Magnetic susceptibility (MS) is a logging method commonly used for investigating lithostratigraphic features in marine sediments or petrophysical parameters. In this paper, an application of MS core logging as a tool identifying paleoenvironment and potential high organic matter content will be presented. The aim of the study was to define the relationship between magnetic methods, Rock-Eval pyrolysis data, and the results of natural gamma-ray wire-line logging. The MS logging was conducted on six drill cores from exploration boreholes in lower Paleozoic shales from the Baltic Basin, northern Poland. In each drill core, an interval of approximately 200 m was examined. The samples for laboratory measurements were taken from two drill cores more than 3500 m deep. The intervals of rock with variable amounts of organic matter were analyzed. The results indicate that MS variations in the analyzed rocks are strongly connected with the percent amount of chlorite and has a positive correlation with oxygen and bioturbation indices while having a negative correlation with total organic carbon. This relationship suggests that MS values were influenced by the redox conditions on the seabed in the early stages of diagenesis. Another important finding was that the MS value is associated with a ferroan chlorite amount. Therefore, it is critical to define the origin of chlorite. When considering the thermal history of studied rocks (buried above 80 degrees C), it is likely that chlorite was of a diagenetic origin. However, to confirm this assumption, Fe-rich clays that are precursors for diagenetic chlorite have to be defined.