|Zusammenfassung||I carried out a magnetotelluric field survey in the southeastern area of the Lower Saxony Basin, in Germany. Eighty-three magnetotelluric stations were deployed along a SW-NE, sixty-three kilometre long profile. The main goals of the survey were of testing this geophysical method for imaging the regional Posidonia black shale formation and evaluating the influence of thermal maturity in its electrical resistivity.
The acquired data were highly affected by cultural noise. For this reason, I discarded the southern profile section between stations one and twenty-seven. However, data processing was successful for the majority of stations between twenty-eight and eighty-three.
The corresponding geo-electric strike estimates point to a predominantly E-W direction, in accordance with regional geology and major faults.
Two-dimensional magnetotelluric inversion results of the acquired data show a series of electrically conductive structures which correlate with brine saturated sediments but also with deeper, meta-anthracitic/graphitized Carboniferous coals. However, none of these structures could be directly related with the Posidonia black shale, which appears to be generally electrically resistive due to its low thermal maturity and, therefore, very difficult to resolve with the magnetotelluric method.
Besides the magnetotelluric experiment, I made a series of laboratory measurements of electrical resistivity on a set of Posidonia black shale samples from the Hils syncline structure found in the Lower Saxony basin. The corresponding rock samples were collected in shallow wells named Wickensen, Harderode and Haddessen. These three wells show immature, oil and gas thermal maturities, respectively. Results showed that the Posidonia black shale samples are electrically resistive, when measured in dry conditions. Furthemore, electrical resistivity is consistently higher when measured against rock bedding, which indicates that the Posidonia black shale is heterogeneous and electrically anisotropic.
I observed no direct correlation between electrical resistivity and thermal maturity: the Harderode samples showed the highest electrical resistivity, whilst the Haddessen samples showed the lowest. This was an unexpected result because the increase of thermal maturity between Wickensen and Haddessen contributed to the aromatization of the organic carbon in the Posidonia black shale. Consequently, this effect should have contributed to a constant
decrease of electrical resistivity in the Posidonia black shale between Wickensen and Haddessen. An ineffective drying process and the presence of moisture could have been the reasons for the observed trend of electrical resistivity.
I tested the dependence of electrical resistivity of the Posidonia black shale samples on water content by saturating them with distilled and saline water solutions. The saturation process led to a constant and significant decrease of electrical resistivity of the experimented samples. Moreover, increasing salinity corresponded to higher drops in electrical resistivity for the Wickensen and Harderode samples. However, the observed decrease of electrical resistivity with distilled water of electrical resistivity was generally lower at the Haddessen samples, meaning the influence of water content in electric conduction could be lower for these samples.
To constrain the influence of water content in electric conduction through the Posidonia black shale, I measured the porosity of all samples and correlated it with electrical resistivity of the water saturated samples. Strong and positive correlations were observed for the Wickensen and Harderode samples. No correlation was observed, however, for the Haddessen samples, possibly due to the lack data points. These observations indicate that the electrical resistivity of Posidonia black shale, at the immature and hydrocarbon generation stages, is porosity controlled.
I also correlated electrical resistivity with the organic carbon content of the samples. No correlation was observed in the Wickensen and Harderode samples, but a strong and positive correlation was observed for the Haddessen samples. This result together with the less prominent influence of water in electric conduction through the Haddessen samples strengthens the possibility of a more influential role of carbon in electric conduction at the gas stage in detriment of pore water and porosity.
I conclude from my research that the electrical conductivity of black shales depends on a variety of factors. At immature and hydrocarbon-generation thermal maturities the Posidonia black shale is not electrically conductive. Furthermore, electric conduction at these maturities seems to be mainly controlled by water content and porosity rather than by carbon content. Because of this, the magnetotelluric method has limited potential for the exploration of such black shales in sedimentary basins such as the Lower Saxony basin. The main difficulty is to distinguish them from other over- and underlying water-rich formations. It is possible, however, to characterize black shales with higher thermal maturities. The deep and highly conductive body present in the inversion models related to meta-anthracitic/graphitized Carboniferous coals, is proof of that possibility.