华北板块奥陶系碳酸盐岩热储的水化学特征及其成因机制

Hydrochemical Features and Formation Mechanism of the Ordovician Carbonate Reservoir in North China Plate

奥陶系碳酸盐岩是华北板块普遍且优质的中低温传导型热储层,以其独特的岩溶作用、高产量、低盐度、易回灌的特性而受到关注。对华北板块27眼奥陶系碳酸盐岩地热井进行水化学组分分析,将华北板块奥陶系碳酸盐岩地热水划分为3组:边山岩溶系统地热水,地热井深约600~2 100 m,地热水的水化学类型多为HCO3-Na、SO4·HCO3-Ca·Na、SO4^2-Ca型水,SO4^2-离子含量较高,说明地热水中除石膏类矿物溶解生成SO4^2-离子外,可能还存在黄铁矿、H2S被氧化的现象;过渡岩溶系统地热水,地热井深约2 100~2 700 m,地热水的水化学类型主要为Cl·HCO3-Na型水,随着井深的增加Cl^-离子含量也相对增高;深埋岩溶系统地热水,地热井深多大于2 700 m,地热水的水化学类型为Cl-Na型水,Cl^-离子含量较高,说明深埋岩溶系统地热水中除岩盐溶解生成Cl^-离子外,且随着温度和压力的改变,碳酸盐、硅酸盐、硫酸盐等矿物的溶解度下降而被析出,局部还可能存在封闭水。此外,结合华北板块奥陶系碳酸盐岩地热水的水化学特征、水化学类型与井深的关系以及特征系数的分析,提出了华北板块奥陶系碳酸盐岩热储形成机制的概念模型。

Ordovician carbonate rock,widely distributed in North China Plate,is a type of high-class medium to low temperature conductive reservoir. Its unique karst process,high yield,low salinity and easy recharge have drawn increasingly attention. Based on the hydrochemical composition analysis of thermal waters from twenty seven geothermal wells in North China Plate,this paper discovers that the geothermal waters from Oridovician carbonate reservoir can be divided into three groups (1) boundary mountain karst system;(2) transitional karst system;(3) deep buried karst system.The hydrochemical types of geothermal waters from boundary mountain karst system,whose well depths are about 600 to 2100 m,are mainly HCO3-Na, SO4·HCO3-Ca·Na, and SO4-Ca. The contents of SO4^2- in boundary mountain karst system waters are comparatively high,which may be due to the dissolution of gypsum and oxidation of pyrite or H2S. The hydrochemical types of geothermal waters from transitional karst system, whose well depths are about 2 100 to 2 700 m,are mainly Cl·HCO3-Na,and the Cl^- concentrations of geothermal waters improve with increasing depth. The hydrochemical types of geothermal waters from deep buried karst system, whose well depths exceed 2 700 m,are mainly Cl-Na. Similarly,the contents of Cl^- are extremely high as well,which may attribute to the dissolution of halite. Furthermore 9 the existence of pent-up water may also be related to the high content of Cl^- in buried karst system geothermal waters.At last,combining the hydrochemical features,relationships between hydrochemical type and depth,and analyses of characteristic coefficient,the formation conceptual model of Ordovician carbonate reservoir in North China Plate was put forward.