Kilometer-scale fault-related thermal anomalies in tight gas sandstones |
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| Institution: | 1. EMR | Energy and Mineral Resources Group, Reservoir-Petrology, RWTH Aachen University, Aachen, Germany;2. Helmholtz Zentrum Potsdam GFZ, German Research Center for Geoscience, Potsdam, Germany;3. Geoscience Center of the University of Göttingen (GZG), Göttingen, Germany;4. Wintershall Holding GmbH, Kassel, Germany;5. Institute of Applied Geosciences, Structural Geology & Tectonics, Karlsruhe Institute of Technology, Karlsruhe, Germany;6. TTE Reservoir-Geology, KIT Campus Transfer GmbH, Karlsruhe, Germany;1. Petroleum Geophysics Program, Department of Geological Sciences, Chiang Mai University, 50400, Thailand;2. Department of Earth Sciences, University of Oxford, UK;3. Shell Global, The Hague, The Netherlands;1. School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK;2. Three Cliffs Geomechanical Analysis Ltd, Roseleigh, Swansea, SA3 2EN, UK;3. School of Civil Engineering, University of Leeds, Leeds, LS2 9JT, UK;1. Dept. of Applied Geosciences and Geophysics, Montanuniversitaet Leoben, 8700 Leoben, Austria;2. Babeș-Bolyai-Universität, Cluj, Romania;3. OMV Exploration & Production GmbH, 1020 Wien, Austria;1. GeoBioTec, Departamento de Geociências, Universidade de Aveiro, Portugal;2. MARE, Departamento de Ciências da Terra, Universidade de Coimbra, Portugal;3. Institut für Geologie, Leibniz Universität Hannover, Germany;4. Institute for Geology, Mineralogy and Geophysics, Ruhr-Universität Bochum, Germany;1. Council for Geoscience, 139 Jabu Ndlovu Street, Pietermaritzburg 3200, South Africa;2. Discipline of Geological Sciences, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Westville, Private Bag X54001, South Africa |
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| Abstract: | Upper Carboniferous sandstones make one of the most important tight gas reservoirs in Central Europe. This study integrates a variety of geothermometers (chlorite thermometry, fluid inclusion microthermometry and vitrinite reflection measurements) to characterize a thermal anomaly in a reservoir outcrop analog (Piesberg quarry, Lower Saxony Basin), which is assumed responsible for high temperatures of circa 300 °C, deteriorating reservoir quality entirely. The tight gas siliciclastics were overprinted with temperatures approximately 90–120 °C higher compared to outcropping rocks of a similar stratigraphic position some 15 km to the west. The local temperature increase can be explained by circulating hydrothermal fluids along the fault damage zone of a large NNW-SSE striking fault with a displacement of up to 600 m in the east of the quarry, laterally heating up the entire exposed tight gas sandstones. The km-scale lateral extent of this fault-bound thermal anomaly is evidenced by vitrinite reflectance measurements of meta-anthracite coals (VRrot ∼ 4.66) and the temperature-related diagenetic overprint. Data suggest that this thermal event and the associated highest coalification was reached prior to peak subsidence during Late Jurassic rifting (162 Ma) based on K-Ar dating of the <2 μm fraction of the tight gas sandstones. Associated stable isotope data from fluid inclusions, hosted in a first fracture filling quartz generation (T ∼ 250 °C) close to lithostatic fluid pressure (P ∼ 1000 bars), together with authigenic chlorite growth in mineralized extension fractures, demonstrate that coalification was not subject to significant changes during ongoing burial. This is further evidenced by the biaxial reflectance anisotropy of meta-anthracite coals. A second event of quartz vein formation occurred at lower temperatures (T ∼ 180 °C) and lower (hydrostatic) pressure conditions (P ∼ 400 bars) and can be related to basin inversion. This second quartz generation might be associated with a second event of illite growth and K-Ar ages of 96.5–106.7 Ma derived from the <0.2 μm fraction of the tight gas sandstones.This study demonstrates the exploration risk of fault-bound thermal anomalies by deteriorating entirely the reservoir quality of tight gas sandstones with respect to porosity and permeability due to the cementation with temperature-related authigenic cements. It documents that peak temperatures are not necessarily associated with peak subsidence. Consequently, these phenomena need to be considered in petroleum system models to avoid, for example, overestimates of burial depth and reservoir quality. |
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| Keywords: | Thermal anomaly Diagenesis Geothermometers Stable isotopes Reservoir outcrop analog Upper Carboniferous Lower Saxony Basin Tight gas sands |
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