Fluid inclusions have been studied in minerals infilling fissures (quartz, calcite, fluorite, anhydrite) hosted by Carboniferous and Permian strata from wells in the central and eastern part of the North German Basin in order to decipher the fluid and gas migration related to basin tectonics. The microthermometric data and the results of laser Raman spectroscopy reveal compelling evidence for multiple events of fluid migration. The fluid systems evolved from a H 2 O-NaCl±KCl type during early stage of basin subsidence to a H 2 ONaCl-CaCl 2 type during further burial. Locally, fluid inclusions are enriched in K, Cs, Li, B, Rb and other cations indicating intensive fluid-rock interaction of the saline brines with Lower Permian volcanic rocks or sediments. Fluid migration through Carboniferous sediments was often accompanied by the migration of gases. Aqueous fluid inclusions in quartz from fissures in Carboniferous sedimentary rocks are commonly associated with co-genetically trapped CH 4 -CO 2 inclusions. P-T conditions estimated, via isochore construction, yield pressure conditions between 620 and 1,650 bar and temperatures between 170 and 300°C during fluid entrapment. The migration of CH 4 -rich gases within the Carboniferous rocks can be related to the main stage of basin subsidence and stages of basin uplift. A different situation is recorded in fluid inclusions in fissure minerals hosted by Permian sandstones and carbonates: aqueous fluid inclusions in calcite, quartz, fluorite and anhydrite are always H 2 O-NaCl-CaCl 2 -rich and show homogenization temperatures between 120 and 180°C. Co-genetically trapped gas inclusions are generally less frequent. When present, they show variable N 2 -CH 4 compositions but contain no CO 2 . P-T reconstructions indicate low-pressure conditions during fluid entrapment, always below 500 bar. The entrapment of N 2 -CH 4 inclusions seems to be related to phases of tectonic uplift during the Upper Cretaceous. A potential source for nitrogen in the inclusions and reservoirs is C org -rich Carboniferous shales with high nitrogen content. Intensive interaction of brines with Carboniferous or even older shales is proposed from fluid inclusion data (enrichment in Li, Ba, Pb, Zn, Mg) and sulfur isotopic compositions of abundant anhydrite from fissures. The mainly light d 34 S values of the fissure anhydrites suggest that sulfate is either derived through oxidation and redeposition of biogenic sulfur or through mixing of SO 4 2À -rich formation waters with variable amounts of dissolved biogenic sulfide. An igneous source for nitrogen seems to be unlikely since these rocks have low total nitrogen content and, furthermore, even extremely altered volcanic rocks from the study area do not show a decrease in total nitrogen content.
Geol Rundsch (1998) 86 : 775-785 Springer-Verlag 1998 OR IGINAL PAPER M. Schild · A. Vollbrecht · S. Siegesmund · Chr. Reutel Microcracks in granite cores from the EPS-1 geothermal drill hole, Soultz-sous-Foreˆts (France): paleostress directions, paleofluids and crack-related Vp-anisotropies Abstract For the modelling of Hot Dry Rock systems the interactions between macrofractures, prominent microcrack sets and the in situ stress field is of fundamental importance. In this study complete 3D analyses of microcrack orientations were carried out on granitoid samples from the Soultz geothermal borehole (EPS-1). The crack population is dominated by healed cracks in quartz forming three orthogonal sets (I, II, III) with strong preferred orientation, which probably result from superposition of internal thermal stresses during cooling and external tectonic stresses. The inferred orientation of paleo-& is NE-SW. Based on the microthermometry of secondary fluid inclusions it can be assumed that cyclic crack/healing events occurred at P/T conditions roughly ranging between 1 and 2 kbar and 200-400°C. A younger generation of open cracks in quartz which coincide with macroscopic fractures can be correlated with the Tertiary N-S direction of & . The distribution of P-wave velocities (Vp) was measured by using the pulse transmission technique in 132 propagation directions at confining pressures up to 200 MPa. These Vp data and their variation with confining pressure are a suitable tool for detecting prominent crack patterns and to estimate the crack-controlled in situ properties. The observed bulk Vp anisotropy roughly reflects the composite microcrack fabric and the relative importance of individual sets. It can be assumed that the prominent crack sets represent planes of weakness and hence may cause a mechanically anisotropic behaviour dependent on their orientation with respect to the recent in situ stresses.
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