Abstract
The Middle Triassic Grenzbitumenzone Formation is a 16-m-thick sequence of finely laminated, organic-rich dolomites and black shales. TOC contents reach 10% in the dolomites and 40% in the shales. Sedimentological and geochemical data suggest deposition in a shallow marine basin under permanently anoxic conditions. The organic matter is of type II. Based on geochemical parameters the Grenzbitumenzone can be divided into two sections. In the lower part, hopanes are present in very high concentrations (up to one order of magnitude higher than steranes) and the δ13C of the TOC varies between -30.0 and -31.8‰ (PDB). Above 13 m from the base hopanes are scarce and the δ13C of the TOC becomes less negative. Sedimentological and geochemical data suggest that a planktonic “bacterial plate” developed at the oxic-anoxic boundary in the water column and that bacteria were a significant contributor to the primary organic matter production in the basin. Variations in the maturity-dependent Ts/Tm ratio and 20S/20R C29 sterane isomerization ratio occur over thicknesses of a few metres. Their correlation with the carbon isotope composition of the extracts suggests that the scatter may be related to changes in organic matter input and/or bacterial activity during early diagenesis and not differences in maturity.
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Bernasconi, S., Riva, A. (1993). Organic Geochemistry and Depositional Environment of a Hydrocarbon Source Rock: The Middle Triassic Grenzbitumenzone Formation, Southern Alps, Italy/Switzerland. In: Spencer, A.M. (eds) Generation, Accumulation and Production of Europe’s Hydrocarbons III. Special Publication of the European Association of Petroleum Geoscientists, vol 3. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-77859-9_15
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DOI: https://doi.org/10.1007/978-3-642-77859-9_15
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