Abstract Investigations on geochemical processes involved in H2 generation in natural reservoirs are important for the present energy transition from fossil fuels. Most studies related to native H2 focus on the serpentinisation of ultra-mafic rocks, in which the source of H2 is linked to oxidation of Fe2+-bearing minerals and a reduction of H2O. In this study, we investigate abiotic H2 generation from a biotite-rich granite using an approach based on a study case of the Soultz-sous-Forets geothermal site in France. Using a geochemical and reactive transport model and an existing database, we simulated the hydrothermal alteration of the granite at 130–200 °C with a redox potential from −100 mV to −300 mV. The simulations show that generation of native H2 is possible in an open system by hydrothermal alteration of biotite as a source of Fe2+, which oxidises to Fe3+ leading to precipitation of Fe3+ minerals and reduction of H+. The amount of H2 produced depends on the type of Fe2+/3+ minerals which precipitate. The optimal conditions for H2 generation are related to magnetite precipitation as in the serpentinisation process. We assessed that the upper bound for H2 production for this site is 102 kt per km3 of granite. This is in case of complete oxidation of all the ferrous iron contained in the biotite of the geothermal reservoir. In practice, only part of this potential may be attained owing to the limited current exploitation methods which are unlikely to result in a complete disequilibrium of the whole biotite. Future research needs to focus on how to reach most of the H2 potential. Simulations with increasing CO2 pressures suggest that CO2 injection can stimulate the H2 production. This study has implications for possible coupling of heat extraction and future exploitation with H2 production.

中文翻译：

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富含黑云母的花岗岩产生非生物氢气：法国 Soultz-sous-Forêts 地热站点的案例研究

摘要 对天然储层中 H2 生成所涉及的地球化学过程的研究对于目前化石燃料的能源转型非常重要。大多数与天然 H2 相关的研究都集中在超基性岩石的蛇纹石化上，其中 H2 的来源与含 Fe2+ 矿物的氧化和 H2O 的还原有关。在这项研究中，我们使用基于法国 Soultz-sous-Forets 地热站点研究案例的方法研究了富含黑云母的花岗岩的非生物 H2 生成。使用地球化学和反应输运模型和现有数据库，我们模拟了花岗岩在 130–200 °C 下的热液蚀变，氧化还原电位从 -100 mV 到 -300 mV。模拟表明，通过黑云母的热液蚀变作为 Fe2+ 的来源，可以在开放系统中生成天然 H2，氧化成 Fe3+ 导致 Fe3+ 矿物沉淀和 H+ 还原。产生的 H2 量取决于沉淀的 Fe2+/3+ 矿物的类型。H2 生成的最佳条件与蛇纹石化过程中的磁铁矿沉淀有关。我们评估了该场地的 H2 产量上限为 102 kt/km3 花岗岩。这是在地热储层黑云母中包含的所有亚铁完全氧化的情况下。在实践中，由于目前有限的开采方法不太可能导致整个黑云母的完全不平衡，因此只能实现部分潜力。未来的研究需要关注如何发挥大部分的 H2 潜力。随着 CO2 压力增加的模拟表明，CO2 注入可以刺激 H2 的产生。