--> New constraints on thermal maturity by means of reflectance and micro-Raman Spectroscopy on organoclasts from Holy Cross Mountains (Central Poland)

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New constraints on thermal maturity by means of reflectance and micro-Raman Spectroscopy on organoclasts from Holy Cross Mountains (Central Poland)


Building on the success in the U.S., in the last decades a great interest in unconventional resources has been developed in Eastern and Northern Europe. Poland is one of the most perspective country and the Lower Paleozoic successions in a wide belt ranging from the Baltic Sea to the Ukraine border comprising the Baltic basin and the Lublin basin became targets for shale-gas exploration. Holy Cross Mountains (HCM) Paleozoic outcrops, in Central Poland, provide a unique opportunity to sample and analyse successions that are similar in age to those buried in Baltic and Lublin basins.

The aim of this work is to provide new thermal maturity data by means of reflectance (Roeq%) and micro-Raman analyses on organoclasts and compare them with maturity data derived from previous works (Belka, 1990; Marynowski et al., 2012; Narkiewicz, 2010; Szczepanik, 2013).

Vitrinite reflectance is by far the most popular indicator for thermal maturity, but is not an useful tool when exploring targets that are devoid of vitrinite such as the old Paleozoic successions of the HCM. However, marine organoclasts abound and reflectance measurements were taken on other organoclasts (scolecodonts, chitinozoans and in particularly graptolites) following Betrand and Malo (2012) method.

In addition micro-Raman analyses were performed on graptolites. Parameters obtained from the treatment of the Raman spectra of dispersed organic matter can be used to describe temperature paths in metamorphic rocks (Beyssac et al., 2002; Beyssac et al., 2003; Lahfid et al., 2010) or thermal maturity in diagenetic conditions (Jehlicka et al., 2003; Kelemen and Fang, 2001; Liu et al., 2012; Romero-Sarmiento et al., 2014). In this study micro-Raman analyses were directly performed on graptolites surface to test if Raman parameters show correlation with reflectance values.

HCM are located eastward in the central part of the Trans-European Suture Zone (TESZ) and comprise Paleozoic sedimentary rocks ranging in age from Cambrian to Early Carboniferous, emerged from below the Permian and Mesozoic cover during the Late Cretaceous movements (Nawrocki et al., 2007). The Paleozoic rocks crop out along a tectonic line, the Holy Cross Fault, that divide the HCM in two different tectonic blocks: the Lysogory region to the North and the Kielce region (part of the Malopolska massif) to the South.

In terms of thermal maturity, our analyses performed on Cambrian to Devonian successions show that the two tectonic blocks undergo different burial history. Petrographic analyses show a decrease of reflectance from the north to the south. Samples from the Kielce block show Roeq.% values between 0.7 and 0,9, whereas samples from Lysogory blocks show values comprise between 1.4 and 1.6 Roeq.%.

When compared with thermal alteration indexes (CAI and AAI) from the works of Szczepanik (2013) and Belka (1990), our results indicate similar thermal maturity values in the Kielce region and systematically lower values in the samples from the Lysogory region.

Finally the micro-Raman analyses confirm the validity of this methods on graptolite fragments. Preliminary correlation of the Raman parameters with reflectance data show that the parameters that better correlated with thermal maturity increase are those related to the area ratio between D and G bands on Raman spectra.