Palynofacies Analysis Illustrates the Source Rock Potential of the Menilite Beds, Polish Outer Carpathians

Abstract

The deposition of the Menilite Beds began during the reorganization of the sea ways and the extensional stage of the Carpathian Basin evolution in the early Oligocene. The beginning of their sedimentation is considered to be isochronous throughout the entire marine basin, which formed a part of the Paratethys and was likely isolated from the global ocean due to tectonic movements. However, the changes in the Carpathian region were likely also influenced by processes associated with the Eocene–Oligocene Transition. The present study focuses on the investigation of the Menilite Beds from two Outer Carpathians (OC) main tectonic units: the Skole (Krościenko section) and Silesian (Korzenna section), using a coupled palynofacies, Rock-Eval pyrolysis, and sedimentological framework. This approach permitted the observation of lateral variations in lithology, sedimentary structures, and palynofacies, recording the changes that took place in the early Oligocene, during the deposition of the Menilite Beds, in regions that are currently separated by great distances. In both sections, the palynofacies of the Menilite Beds are predominantly composed of amorphous organic matter (AOM), suggesting that the Menilite Beds were deposited under strongly oxygen-depleted conditions. The presence of spherical bacteria and AOM with alveolar structures in the Korzenna section indicates that microbial mats were probably locally present at the sediment–water interface during the deposition of the Menilite Beds. Analyses of microplankton with organic walls indicate that brackish conditions were present during the deposition of the lower Menilite Beds. Identified microplankton notably included Botryococcus sp. (Krościenko, Korzenna), Campenia sp. (Krościenko, Korzenna), Leiosphaeridia sp. (Krościenko, Korzenna), Pterospermella sp. (Krościenko, Korzenna), and Cymatiosphaera sp. (Korzenna). These results indicate that some lower Menilite Beds, which are located directly above the Eocene deposits (Krościenko—Complex II, III, and IV; Korzenna—Complex I, II, IV, VI), contain horizons with palynological compositions favorable to hydrocarbon generation, including Botryococcus sp. and Leiosphaeridia sp., which are characterized by a high ability to synthesize and accumulate hydrocarbon. We demonstrate that the integration of sedimentological, palynofacies, and organic matter analyses under UV light supported by Rock-Eval pyrolysis can facilitate the identification of potential hydrocarbon horizons—characterized by elevated algal abundances—within the differentiated lithostratigraphic profile of the Menilite Beds. The formation of these deposits was influenced by a complex interplay of paleogeographical, paleoclimatic, and tectonic factors.