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Magnetic Mineral Composition as a Potential Indicator of Depositional Conditions in Gas-Bearing Silurian Shale Rocks from Northern Poland

Abstract

In our studies we focus on the rockmagnetic properties of two types of Silurian gas-bearing shales from Northern Poland: the Pelplin Formation and the Jantar Member, which both represent a potential source of unconventional hydrocarbons. The analyzed rocks have similar burial evolution, but different amounts of organic matter (in the Pelplin samples the TOC content does not exceed 1.5 percent, while in the Jantar it reaches up to 7 percent). Additionally, spherical carbonate concretions in the Pelplin Fm. were investigated. The differences in magnetic mineral assemblage may help in better understanding the determinants, which influence water chemistry at the bottom of the sedimentary basin and thus the preservation of organic matter. Therefore, in order to recognize nano-particles, not detectible in basic rockmagnetic studies, low temperature SIRM measurements in the 10 – 300 K range were performed. The results show the presence of MD and SP magnetite, which we associate with detrital and chemical origin (smectite illitization or organic maturation), respectively. Furthermore, the most interesting observation is the appearance of hematite in the Pelplin Fm. (mostly SD grains), while in the organic rich Jantar Mb. this mineral is absent. We suggest that hematite in mudstones and concretions is a product of magnetite reaction in oxic conditions. This hypothesis is consistent with the presence of early diagenetic carbonate concretions and also with lower values of organic matter in the Pelplin Fm. Moreover, the hematite preserved in both mudstones and concretions in the Pelplin Fm. suggests that stable oxic conditions were present during sedimentation and early compaction process. As a main conclusion, we propose correlation between hematite and organic matter content in shale rocks, which may be a useful factor in understanding the preservation of organic matter. However, further investigation is necessary to fully recognize this complex problem. I thank the Institute for Rock Magnetism for Visiting Fellowship, which was funded by the US National Science Foundation and the University of Minnesota. The visit has been partially financed from the funds of the Leading National Research Centre (KNOW) received by the Centre for Polar Studies for the period 2014-2018. This work has been also funded by the Polish National Centre for Research and Development within the Blue Gas project (No BG2/SHALEMECH/14). Samples were provided by the PGNiG SA.