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- ItemDetailed Fluid Inclusion and Stable Isotope Analysis on Deep Carbonates from the North Alpine Foreland Basin to Constrain Paleofluid Evolution(London : Hindawi, 2019) Mraz, Elena; Wolfgramm, Markus; Moeck, Inga; Thuro, KuroschThe recent interest on environmentally friendly energy resources has increased the economic interest on the Upper Jurassic carbonate rocks in the North Alpine Foreland Basin, which serves as a hydrogeothermal reservoir. An economic reservoir use by geothermal fluid extraction and injection requires a decent understanding of porosity–permeability evolution of the deep laying Upper Jurassic strata at depths greater than 2000 m. The analysis of paleofluids caught in cements of the rock mass helps to determine the postdepositional reservoir evolution and fluid migration. Therefore, the high- and low-permeability areas of the Upper Jurassic in the North Alpine Foreland Basin referred to as Molasse Basin were analyzed by means of encountered postdepositional cements to determine the reservoir evolution. The cements were sampled at different hydrocarbon and geothermal wells, as well as at outcrops in the Franconian and Swabian Alb. To determine the composition and temperature of the paleofluids, fluid inclusions and cements of the Upper Jurassic carbonate rocks were analyzed by microthermometry and stable isotope measurements. Since drill cuttings are a rather available sample material compared to drill cores, a new microthermometry measurement method was achieved for the around 1 mm drill cuttings. Salinity and formation temperature of paleofluids in fluid inclusions and isotope data are consistent with previous studies and reveal a 5-stage evolution: the main cementation phases are composed of (I) the early diagenesis in limestones (200-400 m, 40-50°C), (II) early diagenetic dolomitization, and (III) burial dolomitization (1-2 km, II: 40-90°C; III: 70-100°C; 40 g/L NaCl equiv.), and (IV) late burial calcification (IIIa: 110-140°C, IIIb: 140-200°C) linked to tectonic features in the Molasse Basin. In the outcrop samples, a subsequent (V) cementation phase was determined controlled by karstification. In the southwest, an increase in salinity of the fluid inclusions in vein calcites, above the salinity of the Jurassic seawater, highlights the influence of basin fluids (diagenetic, evaporitic). In the other eastern wells, vein calcites have precipitated from a low saline fluid of around 10-20 g/L NaCl equiv. The low salinity and the isotope values support the theory of a continuous influence of descending meteoric fluids. Consequently, the Upper Jurassic seawater has been diluted by a meteoric fluid to a low saline fluid (<1 g/L), especially in areas with high permeability. Here, we show how a better understanding of cementation trajectory at depth can help to generate a better understanding of geothermal usability in deep carbonate reservoirs.
- ItemMultiphase fossil normal faults as geothermal exploration targets in the Western Bavarian Molasse Basin: Case study Mauerstetten(Stuttgart : Schweizerbart, 2018) Mraz, Elena; Moeck, Inga; Bissmann, Silke; Hild, StephanMraz, E., Moeck, I., Bissmann, S. & Hild, S. (2018): Multiphase fossil normal faults as geothermal exploration targets in the Western Bavarian Molasse Basin: Case study Mauerstetten. – Z. Dt. Ges. Geowiss., 169: 389–411, Stuttgart. The Bavarian Molasse Basin represents a peripheral foreland basin hosting abundant hydrothermal resources in 3–5 km deep Upper Jurassic carbonate rocks. Faults and facies play a major role in targeting production wells; however the kinematic evolution of fault zones and the classification of carbonate facies of the Upper Jurassic are still debated. At the geothermal prospect Mauerstetten in the Western Bavarian Molasse Basin, a geothermal well and a side track are drilled along and about 650 m off an ENE–WSW striking normal fault. A stratigraphy related fault throw analysis of six 2D seismic sections crossing this fault evidences multiphase normal faulting from Cretaceous to Upper Miocene with a major activity phase in the Oligocene. This fault, inactive since Upper Miocene, is presumably a fossil normal fault in the present-day stress field that has a maximum horizontal stress direction in N–S. Analysis of carbonate facies by thin section petrography of drill cuttings and geophysical borehole logs lead to two major conclusions: (i) the reservoir rock represents low permeable platform limestones, reef detritus and dolostones of the Franconian facies, and (ii) the fault consists of multiple normal faulting steps with higher permeability than in intact rock. This observation suggests a fracture controlled reservoir with permeable damage zones in a tight rock mass along reactivated normal faults.