Due to increasing concerns over the potential impact of shale gas and coalbed methane (CBM) development on groundwater resources, it has become necessary to develop reliable tools to detect any potential pollution associated with hydrocarbon exploitation from unconventional reservoirs. One of the key concepts for such monitoring approaches is the establishment of a geochemical baseline of the considered groundwater systems. However, the detection of methane is not enough to assess potential impact from CBM and shale gas exploitation since methane in low concentrations has been found to be naturally ubiquitous in many groundwater systems. The objective of this study was to determine the methane sources, the extent of potential methane oxidation, and gas-water-rock-interactions in shallow aquifers by integrating chemical and isotopic monitoring data of dissolved gases and aqueous species into a geochemical PHREEQC model. Using data from a regional groundwater observation network in Alberta (Canada), the model was designed to describe the evolution of the concentrations of methane, sulfate and dissolved inorganic carbon (DIC) as well as their isotopic compositions (δ³⁴S_SO4, δ¹³C_CH4 and δ¹³C_DIC) in groundwater subjected to different scenarios of migration, oxidation and in situ generation of methane. Model results show that methane migration and subsequent methane oxidation in anaerobic environments can strongly affect its concentration and isotopic fingerprint and potentially compromise the accurate identification of the methane source. For example elevated δ¹³C_CH4 values can be the result of oxidation of microbial methane and may be misinterpreted as methane of thermogenic origin. Hence, quantification of the extent of methane oxidation is essential for determining the origin of methane in groundwater. The application of this model to aquifers in Alberta shows that some cases of elevated δ¹³C_CH4 values were due to methane oxidation resulting in pseudo-thermogenic isotopic fingerprints of methane. The model indicated no contamination of shallow aquifers by deep thermogenic methane from conventional and unconventional hydrocarbon reservoirs under baseline conditions. The developed geochemical and multi-isotopic model describing the sources and fate of methane in groundwater is a promising tool for groundwater assessment purposes in areas with shale gas and coalbed methane development.

由于人们越来越关注页岩气和煤层气（CBM）开发对地下水资源的潜在影响，因此有必要开发可靠的工具来检测与非常规油藏开采碳氢化合物相关的任何潜在污染。这种监测方法的关键概念之一是建立所考虑的地下水系统的地球化学基线。但是，甲烷的检测不足以评估煤层气和页岩气开采的潜在影响，因为发现低浓度的甲烷在许多地下水系统中自然普遍存在。这项研究的目的是确定甲烷来源，潜在的甲烷氧化程度，通过将溶解气体和含水物类的化学和同位素监测数据整合到地球化学PHREEQC模型中，在浅层含水层中进行气水-岩-岩相互作用。使用来自加拿大艾伯塔省的区域地下水观测网络的数据，设计了该模型，以描述甲烷，硫酸盐和溶解的无机碳（DIC）的浓度及其同位素组成（δ）的演变。³⁴小号_SO4，δ ¹³ Ç _CH4和δ ¹³ Ç _DIC）中进行迁移，氧化的不同场景地下水和原位生成甲烷。模型结果表明，在厌氧环境中甲烷迁移和随后的甲烷氧化会强烈影响其浓度和同位素指纹图，并可能损害甲烷源的准确识别。例如升高的δ ¹³ C ^ _CH4该值可能是微生物甲烷氧化的结果，并且可能被误解为产热甲烷。因此，量化甲烷氧化程度对于确定地下水中甲烷的来源至关重要。该模型的含水层中艾伯塔示出了应用程序，一些情况下的升高的δ ¹³ C ^ _CH4该值是由于甲烷氧化导致甲烷的伪热同位素同位素指纹。该模型表明，在基准条件下，常规和非常规油气藏的深层热甲烷对浅层含水层没有污染。描述页岩气和煤层气开发地区的地下水评估目的的发达的地球化学和多同位素模型描述了地下水中甲烷的来源和去向。