Organic geochemical study of the lower miocene kremna basin, Serbia

Abstract

The Kremna Basin is located in south west Serbia, in the Zlatibor area, which is part of the Internal Dinarides. This basin is note worthy because of the type of bedrock drainage, which it represents. It was formed on ultrabasic rocks and volcanic materials that influenced the occurrence of organic matter (OM) in the basin fill. The objective of the study was to determine the organic geochemical characteristics of sediments from the central part of the Kremna Basin. The sediments studied belong to an intrabasinal facies, in which two sequences were distinguished. The lower sequence occurs at depths of 216-343 m, while the upper sequence is found from 13.5-216 m. At the start of basin development (265-343 m) sedimentation took place in shallow alkaline water, rich in Mg ions. Through time, a slight deepening of the basin occurred. This was followed by chemical deposition of carbonates (216-265 m). The most important change in the sedimentary environment occurred with the formation of sediments marking the transition between the sequences (at about 200 m). Sediments from the lower sequence are characterized by the dominance of dolomite and magnesite. There are indications of volcanic activity, i.e. tuff layers and the presence of searlesite. The upper sequence is characterized by the prevalence of calcite and dolomite. The amounts of MgO, Na2O and B are higher in the lower sequence, whereas the CaO content is higher in the upper sequence. The sediments contain different amounts of immature OM (late diagenesis). Biomarker analysis shows diverse precursors of the sedimentary OM: methanogenic archaea, photosynthetic green sulfur bacteria (Chlorobiaceae), bacterivorous ciliates, various bacteria, both photosynthetic and non-photosynthetic, the green unicellular microalga, Botryococcus braunii race A (exclusively in the upper sequence) and terrestrial plants. The lower sequence contains lower amount OM, composed primarily of kerogen II/III and III types, indicating a higher contribution of the allochtonous biomass of land plants from the lake catchment, particularly in the lower part. The sediments of the upper sequence are enriched in autochthonous aquatic OM, which comprises mostly kerogen I, I/II and II types. The transition from the lower sequence to the upper one is associated with a decrease in pristane to phytane ratio, gammacerane index and content of C-28 steroids, absence or significantly lower amount of squalane, absence of C-24 and C-25 regular isoprenoids, 8-methyl-2-methyl-2-(4,8,12-trimethyltridecyl) chroman and C-30 hop-17(21)-ene. Pyrolytic experiments showed that the sediments of the upper sequence, rich in aquatic OM, at a catagenetic stage could be a source of liquid hydrocarbons. The values of hopane, sterane and phenanthrene maturation parameters indicate that through pyrolysis at 400 degrees C the samples investigated reached a value of vitrinite reflectance equivalent of approximately 0.70%. It was estimated that the sediments should be found at depths of 2300-2900 m in order to become active source rocks. The calculated minimum temperature, necessary for catagenetic hydrocarbon generation, is between 103 and 107 degrees C.