Oxidation of Organic Matter in the Transition Zone of the Zechstein Kupferschiefer from the Sangerhausen Basin, Germany

Abstract

Thirteen samples of the Permian Kupferschiefer (Sangerhausen Basin, Germany) were studied by microscopy, stable isotope analysis, Rock-Eval analysis, Fourier transform infrared (FT-IR) spectroscopy, and additional geochemical methods in order to understand the oxidation processes of organic matter in a profile of mineralized Kupferschiefer. Relatively intense oxidation occurred at the bottom of the profile, where bitumens were oxidized by ascending brines, and altered to pyrobitumen and “kir” bitumen. Because of organic matter oxidation Corg-values, extract yields, and HI values decrease at the bottom of the profile. The oxidation resulted in release of δ12C-rich organic constituents and enrichment of the residual organic matter in δ13C organic carbon in the extracts of the oxidized Kupferschiefer. In the bottom sample, major IR absorption bands between 1207 and 900 cm-1 (C−O−C, C−O−H) were caused by oxidation of saturated hydrocarbons. A reduced absorption at 2955−2869 cm-1 relative to less oxidized samples indicates oxidation of aliphatic compounds. The intensity of the carbonyl band at 1700 cm-1 (CO) relative to the band at 1600 cm-1 (CC) decreases from the bottom to the top of the profile, indicating a preferential oxidation in the bottom part of the profile. Results from the analysis of extracts by gas chromatography−mass spectrometry (GC-MS) indicate that these major absorption bands are due to the oxidation of long-chain saturated hydrocarbons and alkylated aromatic hydrocarbons. The oxidation products from alkylated aromatic compounds include naphthalene, biphenyl, phenanthrene, and dibenzofuran. The oxidation products of saturated hydrocarbons include aliphatic acids, alcohols, and esters. The oxidation may have occurred after Kupferschiefer deposition.