Li Yang, Chen Jian, Tang Shuheng, Zhang Songhang, Xi Zhaodong
School of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, Anhui, China.
State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Anhui University of Science & Technology, Huainan 232001, Anhui, China.
ACS Omega. 2022 Feb 22;7(9):7715-7724. doi: 10.1021/acsomega.1c06496. eCollection 2022 Mar 8.
The exploration and exploitation of coalbed methane (CBM), an essential unconventional gas resource, have received much attention. In terms of shallow groundwater assessment during CBM production, biogenic methane natural formation in situ and methane migration from deep sources into shallow aquifers need to be of most concern. This study analyzes geochemical surveys including ions, isotopes, and dissolved methane concentrations in 75 CBM coproduced water samples in the southern Qinshui Basin. Most of these water samples are weakly alkaline. Some samples' negative oxidation/reduction potential (ORP) values reveal that the CBM reservoir water samples are mainly produced from reductive groundwater environments. Cl, Na, and HCO are the dominant ionic constituents of the water samples, which are usually associated with dissolved methane concentrations. The biogeochemical parameters and isotopic features provide an opportunity to assess the origin, migration, and oxidation of biogenic or thermogenic methane. Some water samples suggest biogenic methane formation in situ characterized by negligible SO and NO concentrations and low δC. Only a few water samples indicate the migration of biogenic methane into shallow aquifers without oxidation based on elevated SO , NO , and δC and low δC. A few cases characterized by elevated δC, negative δC values, and negligible SO and methane concentrations suggest the oxidation of biogenic methane rather than the migration of thermogenic methane. A significant number of cases mean methane migration to shallow aquifers. Partial oxidation of thermogenic or mixed methane is evaluated by negligible SO , NO , and methane concentrations and elevated δC. Dissolved methane isotopic compositions and aqueous biogeochemical features help study methane formation and potential migration in shallow groundwater.
煤层气作为一种重要的非常规天然气资源，其勘探与开发备受关注。在煤层气开采过程中的浅层地下水评估方面，原地生物成因甲烷的自然形成以及甲烷从深部来源向浅层含水层的运移是最需要关注的问题。本研究分析了沁水盆地南部75个煤层气产出水样中的地球化学调查数据，包括离子、同位素和溶解甲烷浓度。这些水样大多呈弱碱性。一些样品的负氧化还原电位（ORP）值表明，煤层气储层水样主要源自还原型地下水环境。Cl、Na和HCO是水样中的主要离子成分，它们通常与溶解甲烷浓度相关。生物地球化学参数和同位素特征为评估生物成因或热成因甲烷的来源、运移和氧化提供了契机。一些水样表明原地存在生物成因甲烷形成，其特征是SO和NO浓度可忽略不计且δC较低。只有少数水样显示，基于SO、NO和δC升高以及δC较低，生物成因甲烷在未被氧化的情况下运移到浅层含水层。少数以δC升高、δC值为负以及SO和甲烷浓度可忽略不计为特征的情况表明是生物成因甲烷被氧化，而非热成因甲烷运移。大量情况意味着甲烷运移到浅层含水层。通过可忽略不计的SO、NO和甲烷浓度以及升高的δC来评估热成因或混合甲烷的部分氧化情况。溶解甲烷同位素组成和水相生物地球化学特征有助于研究浅层地下水中甲烷的形成和潜在运移。
