The importance of research into clean and renewable energy solutions has increased over the last decade. Geothermal energy provision is proven to meet both conditions. Therefore, conceptual models for deep geothermal applications were developed for different field sites regarding different local conditions. In Bavaria, Germany, geothermal applications were successfully carried out in carbonate horizons at depths of 4000 to 6000 m. Matrix permeability and thermal conductivity was mainly studied in karstified carbonates from the Late Jurassic reef facies. Similar to Bavaria, carbonates are located in the east of the Rhenohercynian Massif, in North Rhine-Westphalia (NRW), for which quantification of the geothermal potential is still lacking. Compared to Bavaria, a supraregional carbonate mountain belt is exposed at the Remscheid-Altena anticline (in NRW) from the Upper Devonian and Lower Carboniferous times. The aim of our study was to examine the potential geothermal reservoir by field and laboratory investigations. Therefore, three representative outcrops in Wuppertal, Hagen-Hohenlimburg, and Hönnetal were studied. During field surveys, 1068 discontinuities (139 open fractures without any filling, 213 joints, 413 veins filled with calcite, and 303 fractures filled with debris deposits) at various spatial scales were observed by scanline surveys. These discontinuities were characterized by trace length, true spacing, roughness, aperture, and filling materials. Discontinuity orientation analysis indicated three dominant strike orientations in NNW–SSE, NW–SE, and NE–SW directions within the target horizon of interest. This compacted limestone layer (Massenkalk) is approximately 150 m thick and located at 4000 to 6000 m depth, dipping northwards at a dip angle of about 30 to 40^∘. An extrapolation of the measured layer orientation and dip suggests that the carbonate reservoir could hypothetically extend below Essen, Bochum, and Dortmund. Our combined analysis of the field and laboratory results has shown that it could be a naturally fractured carbonate reservoir. We evaluated the potential discontinuity network in the reservoir and its orientation with respect to the prevailing maximum horizontal stress before concluding with implications for fluid flow: we proposed focusing on prominent discontinuities striking NNW–SSE for upcoming geothermal applications, as these (1) are the most common, (2) strike in the direction of the main horizontal stress, (3) have a discontinuity permeability that significantly exceeds that of the reservoir rock matrix, and (4) only about 38 % of these discontinuities were observed with a calcite filling. The remaining discontinuities either showed no filling material or showed debris deposits, which we interpret as open at reservoir depth. Our results indicate that even higher permeability can be expected for karstified formations related to the reef facies and hydrothermal processes. Our compiled data set, consisting of laboratory and field measurements, may provide a good basis for 3D subsurface modelling and numerical prediction of fluid flow in the naturally fractured carbonate reservoir.

中文翻译：

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莱茵-鲁尔都会区（德国）中用于地热的潜在储层岩石的不连续性表征

在过去的十年中，研究清洁和可再生能源解决方案的重要性日益提高。事实证明，地热能供应可以满足这两个条件。因此，针对不同的现场条件针对不同的现场开发了深层地热应用的概念模型。在德国巴伐利亚州，在4000至6000 m深度的碳酸盐岩层中成功进行了地热应用。基质渗透率和热导率主要研究于侏罗纪晚期礁相的喀斯特碳酸盐岩中。与巴伐利亚州相似，碳酸盐岩位于北莱茵-威斯特法伦州（NRW）的Rhenohercynian断层块的东部，其地热潜力仍缺乏量化。与巴伐利亚相比 在上泥盆纪和下石炭世时期，雷姆沙伊德-阿尔滕纳背斜（在北威州）暴露了一个超区域性碳酸盐山带。我们的研究目的是通过野外和实验室调查来检查潜在的地热储层。因此，对伍珀塔尔，哈根-霍亨利堡和洪内塔尔的三个代表性露头进行了研究。在野外调查期间，通过扫描线调查在各种空间尺度上观察到了1068个间断点（139个未填充的开放裂缝，213个接头，413个方解石填充的静脉和303个充满碎屑沉积的裂缝）。这些不连续性的特征是走线长度，真实间距，粗糙度，孔径和填充材料。不连续性方向分析表明，NNW–SSE，NW–SE，以及目标目标范围内的NE–SW方向。该压实的石灰岩层（Massenkalk）约150 m厚，位于4000至6000 m深度，向北倾斜约30至40度的倾角^∘。对测得的层方位和倾角的外推表明，碳酸盐岩储层可能会在埃森，波鸿和多特蒙德以下延伸。我们对现场和实验室结果的综合分析表明，它可能是天然裂缝的碳酸盐岩储层。在结束对流体流动的影响之前，我们评估了储层中潜在的不连续网络及其相对于主要最大水平应力的方位，并最终得出结论：对未来的地热应用，着眼于突出NNW-SSE的突出间断，因为这些（1）是最常见的是（2）沿主水平应力方向走向；（3）的不连续渗透率大大超过了储集岩矩阵的不渗透率，（4）方解石充填仅观察到了这些不连续性的约38％。其余的不连续性要么没有填充物，要么没有碎屑沉积，我们将其解释为在储层深度处是开放的。我们的结果表明，与礁岩相和热液过程有关的岩溶化地层可望获得更高的渗透率。我们收集的由实验室和现场测量组成的数据集可能 为天然裂缝碳酸盐岩储层的3D地下建模和流体流动的数值预测提供了良好的基础。