Gas seepages in Germany: Revisited subsurface permeabilities in the German mining district

Abstract

The German mining district, the Ruhr area, has had a coal mining history for nearly two centuries. Even with this history, all the geological processes in the region are still not fully understood. For the coal mining, extensive exploration work was necessary, and thousands of wells were drilled. These wells targeted Carboniferous coal layers and were used to provide an understanding of its geology and tectonic structure. In the southern region, the Carboniferous pinches out at the surface. In the north, theses rocks are deeply buried under approximately 1200 m (3937 ft) of Mesozoic, mainly Cretaceous, layers. Although the Carboniferous is covered in the north, areas with methane seepages can be found. Because of regional and local changes in the lithology of the overlying Cretaceous successions, whether these gas seepages link the Carboniferous to the surface through the permeable Cretaceous has not been clear. The scope of this study was to understand the gas seepages at the surface and to identify potential pathways of a methane migration in the subsurface. Input data sets, which vary in availability and quality, include regional digital subsurface models. To improve the subsurface understanding, permeability distributions in the Cretaceous were simulated and an assessment of the known fault system was made using the modeling software Petrel. Constructing a detailed static subsurface model showed that the shallow subsurface has, in general, a high probability of low permeabilities and sealing faults. Streaks of higher permeability along the fault planes, however, could create pathways for a possible migration of methane. The methane migration and the degassing of the deeper subsurface could potentially influence further exploration activities in the region. The results could also be used to identify shallow gas hazards for drilling operations.