In regions where water resources are scarce and in high demand, it is important to safeguard against contamination of groundwater aquifers by oil-field fluids (water, gas, oil). In this context, the geochemical characterisation of these fluids is critical so that anthropogenic contaminants can be readily identified. The first step is characterising pre-development geochemical fluid signatures (i.e., those unmodified by hydrocarbon resource development) and understanding how these signatures may have been perturbed by resource production, particularly in the context of enhanced oil recovery (EOR) techniques. Here, we present noble gas isotope data in fluids produced from oil wells in several water-stressed regions in California, USA, where EOR is prevalent. In oil-field systems, only casing gases are typically collected and measured for their noble gas compositions, even when oil and/or water phases are present, due to the relative ease of gas analyses. However, this approach relies on a number of assumptions (e.g., equilibrium between phases, water-to-oil ratio (WOR) and gas-to-oil ratio (GOR) in order to reconstruct the multiphase subsurface compositions. Here, we adopt a novel, more rigorous approach, and measure noble gases in both casing gas and produced fluid (oil-water-gas mixtures) samples from the Lost Hills, Fruitvale, North and South Belridge (San Joaquin Basin, SJB) and Orcutt (Santa Maria Basin) Oil Fields. Using this method, we are able to fully characterise the distribution of noble gases within a multiphase hydrocarbon system. We find that measured concentrations in the casing gases agree with those in the gas phase in the produced fluids and thus the two sample types can be used essentially interchangeably.

EOR signatures can readily be identified by their distinct air-derived noble gas elemental ratios (e.g., ²⁰Ne/³⁶Ar), which are elevated compared to pre-development oil-field fluids, and conspicuously trend towards air values with respect to elemental ratios and overall concentrations. We reconstruct reservoir ²⁰Ne/³⁶Ar values using both casing gas and produced fluids and show that noble gas ratios in the reservoir are strongly correlated (r² = 0.88–0.98) to the amount of water injected within ~500 m of a well. We suggest that the ²⁰Ne/³⁶Ar increase resulting from injection is sensitive to the volume of fluid interacting with the injectate, the effective water-to-oil ratio, and the composition of the injectate. Defining both the pre-development and injection-modified hydrocarbon reservoir compositions are crucial for distinguishing the sources of hydrocarbons observed in proximal groundwaters, and for quantifying the transport mechanisms controlling this occurrence.

在水资源稀缺且需求量很大的地区，重要的是要防止油田流体（水、气、油）污染地下水含水层。在这种情况下，这些流体的地球化学特征至关重要，以便可以轻松识别人为污染物。第一步是表征开发前地球化学流体特征（即那些未受油气资源开发修改的特征），并了解这些特征可能如何受到资源生产的干扰，特别是在提高石油采收率 (EOR) 技术的背景下。在这里，我们展示了美国加利福尼亚州几个水资源紧张地区的油井生产的流体中的惰性气体同位素数据，这些地区普遍采用 EOR。在油田系统中，由于气体分析相对容易，即使存在油相和/或水相，通常也只收集和测量套管气体的惰性气体成分。然而，这种方法依赖于许多假设（例如，相之间的平衡、水油比 (WOR) 和气油比 (GOR) 以重建多相地下成分。在这里，我们采用了新颖、更严格的方法，并测量来自 Lost Hills、Fruitvale、North and South Belridge（圣华金盆地，SJB）和 Orcutt（圣玛丽亚盆地）的套管气和采出液（油-水-气混合物）样品中的惰性气体) 油田. 使用这种方法，我们能够全面表征多相烃系统中惰性气体的分布。

EOR 特征可以很容易地通过其独特的空气衍生惰性气体元素比（例如，²⁰ Ne/ ³⁶ Ar）来识别，与开发前的油田流体相比，该比值升高，并且相对于元素比值明显趋向于空气值和整体浓度。我们使用套管气和采出液重建储层²⁰ Ne/ ³⁶ Ar 值，并表明储层中的惰性气体比率与 井约 500 m 内注入的水量密切相关 (r ² = 0.88–0.98)。我们建议²⁰ Ne/ ³⁶注入导致的 Ar 增加对与注入物相互作用的流体体积、有效水油比和注入物组成敏感。定义开发前和注入改性的油气藏组成对于区分在近端地下水中观察到的油气来源以及量化控制这种发生的传输机制至关重要。