热成因煤层气区煤层水同位素分异演化研究

South Qinshui Basin is a successful example of thermogenic or secondary thermogenic coalbed methane (CBM) development in the high rank coal of China. The produced water (mainly coal seam water) of the gas producing CBM wells in the region is characterized by the upper deviation of hydrogen and oxygen isotope values and positive dissolved inorganic carbon isotope values. These isotope features normally is explained by the biogenic methane generation, which contradicts with the thermogenic CBM in the high rank coal. In this research, the core issue that the mechanism of coal seam water isotope differentiation and evolution in thermogenic CBM region will be studied. Today's coal seam water isotopic characteristics is analyzed by CBM produced water; ancient coal seam water isotopic characteristics is analyzed by groups of fluid inclusions; and the variation characteristics of coal seam water isotopic with coal maturity is analyzed by saturated water thermal simulation of coal in closed system. Furthermore, with the rich CBM geology and CBM well production data of eastern margin of Ordos Basins and Qinshui Basin, the differentiation, evolution mechanism of seam water isotopes will be analyzed, and the model of coal seam water isotopic differentiation, evolution in thermogenic CBM region will be established. Finally, the implications of coal seam water isotope on CBM exploration and development will be discussed. This research is helpful to rich the CBM geology theory.

沁水盆地南部是我国高煤阶热成因或次生热成因煤层气成功开发的范例，区内产气煤层气井的产出水（以煤层水为主）具有氢、氧稳定同位素值上偏，溶解无机碳同位素为正的特征。当前一般将上述同位素特征解释为生物成因，与区内煤层气为热成因相矛盾。本项目拟针对“热成因煤层气区煤层水同位素分异演化机理”这一核心问题，通过煤层气井产出水分析现今煤层水的同位素特征，通过群体流体包裹体分析古煤层水的同位素特征，通过煤岩封闭体系饱和水热模拟实验分析煤生气过程中煤层气水同位素的变化特征，同时结合鄂尔多斯盆地东缘及沁水盆地南部丰富的煤层气地质和生产资料，分析煤层水同位素的分异、演化机理，建立热成因气区煤层水同位素的分异演化模型，探讨煤层水同位素在煤层气勘探开发中的指示意义，丰富煤层气地质理论。

热成因气区煤层水“氢、氧稳定同位素值上偏，溶解无机碳同位素为正”特征的成因及演化机理尚无明确的解释。项目针对该问题，开展沁水盆地柿庄南区块煤层气井产出水和封闭体系煤-水热模拟实验产物的水化学和同位素研究，同时结合柿庄南区块煤层气地质和生产资料，分析煤层水同位素的分异、演化机理，建立热成因气区煤层水同位素的分异演化模型，探讨煤层水同位素在煤层气勘探开发中的指导作用。取得主要的成果认识如下：(1)煤层气同为素分析表明研究区煤层气为次生热成因气。煤层气井氢、氧同位素分析表明，明产出水主要为大气降水来源，存在一定的同位素偏移效应。（2）热模拟煤成烃过程中水-岩反应以溶滤作用为主，当温度高于300℃时存在硫酸盐还原作用；水产物中SO42-和多数Ca2+、Mg2+来源于硫酸盐溶解，少部分来源于碳酸盐溶解，HCO3-来源于碳酸盐溶解及硫酸盐还原作用，Na+、K+及部分Cl- 来源于盐岩溶解；250℃～450℃水型分别为Ca·Mg-Cl·SO4、Ca·Mg-SO4、Ca·Mg-HCO3·Cl、Ca·Mg-HCO3、Ca·Mg-HCO3；水产物中δD、δ18O随模拟温度升高逐渐变重；低于400℃时为氧同位素漂移，高于400℃时为氢同位素漂移。 (3) 8O漂移与相邻含水层相关, D漂移表明水来源于煤储层。δ13CDIC和碱度(HCO3-)有明显正相关。δ13CDIC＜0主要归因于有机物分解或碳酸盐溶解，通常煤层气产量较低；δ13CDIC＞0 主要由于微生物活动，特别是产甲烷菌的产甲烷活动，0‰＜δ13CDIC＜25‰多为高产气井。煤层沟通无越流补给高渗透含水层时，增大气井日排水量更有利于煤层气的增产。总之，煤层水中蕴含丰富的地质信息，对煤层气有利区块优选和煤层气生产具有重要的工程指导意义。