Origin and preservation processes of amorphous organic matter in the Maykop Series (Oligocene-Lower Miocene) of Precaucasus and Azerbaijan

Abstract

Abstract Introduction. – The clay sediments of the Maykop Series accumulated during the Oligocene-Lower Miocene in eastern Paratethys are considered as one of the major source rocks of Precaucasus and Azerbaijan. As numerous marine source rocks, petrographical observations showed that amorphous organic matter (AOM) is widely predominant in these sediments [Fadeeva, 1973; Saint-Germès, 1998]. However, the geochemical characteristics (TOC, HI) of the Maykop Series vary widely [Polster et al. 1960; Sidorenko, 1964; Larskaya, 1966; Saint-Germès et al., 2000a, b]. The integrated geochemical and petrographical investigations were carried out in order to establish the origin of AOM in the Maykopian sediments. Moreover, the organic matter preservation in these sediments deposited in the dysoxic-anoxic paleoenvironments may result from several processes as revealed recently [Sinninghe-Damsté et al., 1989; Largeau et al., 1990; Derenne et al., 1991; Lee, 1992; Keil et al., 1994]. Analysis of the sulphur content in the kerogens and ultrastructural observations by transmission electron microscope (TEM) of AOM particles were performed to determine the involved preservation processes. Materials and methods. – The studied sediments come from western and eastern Precaucasus and Azerbaijan where the Maykop Series is mainly composed of grey-brown clay with sandstone and silts (fig. 1). In Azerbaijan, over several dozens metres of interbedded of grey-greenish carbonated clay (10–30 cm) and black shales (< 1 cm) were observed in the basal part of the series (fig. 2). In all regions, only the samples containing immature or low mature organic matter were selected by Rock-Eval pyrolysis (259 samples) and are examined in this paper. In addition, analyses of the palynofacies (85 samples), Rock-Eval pyrolysis (50 samples) and elemental analyses (15 samples) of kerogens were performed. Finally, TEM observations and GC-MS analyses of the extracted hydrocarbons were carried out on samples with contrasted morphological and geochemical features. Results. – The palynofacies analysis shows that in the Precaucasus the AOM widely predominates (60–100 %) in the Oligocene sediments. On the other hand, the morphologically recognizable fraction of continental origin is more abundant (> 50 %) in the Lower Miocene sediments. In Azerbaijan, AOM predominates (> 80 %) in all the sediments. The palynofacies of the black shales of the basal part of the Maykop Series only comprises AOM (fig. 3A). In grey clays the morphologically recognizable fraction occurs in very limited amount (< 5 % ; fig. 3B). Rock-Eval pyrolysis shows variable TOC contents (0.7–7.2 %) and HI values (30–580 mg HC/g TOC) in Oligocene sediments of Precaucasus, whereas in the Lower Miocene sediments, the TOC contents (0.4–2.3 %) and HI values (30–200 mg HC/g TOC) are lower. In Azerbaijan samples, TOC contents range from 0.4 to 5.7 % and HI values from 40 to 530 mg HC/g TOC. The black shales always exhibit high TOC contents (9.3–17.8 %) and important HI values (580–670 mg HC/g TOC; fig. 2), whereas in the carbonated grey clays the TOC (< 1 %) and HI (< 150 mg HC/g TOC) values are low. Indeed, these studies show important variations of the geochemical parameters in the samples although the latter are always highly dominated by the AOM. However, a large effect of the mineral matrix in the Maykopian sediments is revealed by the Rock-Eval pyrolysis of the isolated kerogens (fig. 6). This effect decreases with increasing TOC concentrations ; however, it remains significant up to about 7 %. The HI values of the isolated kerogens of the grey clays of the lowermost sediments of Maykop Series in Azerbaijan are much more important that the ones observed in the bulk rocks, whereas in the black shales, the mineral matrix has no influence (fig. 7). Comparison of the HI values in the kerogens with the percentage of AOM in the palynofacies shows a positive correlation between these two parameters (fig. 5). According to the elemental analyses of the kerogens, the H/C atomic ratios range from 1.1 to 1.3 in the Precaucasus and from 1.2 to 1.4 in Azerbaijan (table I) and show a predominant marine origin for the OM. The comparison between the Sorg/C atomic ratios which vary from 0.01 to 0.07 in the studied kerogens and the HI values of kerogens shows that a positive trend can be