Microfossil assemblages (Diatoms, calcareous nannofossils, and silicoflagellates), paleoenvironment, and hydrocarbon source rock potential of the oligocene ruslar formation at karadere, Bulgaria

Abstract

The Oligocene Ruslar Formation, an equivalent of the Maykop Suite, is a potential hydrocarbon source rock in the western Black Sea Basin. In contrast to the offshore areas, the depositional environment and hydrocarbon source rock potential of onshore Bulgaria sediments are largely unknown. Hence, a 14-m-thick section of the Ruslar Formation, exposed near Karadere (Black Cape) along the Black Sea coast, provides an excellent opportunity to study the upper part of the Ruslar Formation. Here, laminated diatom-rich mudstones with frequent thin sandstone beds and a prominent concretion horizon are exposed. Furthermore, the fossil diatom assemblages provide a key component to understand the paleoenvironment. Overall, Paleogene diatoms are understudied in the Black Sea Basin and therefore only a genus-level study is undertaken here. The studied Ruslar Formation contain remarkably diverse and well-preserved diatom assemblage with 23 different genera. The most frequent genera are Paralia, Distephanosira, and Stephanopyxis. Common genera include Coscinodiscus, Hemiaulus, Pseudopodosira, Rouxia, and Xanthiopyxis. Rare taxa include Actinoptychus, Asterolampra, Azpeitia, Delphineis, Distephanosira, Diploneis, Eunotogramma, Eurossia, Lyrella, Liradiscus, Plagiogramma, Radialiplicata, Rutilaria, Saeptifera, and Triceratium. The diatom assemblages together with calcareous nannoplankton, silicoflagellates, and the presence of rare foraminifera indicate a fully marine neritic environment without major salinity variations. The calcareous nannoplankton investigated can be assigned to biozone NP23 (Early Oligocene). The exposed fragment of the Ruslar Formation was deposited after the low salinity “Solenovian event”, which represents the maximum isolation of the Paratethys present in the lower part of the NP23. Bulk geochemical parameters from 35 samples (avg. TOC: 1.80% wt.; avg. HI: 226 mg HC/g TOC) show that the exposed part of the Ruslar Formation contains type II-III kerogen and a fair to good hydrocarbon potential. The Ruslar Formation is immature (avg. Tmax: 424 °C), but may generate about 0.5 tons of hydrocarbons per square meter if mature. Biomarker proxies support the low maturity and are characterized by diatom-related biomarkers (24-norcholestane; C25 HBI alkanes and thiophenes). Land-plant-derived biomarkers suggest a significant input of angiosperms. Based on biomarker ratios, the depositional environment was oxygen-depleted but probably not strictly anoxic. Reworking of biomass by chemoautotrophic bacteria is suggested by the presence of 28,30-bisnorhopane.