Print this pageSourgas Generation during Hydrocarbon Migration through Zechstein Evaporites (Upper Permian of Germany)
STEINHOFF, INGO, NICHOLAS E. PINGITORE JR., CHRISTIAN STROHMENGER and KONRAD ROCKENBAUCH
The Zechstein 2 Carbonate (Ca2) in NW Germany is an important natural gas reservoir. Here the Ca2 is stratigraphically underlain by the Werra Anhydrite (A1, up to 300 m thick) and overlain by the Basal Anhydrite (A2). Since different amounts of H[2]S are encountered within the reservoir rocks, it is of economic interest to understand the mechanism of H[2]S generation in this setting.
In reservoirs below the Werra Anhydrite only sweet gas is encountered. Above the Werra Anhydrite, no evidence for generation of major amounts of H[2]S by thermochemical sulfate reduction (TSR) was found in the diagenetic record of the Ca2 reservoir. Calcitized sulfite mineralogies in the Ca2 are too old to be related to TSR. In contrast, calcite and sulfides, both possibly products of TSR, were documented by petrography, electron microprobe and XRD in slope/turbidite facies of the Werra Anhydrite. It is believed that longer hydrocarbon chains (C[2]-C[7]) preferably react during TSR, whereas methane behaves relatively inertly. Comparison of Upper Carboniferous gases with samples from the Ca2 in the same wells has revealed a dramatic decrease of longer hydrocarbons. The amount of methane is slightly increased. Analysis of gases released after dissolution of Al samples indicated that H[2]S occurs in the sulfate depositional system. The low permeability of the A1 may have slowed migration, forcing long, intimate conduct between the gas and the enclosing anhydrite "plumbing." Paleotemperature estimates place conditions in the Al during migration within the TSR window, making it the most likely site of major H[2]S generation.