detected between these two parameters (fig. 8). Nevertheless, some samples with high HI values (e. g. A and A1) show highly variable contents in organic sulphur. Moreover, 3 samples of the lowermost sediments of the Maykop Series in Azerbaijan (A-C, fig. 2) with palynofacies comprising more than 95 % of AOM, but with sharply different geochemical characteristics, were observed by TEM. This study showed that the AOMs are set apart by their ultrastructural characteristics (fig. 9) and that a relationship appears between the geochemical characteristics and the nanoscopic aspect of the studied organic matter. The increase in the HI values goes together with the abundance of the ultralaminar structures. Finally, GC-MS analysis of the saturated hydrocarbons of the black shales (A) and grey clay (C) showed similarities in their molecular composition (fig. 10 and 11). Thus, the contrasted morphological features revealed by TEM cannot be explained by differences in organic matter sources. Discussion Origin and type of organic matter The maykopian sediments are characterized by very different, but generally middle and low TOC contents and HI values. However, the apparent contradiction between the low HI values of the bulk rocks and the AOM abundance in the palynofacies is mainly explained in these sediments by the absorption onto the argillaceous matrix of the hydrocarbon compounds constituting the S2 peak. After destruction of the mineral matrix, the isolated kerogens generally show considerably higher HI values, which points to the presence of organic matter of mainly marine origin in the maykopian sediments. The existing correlation between the HI values of the kerogens and the AOM percentages in the palynofacies (fig. 5B) shows that petroleum quality of the maykopian sediments is directly due to the AOM preserved in these sediments. The high H/C atomic ratios confirm that the AOM prevalent in the Oligocene sediments of the deep zones of the maykopian basin is primarily of marine origin. In the lowermost sediments of the Maykop series in Azerbaidjan, the prevalence of the marine organic matter in the samples having different TOC contents and HI values is also highlighted by the GC-MS analysis of the saturated fraction. Preservation pathways of organic matter . – There is, in the Maykop Series, an overall positive trend between HI values and organic sulphur contents (fig. 8) which suggests that the incorporation of sulphur in the lipidic compounds, enabling them to acquire a greater resistance to microbial degradation [Sinninghe Damsté et al., 1989], would have favoured the formation of some kerogens with high oil potential, in the dysoxic-anoxic sediments of the Maykop Series. On the other hand, the low organic sulphur content observed in sample A of black shale from Azerbaïdjan with high HI value shows that the natural sulfuration of the kerogens is not the only mechanism having led to the preservation of the organic matter in the maykopian sediments. TEM observations show that the increase of the HI values in these sediments is related to an increase in the abundance of laminar ultrastructures in the kerogens. Consequently, the high HI values observed in these sediments must also reflect the better preservation of the biomacromolecules of algal origin. Indeed, these macromolecules are highly aliphatic and thus lead to kerogens characterised by high HI values [e.g. Derenne et al., 1991]. In addition to these two mechanisms, our studies highlighted that the argillaceous minerals of the maykopian sediments have a strong ability of absorption of organic compounds. This makes possible to suppose that absorption on the argillaceous matrix should have also played a significant role in the preservation of organic matter in the maykopian sediments. Conclusion. – The present studies show that the AOM which prevails in the Oligocene sediments of Precaucasus and in the maykopian sediments of various ages of Azerbaidjan is mainly of marine origin. The low values of HI observed in most of these sediments result from a significant mineral matrix effect. This explains the apparent discordance between the data of the Rock-Eval pyrolysis of the bulk rocks and the AOM abundance in the palynofacies. Several mechanisms alternated, even coexisted, leading to the preservation of the marine organic matter in the maykopian sediments: the natural sulfuration, the more limited degradation of aliphatic macromolecules, as well as protection by the argillaceous matrix.