Petroleum-Fueled Microbially-Mediated Carbonate and Native Sulfur Authigenesis in a Gypsum Caprock of a Zechstein Diapir, Northern Germany
Henning Peters and Joern Peckmann
Bremen University, Research Center for Ocean Margins (RCOM), Bremen, Germany
Due to microbial activity fueled by hydrocarbons salt dome environments generated an economically important amount of epigenetic minerals in the past. In our study we focus on a gypsum caprock in Northern Germany, which contains native sulfur and authigenic carbonates. The salt diapir is located at the southern rim of the Lower Saxony Basin, the former main oil province in Germany. The onset of diapirism of Zechstein salt was induced by extensional tectonics in the Late Oxfordian. The cap rock consists of the following lithologies from base to top: (1) a basal succession of condensed anhydrite cross-cut by bitumen filled selenitic fractures (2) massive to nodular gypsum which contains sulfur veins and carbonate nodules, (3) a layered, bitumen-impregnated limestone and (4) a carbonate cap consisting of loosely packed, subhedral to euhedral calcite with the interstitial porespace filled by pyrobitumen. A unique assemblage, that records the epigenetic processes, includes sulfur intimately associated with spheroidal and spindle-shaped calcite and accessory pyrite embedded in a bituminous matrix. In order to constrain if there is a causal link between prolonged biodegradation, uptake of sulphur species and the elemental sulfur precipitation, we apply sulfur, oxygen and carbon isotopes of the mineral assemblages together with organic geochemical analysis of rock and bitumen samples at different stages of biodegradation. An exclusively anaerobic mechanism favored by high C/H ratios in the biodegraded petroleum is the most likely explanation.