The Danish onshore subsurface contains very large geothermal resources that have the potential to make a significant contribution to transforming Danish energy consumption toward a more sustainable energy mix. Presently, only a minor fraction of this green energy is exploited in three small plants. The main factors that have hampered and delayed larger-scale deployment are related to uncertainties in the geological models, which inevitably lead to high economic risks that are difficult for smaller district heating companies to mitigate without support from a compensation scheme. To facilitate and stimulate much wider use of the Danish geothermal resources, the Geological Survey of Denmark and Greenland (GEUS) and other research institutes have conducted several regional research projects focusing on the geological and geochemical obstacles with the principal objective of reducing the exploration risks by selecting the best geological reservoirs.One of the most important geological factors causing uncertainty is the quality of the reservoirs and their ability to produce the expected volume of warm geothermal brine. Thus, great emphasis has been placed on investigating and understanding the relationships between reservoir sandstone, porosity, permeability, petrography, diagenetic processes and alterations related to variable sediment sources, basin entry points, depositional systems and climate, burial and thermal history. Mesozoic sandstones comprise the most important geothermal reservoirs in Denmark. Details concerning the reservoir quality are compiled and compared for the Lower Triassic Bunter Sandstone, Triassic Skagerrak, Upper Triassic – Lower Jurassic Gassum and Middle Jurassic Haldager Sand formations. The Bunter Sandstone Formation contains extensive aeolian and more confined fluvial sandstones with high porosity and permeability. However, highly saline formation water could be unfavourable. The Skagerrak Formation comprises well-sorted braided stream sandstones in the centre of the basin, and is otherwise characterised by muddy sandstones and alluvial fan conglomerates. An immature mineralogical composition has caused intensive diagenetic changes in the deepest buried parts of the basin. The Gassum Formation consists of shoreface, fluvial and estuarine sandstones interbedded with marine and lacustrine mudstones. In the upper part of the formation, the sandstone beds pinch out into mudstones towards the basin centre. Pervasive siderite- and calcite cement occurs locally in shallowly buried sandstones, and with burial depth the maximum abundances of quartz and ankerite cement increase. Sandstones of shallow burial represent excellent reservoirs. The relatively coarse grain size of the Haldager Sand Formation results in high porosity and permeability even at deep burial, so the formation comprises a high-quality geothermal reservoir.Substantial progress has been made, and a well-established regional geological model combined with reservoir quality is now available for areas with cored wells. This has enabled an improved estimation of reservoir quality between wells for exploration of geothermal reservoirs.

中文翻译：

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丹麦潜在地热储层概览

丹麦陆上地下含有非常大量的地热资源，这些资源有可能为将丹麦的能源消费转变为更可持续的能源组合做出重大贡献。目前，只有一小部分绿色能源在三个小型工厂中得到利用。阻碍和延迟大规模部署的主要因素与地质模型的不确定性有关，这不可避免地导致较高的经济风险，小型区域供热公司在没有补偿计划支持的情况下难以缓解。为了促进和刺激更广泛地利用丹麦地热资源，丹麦和格陵兰地质调查局 (GEUS) 等研究机构开展了多个区域研究项目，重点关注地质和地球化学障碍，主要目标是通过选择最佳地质储层来降低勘探风险。最重要的地质因素之一造成不确定性的是储层的质量及其生产预期体积的温暖地热盐水的能力。因此，研究和理解储层砂岩、孔隙度、渗透率、岩相学、成岩过程以及与可变沉积物来源、盆地入口点、沉积系统和气候、埋藏和热历史相关的变化之间的关系非常重视。中生代砂岩是丹麦最重要的地热储层。汇编并比较了下三叠统 Bunter 砂岩、三叠统 Skagerrak、上三叠统 - 下侏罗统瓦斯岩和中侏罗统 Haldager 砂岩地层的储层质量细节。Bunter 砂岩组包含广泛的风成砂岩和更封闭的河流砂岩，具有高孔隙度和渗透率。然而，高含盐地层水可能是不利的。Skagerrak 组在盆地中心由分选良好的辫状流砂岩组成，其他特征是泥质砂岩和冲积扇砾岩。一种不成熟的矿物成分在盆地的最深处引起了强烈的成岩作用。Gassum 组由滨面、河流和河口砂岩与海相和湖相泥岩互层。在地层的上部，砂岩层向盆地中心尖伸成泥岩。普遍的菱铁矿和方解石胶结物局部存在于浅埋砂岩中，随着埋藏深度的增加，石英和铁白石膏胶结物的最大丰度增加。埋藏较浅的砂岩是优良的储层。Haldager砂组粒度相对粗大，即使在深埋处也具有较高的孔隙度和渗透率，构成了优质地热储层。取得了实质性进展，建立了与储层质量相结合的区域地质模型现在可用于有取芯井的地区。