为地层原生孔隙水(海水)与超压带内低矿化度热流体混合场所,地层水矿化度越低,说明来自深部超压热流体所占比例越大。不同压力地层中的水化学特征可以很好地用来识别流体运移路径和可能的水-岩反应。
In an overpressured system, chemical features of formation water record information on origin and evolution of basin fluids. In this study, more than 100 pore water geochemical analyses from 26 wells in the Yinggehai basin, northern South China Sea margin have been carried out. Three pore water salinity domains recognized： non-diapiric, diapiric within normal pressure and overpressured zones. Salinity of water in the non-diapiric area is close to that of normal seawater. In the overpressured system, however, NaHCO3-dominated formation water has much lower salinity about 9.9 - 31.6g/L and lower ion concentrations （ except bicarbonate and carbonate） than normal seawater. The largest variation in salinity, ranging from 10. 3g/L to 35.9g/L, occurs in the diapiric area with normal pressure. In general, formation water closed to seawater occurs on the flank of the diapiric structures or in the interval at a depth less than 1200m, which is dominated by MgCl2-typed water. The results indicate that geochemical variation of pore water in the diapiric structures shows episodic expulsion of overpressured fluids along vertical faults. Strong expulsion of fluid from deep overpressured sediments with low salinity NaHCO3 -dominated water could explain the variation in salinity at the top of the overpressured zone in the diapiric structures. The magnitude of TDS, and HCO3^- and CO3^2- concentrations depend on the distance from the vertical faults connected with the overpressured compartments. The shorter the distance to the vertical faults, the smaller the TDS content is, and the greater the HCO3^- and CO3^2- concentrations. Moreover, hot thermal fluid expulsion enhances this transformation of clay minerals and dissolution of albite. Therefore, variation of salinity and ion concentrations in different pressured systems provides a clue to the flow pathways and possible diagenetic reactions.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第8期2243-2248,共6页
Acta Petrologica Sinica
基金
教育部跨世纪人才基金项目资助课题
关键词
地层水
超压体系
水-岩相互作用
底辟带
莺歌海盆地
Formation water, Overpressured system, Water-rock interaction, Diapiric structure, Yinggehai basin
