Fluid and gas migration in the North German Basin: fluid inclusion and stable isotope constraints

Abstract

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.

REY and Trace Element Chemistry of Fluorite from Post-Variscan Hydrothermal Veins in Paleozoic Units of the North German Basin

“…Due to a limited number of analyses the trend of LREE enrichment and HREE depletion in chondrite-normalized patterns is less pronounced but still visible in fluorite from ABB Well D (Figure 5d). Fluorite from ABB Well E displays a flat C1 chondrite-normalized signature and is overall quite similar to the signatures found in fluorite from the LSB (Figure 5d,e) which also correspond with previously published data by Duschl et al [12]. Negative Eu CN anomalies were also in some of the samples (ABB Well AI, CI, EI) but are not particularly pronounced.…”

“…Changing tectonic settings, from extensional to divergent movements and the stepwise and long-term subsidence followed by a later inversion of the basin enabled fluid migration along structurally controlled pathways and led to the formation of several hydrothermal vein systems in the NGB [2,[11][12][13][14][15]. The possible fluid sources (e.g., meteoric or magmatic water) and the heat source for the enhanced geothermal gradients are also discussed by these authors.…”

“…In particular the fluorite-barite mineralization and REE-enriched hematite veins in the ABB and FCB have been attributed to the latter phase [13,17]. The Harz Mountains have historically been an important mining area for fluorite and other commodities and post-Variscan hydrothermal mineralization in this area shows many similarities to the hydrothermal veins found in sedimentary rocks and volcanic units further north in the FCB and the ABB [12,18]. For example, sulphur isotopic ratios measured by Schmidt Mumm and Wolfgramm [13] are strikingly similar and could reflect a mutual source, that is, Rotliegend volcanics and underlying Carboniferous sedimentary rocks.…”

“…In particular in the ABB, where a suite of volcanogenic units of variable compositions [7] is part of the basin's Paleozoic rock inventory, the possibility that percolating basinal brines migrated through these volcanic rocks along steep and deep-reaching faults [94] and picked up their distinct REY signature, including a typical negative Eu anomaly, is a plausible explanation for the observed REY CN signatures. Lüders et al [12], for example, provide evidence from fluid inclusion investigations that indicated high water-rock interactions with Permian volcanic and sedimentary rocks in the ABB. However, a primary volcanogenic fluid source can be discarded for both the ABB and LSB [2,12,18,93].…”

“…Lüders et al [12], for example, provide evidence from fluid inclusion investigations that indicated high water-rock interactions with Permian volcanic and sedimentary rocks in the ABB. However, a primary volcanogenic fluid source can be discarded for both the ABB and LSB [2,12,18,93]. The flat REY CN signature [95,96] and the overall low concentrations are strong indicators that basinal brines derived from marine water represent the mineralizing fluid in the LSB.…”