东海盆地西湖凹陷中部古近系地层水化学特征及地质意义

Chemical characteristics and geological significance of Palaeogene formation water in central Xihu Depression,East China Sea Basin

作为含油气盆地中与油气共存的重要流体,地层水化学特征对油气保存条件的评价具有重要意义。通过对西湖凹陷中部古近系平湖组和花港组可靠的地层水数据进行处理分析,发现古近系地层水具有矿化度较低,平均值为21 657.37mg/L,Na^++K^+、Cl^-、HCO_3^- 占优势,以NaHCO_3水型为主,钠氯系数、脱硫系数、镁钙系数较大,变质系数较小等特征,反映出古近系地层水浓缩变质程度较弱,处于半开启—半封闭的区域水动力缓慢交替区。地层水的来源除湖盆的淡水—微咸水蒸发浓缩作用外,还存在海水沿开放型构造向下伏地层渗入造成与同沉积水混合而咸化的作用。盐岩的溶解是Na^+、Cl^-的主要成因,钠长石的高岭石化对HCO_3^-的形成有重要贡献。对地层水化学组成变化与天然气藏分布关系研究表明,地层水矿化度低值区(<25 000mg/L)、NaHCO_3型水分布区,钠氯系数为1~1.6分布区、脱硫系数低值区(<8)、变质系数为-80^-0.5分布区、镁钙系数低值区(<1.5)等区域更有利于天然气的保存。

Formation water is an important fluid coexisting with oil and gas in petroliferous basins,its chem ical characteristics are of great significance to the evaluation of oil and gas preservation conditions.Reliable Pinghu-Huagang Formations water data suggest that： The total dissolved solid （TDS） is low,with average of 21 657.37mg/L. Na＋-K＋, CI-, HCO3- are dominant ions and water type is mainly NaHCO3 type, r（Na＋）/r（C1-）,r（SO1-））〈100/r（C1 ）,and r（Mg2＋ ）/r（Ca2＋） are high,and r（C1 -Na ）/r（Mg2＋ ） is low.These characteristics reflect weak concentration and deterioration of formation water and it is in half open-half close regional hydrodynamic slow alternation zone. Except the evaporation and concentration of freshwater-brackish water of lake basin,the source of formation water also has salinization that seawater a- long the open structure infiltrates into synsedimentary water of underlying strata. Halite dissolution is mainly origin of Na＋ and C1- , kaolinization of albite makes important contribution to HCO3- formation. There are close relations between chemical compositions of the formation water and natural gas reservoirs. While the formation water is of low TDS（〈25 000mg/L）,NaHCO3 water type, low r （SO2-））4 100/ r（Cl-） （〈8）,low r（Mg2＋）/r（Ca）2） （〈1.5） and r（Na＋）/r（C1-） is at 1-1.6,r（C1 -Na＋）/r（Mge2） is from --80 to --0.5,there may be beneficial to preserve natural gas.