Late Triassic (Rhaetian) black shales in NW-Germany – The paleoenvironment around the Triassic/Jurassic-boundary and the control on the deposition of an “unconventional hydrocarbon play” Martin Blumenberg1,*, Jolanta Kus1, Georg Scheeder1, Carmen Heunisch2, Kristian Ufer1, Carsten Helm1, Klaus-G. Zink1 1
Federal Institute for Geosciences and Natural Resources, Hannover, 30655, Germany 2 State Authority for Mining, Energy and Geology, Hannover, 30655, Germany (* corresponding author:
[email protected])
Unconventional plays for oil and gas may be crucial energy resources for our immediate and long time future. In the frame of the NiKo-project, the German Federal Institute for Geosciences and Natural Resources investigated the potential of shales in Germany for shale oil and gas production. Among other potential plays (most notably the Posidonia (Late Liassic), Wealden (Lower Cretaceous), and marine Lower Carboniferous shales), bituminous shales in NW-Germany of Late Triassic (Rhaetian) age were found to reveal abundant TOC (up to 8 %) as well as a partially excellent hydrocarbon potential (S2 from Rock Eval pyrolysis up to 40 mgHC/gRock and a hydrogen index of > 500 mgHC/gTOC). Those black shales demonstrate that the potential for tight oil or shale oil in the Rhaetian rocks is comparably high, with maturities being also sufficient for considerable shale gas generation (VR % 0.6 to 2.8). Recovery of oil (and gas), however, may be low due to a potentially low brittleness. Studies of potent gas and oil shales from the US showed that a good frackibility coincides with clay contents 60 % (45 to 67 %), indicating that hydraulic fracturing may be not as favorable as in the key unconventional hydrocarbon plays in the US. Further to the promising tight oil/shale gas-potential of Rhaetian shales, in terms of environmental changes the Rhaetian is also a highly interesting time interval in Earth history. Shortly after the deposition of the Rhaetian black shales in NW-Germany, the Triassic/Jurassic (Tr/J) mass-extinction event, one of the `big five' extinctions in Earth history, occurred and the deposition of the black shales may be ultimately linked to rapid environmental changes triggered by the volcanic activities characterising the Central Atlantic Magmatic Province (CAMP) in the Late Rhaetian. Of particular interest for our studies of the paleoenvironmental changes were samples collected from an as yet unstudied drill core from East of Meppen in NW Germany, located close to the border to the 13 Netherlands. The coverage of the T/J-boundary in this core is recorded in the δ Corg-curve, which demonstrates a negative excursion at about 1600 m depth. This so-called initial carbon isotope excursion (ICIE) marks the end Triassic extinction event (ETE) and is present in respective sections worldwide (Hesselbo et al., 2004; Schoene et al., 2010). These changes most likely record perturbations in the biogeochemical cycles of sulfur and carbon due to massive SO2 and CO2 eruptions and H2S-poisoning (Richoz et al., 2012), global cooling and sea-level change (Schoene et al., 2010). Strong geochemical and organismal changes were also found in the studied core. Carbon to sulfur ratios demonstrate generally low values compared to normal marine depositional environments and the decoupling of both cycles, shortly before the Tr/J-boundary (Corg/S = 0.03). Dinosteranes were found throughout the core and most likely represent early Dinoflagellates (Thomas et al., 1993). However, dinosteranes were highest in Rhaetian samples deposited before the Tr/J-boundary, which corroborates Dinoflagellate fluctuations at other areas at that time (van de Schootbrugge et al., 2013). The existence of an oxic-anoxic transition zone is indicated by abundant gammacerane. Further, the importance of anaerobic sulfur cycling and photic zone euxinia is reflected by high occurrences of isorenieratane. However, this and other signatures in the studied core are most pronounced in the latest Rhaetian black shales and not at or after the ICIE. 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