The chemical characteristics of coal reservoir water are important for studying the formation and enrichment of biogenic coalbed methane (BCM). Based on geological and sampling test data, this paper studied the geochemical characteristics and formation mechanisms of high-salinity coal reservoir water (CRW) in the Jurassic Yan'an Formation of the Binchang area in the southern Ordos Basin. The results show that the TDS contents of the CRW in the Binchang area are between 7577.38 and 15,138.61 mg/L (av. 13,268.95 mg/L), which is high-salinity brackish water. The ion types of CRW are mainly Na, Cl and HCO, and the correlations between TDS and Na and Cl are close to 1. The Piper trilinear diagram indicates that the evolution direction of the CRW is deep concentrated brine, and the hydrochemical type is the NaCl type. The I concentrations of CRW are between 285 and 484 μg/L, which are much higher than the values of 55.88 μg/L for seawater. The results of I dating show that the minimum age of the CRW in the study area is between 6.7 Ma and 39.97 Ma, which is much younger than the actual geological age of the Yan'an Formation. The hydrogen and oxygen isotope results show that the CRW in the study area experiences an apparent oxygen drift, indicating that the coal reservoir of the Yan'an Formation has good sealing and a long retention time for the CRW. The hydrodynamic factors show that the hydrodynamic conditions of the coal reservoir are weak, and the primary ions in the CRW originated from the dissolution of salt rocks. The main ion differentiation indices show that high-salinity coal seam water is mainly formed by evaporation, and the ion exchange between CRW and the surrounding rock and the alternating adsorption of cations in water are very weak. Evaporation and diagenesis lead to an increase in the contents of Na, Cl and I in coalbed water, which in turn leads to an increase in the total dissolved solids contents of CRW and its evolution toward concentrated brine. The genesis and evolution of the CRW in the study area are affected by the combination of the relationships among the paleoclimate, aquifers and aquifuges, and tectonic evolution processes. The CRW in the study area has experienced five evolution stages, i.e., sedimentary water and diagenetic water, high-salinity infiltration water, primary mixed water, paleoatmospheric precipitation recharge water, and secondary mixed water. The above understanding can provide a basis for studying the formation period and accumulation mechanism of BCM and provide a hydrogeological basis for water resource utilisation and pollution prevention and the control of high-salinity water.

中文翻译：

---

鄂尔多斯盆地南部宾厂地区高矿化度煤储层水化学特征、形成机制及地质演化过程


煤储层水的化学特征对于研究生物成因煤层气（BCM）的形成和富集具有重要意义。基于地质和取样测试资料，对鄂尔多斯盆地南部宾厂地区侏罗系延安组高矿化度煤储水地球化学特征及形成机制进行了研究。结果表明，宾长地区CRW的TDS含量在7577.38～15138.61mg/L(平均13268.95mg/L)之间，属于高盐度苦咸水。 CRW的离子类型主要为Na、Cl和HCO，TDS与Na、Cl的相关性接近1。Piper三线图表明CRW的演化方向为深层浓盐水，水化学类型为氯化钠型。 CRW 的 I 浓度在 285 至 484 μg/L 之间，远高于海水的 55.88 μg/L 值。 I测年结果显示，研究区CRW最小年龄在6.7~39.97 Ma之间，远低于延安组的实际地质年龄。氢氧同位素结果表明，研究区CRW出现了明显的氧漂移，表明延安组煤储层封闭性好，CRW滞留时间长。水动力因素表明煤储层水动力条件较弱，CRW中的原生离子来源于盐岩的溶蚀作用。主要离子分异指标表明，高矿化度煤层水主要由蒸发形成，CRW与围岩之间的离子交换和水中阳离子的交替吸附非常弱。 蒸发和成岩作用导致煤层水中Na、Cl、I含量增加，进而导致CRW总溶解固体含量增加，并向浓盐水演化。研究区古气候、含水层与隔水层之间的关系以及构造演化过程共同影响着研究区CRW的成因和演化。研究区CRW经历了沉积水和成岩水、高盐度渗透水、原生混合水、古大气降水补给水和二次混合水5个演化阶段。上述认识可为研究BCM的形成时期和积累机制提供依据，为水资源利用和污染防治及高盐度水体控制提供水文地质依据。