Abstract Between 2015 and 2018, the Fallon FORGE EGS (Engineered or Enhanced Geothermal System) site in Churchill County, Nevada, USA, underwent detailed geoscientific characterization to evaluate its suitability to operate as a field demonstration site to test and develop EGS technologies. This included the compilation, synthesis and acquisition of geophysical, geological, geochemical, hydrological, down-hole log, and mineralogy datasets. Site activities culminated in the drilling of a 2481 m-deep EGS observation well (21−31) to further verify the geology and rock properties of the proposed EGS reservoir formations, which are older than the Miocene and include felsic intrusions, felsic volcanics, and meta-sedimentary lithologies. In this paper, we integrate and evaluate borehole data from well 21−31, including drill cuttings and sidewall cores (petrographic, XRD, hyperspectral), wireline geophysical logs (e.g., gamma, neutron porosity, resistivity, sonic scanner), borehole imagers (FMI, BHTV), and pressure and temperature logs. Hydrothermal alteration and veining is found throughout the entire pre-Miocene interval of well 21−31, and includes epidote mineral assemblages that suggest past temperatures >250 °C. This is higher than the hottest measured equilibrated temperature at similar depths in other wells on the FORGE site today, and suggests that the system was higher temperature in the past. Overall lithologic variability is high, with volcanic, intrusive, metamorphic, and sedimentary rocks present in the bottom 700 m of the well. Natural fractures are abundant throughout the pre-Miocene section, but are more frequent in the felsic intrusive and felsic volcanic lithologies. The dominant orientation of these natural fractures observed in FMI logs is NNE-SSW with an average strike of 010° ± 40°, subparallel to the interpreted SHmax orientation of 009° ± 14° that is indicated by drilling-induced structures in the wellbore in the BHTV log. Therefore, natural fractures appear optimally oriented for reactivation via hydraulic stimulation. Mud loss locations and a non-equilibrated temperature log suggest that there are some zones with relatively-higher permeability and/or open fractures. However injection test results indicate an injectivity index of 0.025 L/min/kPa, which is an order of magnitude lower than a typical threshold for commercial wells. Analytical modeling of injection test results indicate that the majority of the well is low permeability (

中文翻译：

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热液蚀变、天然裂缝和渗透率之间的联系：美国内华达州 Fallon FORGE EGS 站点用于储层表征的钻孔数据集成

摘要 2015 年至 2018 年间，美国内华达州丘吉尔县的 Fallon FORGE EGS（工程或增强型地热系统）站点进行了详细的地球科学表征，以评估其作为现场示范站点运行以测试和开发 EGS 技术的适宜性。这包括地球物理、地质、地球化学、水文、井下测井和矿物学数据集的编译、合成和获取。现场活动最终钻探了一口 2481 m 深的 EGS 观测井 (21-31)，以进一步验证拟议的 EGS 储层的地质和岩石特性，这些储层比中新世更古老，包括长英质侵入体、长英质火山岩和变质沉积岩性。在本文中，我们整合和评估了 21-31 井的钻孔数据，包括钻屑和侧壁岩心（岩相、XRD、高光谱）、电缆地球物理测井（例如伽马、中子孔隙度、电阻率、声波扫描仪）、钻孔成像仪（FMI、BHTV）以及压力和温度测井。在 21-31 井的整个前中新世区间都发现了热液蚀变和脉动，包括绿帘石矿物组合，表明过去的温度 >250 °C。这高于今天在 FORGE 站点的其他井中在类似深度测得的最热平衡温度，并表明该系统过去的温度更高。总体岩性变异性大，井底 700 m 存在火山岩、侵入岩、变质岩和沉积岩。整个前中新世部分天然裂缝丰富，但在长英质侵入岩和长英质火山岩中更为常见。在 FMI 测井中观察到的这些天然裂缝的主要方向是 NNE-SSW，平均走向为 010°±40°，与 009°±14° 的解释 SHmax 方向次平行，该方向由井眼中的钻井诱导结构指示BHTV 日志。因此，天然裂缝似乎是通过水力增产重新激活的最佳方向。泥浆流失位置和非平衡温度测井表明存在一些具有相对较高渗透率和/或开放裂缝的区域。然而，注入测试结果表明注入指数为 0.025 L/min/kPa，这比商业井的典型阈值低一个数量级。