Abstract In sedimentary basins geothermal resources may coexist alongside hydrocarbon resources. The latter can represent a risk to geothermal exploration as experienced during the drilling of the deep St. Gallen geothermal well in eastern Switzerland. In this case, the unexpected occurrence of substantial amounts of natural gas, along with other external factors, prevented the continuation of the geothermal project. Therefore, this work aims at resolving the origin of the gas alongside evaluating the processes activating the petroleum system in the study area. In order to characterize the petroleum system of St. Gallen, we performed a basin analysis study aimed at reconstructing the thermal history of the basin and quantifying the main variables controlling the temperature in the basin: the paleo-heat flow and the magnitude and timing of the most relevant erosion events. The findings from this study indicate that the thermal conditions attained in the area were mostly controlled by the deposition of the Molasse units during the Oligocene-Miocene time. Older thermal events could not be detected by organic paleothermometers. An erosion thickness of 1800−2000 m was estimated for the Molasse deposits, related to the uplift of the Northern Alpine foreland, occurring in the area most likely at 8−5 Ma. Results of thermal modelling revealed favourable conditions for the activation of a petroleum system in the St. Gallen area. The source rocks, located most likely in the Permo-Carboniferous grabens, are in the gas window. The model suggests that most of the hydrocarbons generated in the study area migrated northward, because of the southward dipping of the basin. According to the model only a small percentage of hydrocarbons were trapped in reservoirs, mostly located in the uppermost Permo-Carboniferous and basal Mesozoic units. Accumulations were simulated a few hundred meters below the final depth reached by the geothermal St. Gallen GT-1 well. From here the gas likely migrated into the overlying Mesozoic units, the target area of the well, where effectively the model predicts high petroleum saturation levels. This work demonstrates that the basin-scale thermal modelling approach adopted in this study should be incorporated into the feasibility and planning phase of future geothermal exploration campaigns to de-risk the subsurface manifestation of hydrocarbons.

中文翻译：

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减轻地热能源勘探风险的盆地热建模方法：圣加仑案例研究（瑞士东部）

摘要 在沉积盆地中，地热资源可能与油气资源共存。后者可能对地热勘探构成风险，正如在瑞士东部圣加仑深地热井钻探过程中所经历的那样。在这种情况下，大量天然气的意外出现以及其他外部因素阻止了地热项目的继续进行。因此，这项工作旨在解决天然气的来源，同时评估激活研究区石油系统的过程。为了表征圣加仑的石油系统，我们进行了盆地分析研究，旨在重建盆地的热历史并量化控制盆地温度的主要变量：古热流以及最相关侵蚀事件的规模和时间。这项研究的结果表明，该地区获得的热条件主要受渐新世-中新世时期磨拉石单元的沉积控制。有机古温度计无法检测到较旧的热事件。估计 Molasse 矿床的侵蚀厚度为 1800-2000 m，与北阿尔卑斯前陆的隆起有关，最有可能发生在 8-5 Ma 的区域。热模拟结果揭示了圣加仑地区石油系统活化的有利条件。烃源岩最有可能位于二叠纪—石炭系地堑，位于气窗内。该模型表明，研究区产生的大部分碳氢化合物向北迁移，由于盆地向南倾斜。根据该模型，只有一小部分油气被困在储层中，大部分位于二叠纪-石炭纪和基底中生界单元。在地热 St. Gallen GT-1 井达到的最终深度以下几百米处模拟了堆积物。气体很可能从这里迁移到上覆的中生界单元，即井的目标区域，模型有效地预测了高石油饱和度水平。这项工作表明，本研究中采用的盆地规模热建模方法应纳入未来地热勘探活动的可行性和规划阶段，以降低碳氢化合物的地下表现形式的风险。根据该模型，只有一小部分油气被困在储层中，大部分位于二叠纪-石炭纪和基底中生界单元。在地热 St. Gallen GT-1 井达到的最终深度以下几百米处模拟了堆积物。气体很可能从这里迁移到上覆的中生界单元，即井的目标区域，模型有效地预测了高石油饱和度水平。这项工作表明，本研究中采用的盆地规模热建模方法应纳入未来地热勘探活动的可行性和规划阶段，以降低碳氢化合物的地下表现形式的风险。根据该模型，只有一小部分油气被困在储层中，大部分位于二叠纪-石炭纪和基底中生界单元。在地热 St. Gallen GT-1 井达到的最终深度以下几百米处模拟了堆积物。气体很可能从这里迁移到上覆的中生界单元，即井的目标区域，模型有效地预测了高石油饱和度水平。这项工作表明，本研究中采用的盆地规模热建模方法应纳入未来地热勘探活动的可行性和规划阶段，以降低碳氢化合物的地下表现形式的风险。在地热 St. Gallen GT-1 井达到的最终深度以下几百米处模拟了堆积物。气体很可能从这里迁移到上覆的中生界单元，即井的目标区域，模型有效地预测了高石油饱和度水平。这项工作表明，本研究中采用的盆地规模热建模方法应纳入未来地热勘探活动的可行性和规划阶段，以降低碳氢化合物的地下表现形式的风险。在地热 St. Gallen GT-1 井达到的最终深度以下几百米处模拟了堆积。气体很可能从这里迁移到上覆的中生界单元，即井的目标区域，模型有效地预测了高石油饱和度水平。这项工作表明，本研究中采用的盆地规模热建模方法应纳入未来地热勘探活动的可行性和规划阶段，以降低碳氢化合物的地下表现形式的风险。