The distribution of Pre-Westphalian source rocks in the North German Basin – Evidence from magnetotelluric and geochemical data

For the first time this project attempts to directly correlate magnetotelluric and geochemical data with the aim of creating a model on the regional distribution of potential pre-Westphalian source rocks deposited in marine environments in the North German basin.Analysis of the magnetotelluric data shows, that there is a deep good conductor at the north-eastern fringe of the North German basin around the islands of Rügen and Usedom and on the mainland north east of the Anklam Fault. Through integration with seismic data and the offshore well G14 the conductor can be correlated with the Cambro-Ordovician Scandinavian Alum shales. To the south an adjoining area approximately corresponding to the depo-centre of the Rotliegend basin lacks a deep good conductor. Therefore it can be assumed that a regional distribution of comparable source rocks is unlikely. Another excellent and important conductor starts to the south west of the Lower Elbe Line extending along the Dutch-German border into the North Sea, and into the Münsterland. Its place in the local stratigraphy has not been adequately established. It is most likely that this good conductor corresponds to the black shales of the Early Namurian and the Dinantian, which is the case in the boreholes Münsterland 1 and Pröttlin 1 for example. In this paper they are collectively called Rhenohercynian Alum shales. On the Dutch-German border a transition into the “Bowland Shale” facies or equivalents is to be expected. It cannot be ruled out that even stratigraphically older black shales, possibly from the Cambro-Ordovician could contribute to the high integrated conductivity of the deep good conductor.The evidence of highly conductive layers in the deep subsurface poses the question whether these layers could be potential source rocks for the gases in the north German gas fields. This question can be answered with a clear yes. Gas and isotope geochemical studies on gases from producing Rotliegend deposits have shown that Rhenohercynian Alum shales have been a significant source for these fields. This will be illustrated in detail using the gas fields from the production province “Ems Estuary” as an example.