Keywords

1 Introduction

South Caspian Basin is one of the wealthiest hydrocarbon provinces in the world, with more than 100 years of oil and gas production history. It contains more than 20 km of Mesozoic and Tertiary sediments deposited on oceanic or thinned continental crust. The primary production in the basin is from the Pliocene productive series of fluvial-lacustrine, deltaic sandstones deposited after the Miocene base level fall that caused the isolation from the global ocean. Principal source rocks for the oil and gas fields are organic-rich Oligocene–Miocene Maykop and Diatomaceous shales deposited in deep water environments. Raw data on source rocks predominantly come from the onshore well penetrations and natural outcrops. This work aims to share preliminary results from the geochemical work done on the oil samples collected from the various oil fields in the Absheron archipelago. The focus was on the composition and distribution of adamatanes, their homologs, and other biomarkers, as well as evaluating the role of adamantanes in hydrocarbon formation. Adamantane is the chemical name for “diamond-type hydrocarbons.” It is a bridged hydrocarbon of the C10H16 composition, consisting of three cyclohexane rings in the “armchair” conformation. An essential feature of adamantane hydrocarbons is their thermodynamic stability, which helps a lot in the case of studying highly mature or biodegraded oils.

Oil samples from various depths and stratigraphic formations of the fields in the Absheron archipelago, such as Darvin-kupesi, Pirallahi, Palchig Pilpilesi, Neft Dashlari, Guneshli, Chilov, and Sangachal-Deniz fields are the subjects of the current study (Fig. 1).

Fig. 1
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Study area location map

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2 Materials and Methods

Gas-chromatography-mass-spectrometry (Perkin-Elmer Clarus 680) was used to analyze the oil samples selected: carrier gas—helium, 60 m capillary column/0.25 mm diameter. Temperature programming steps 2 °C/min from 80 to 300 °C, and source temperature is 250 °C. Chromatograms of hydrocarbons were obtained by total ion current (TIC) and specific ion fragments (SIR). The individual hydrocarbons were identified through a computer search of the National Institute of Standards NIST-08 library (more than 130,000 mass-spectrums of organic compounds), according to published data and based on the reconstruction of ion fragments that have been identified. Adamantane hydrocarbons were detected by scanning fragmented ions m/z 136, 135, 149, and 163.

3 Results

More than 250 individual hydrocarbons have been identified in the oil samples analyzed. The important ones are the biomarkers—terpenes, steranes, hopanes, and adamantanes (Peters & Moldowan, 1993). In addition to adamantanes, their homologs with C11–C15 composition have also been identified in the oil samples. It should be noted that the samples analyzed represent oils of different stratigraphic ages. The distribution of adamantanes in the oil samples from the Guneshli field is given as an example in Table 1.

Table 1 Biomarkers in oils—guneshli field, absheron archipelago
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The distribution of adamantanes C10–C13 in oils from the Guneshli and Neft Dashlari fields shows that the highest amount of alkyl-adamantanes corresponds to its dimethyl-transitioned homologs. In contrast, the concentration of the adamantanes that have not been transformed is the lowest. We also observe high content of 2-monomethyl-adamantanes (2-MAd) in C12. Generally, there is the following trend in the distribution of adamantane hydrocarbons in oils analyzed: 2-MAd ˃ 1-MAd ˃ 3-MAd ˃ Ad., i.e., we see a higher concentration of thermodynamically stable adamantanes.

4 Discussion

It is known that the amount of adamantanes in oils directly depends on their chemical composition. The highest content of adamantane is typical for the naphthenic type of oils. Our data clearly show a direct relationship between the concentration of the naphthenic hydrocarbons and adamantanes in oils, i.e., naphthenic oils have a higher amount of adamantanes. It indicates that the degree of transformation of the polycyclic hydrocarbons into adamantanes and their isomers is much higher in a naphthenic type of oil. A high concentration of adamantanes in some of the samples could be due to intensive transformations due to thermal destructions or biodegradation. The correlation of the distributions of adamantane hydrocarbons and their isomers from various oil samples shows a good relationship between individual isomers that could suggest common mechanisms for forming adamantanes in the study area. As per the formation of adamantanes, there could be different mechanisms to consider here, including thermal cracking of n-alkanes, saturated hydrocarbons, and polar components (Chen et al., 1996; Giruts & Gordadze, 2007; Giruts et al., 2014; Goodwin et al., 2020). This will require high temperatures and thus deep-buried source rocks in the cold South Caspian Basin case. The formation of adamantanes could occur during the migration of hydrocarbon fluids from the deeper strata due to chemical transformations and isomerization processes. The presence of acid catalysts (clays) is an important criterion here. This subject requires further analytical work and modeling.

5 Conclusions

  • Adamantanes and their homologs have been identified in soil samples from various depths, stratigraphic ages, and locations in the Absheron archipelago and South Caspian Basin fields.

  • Chromato-mass spectrometry data show the following distribution of the adamantane and its methyl substitutions: 2–MAd > 1-MAd > 3-MAd > Ad. The such distribution suggests the formation of adamantanes due to thermal transformations of normal alkanes.

  • A high amount of adamantanes, low geothermal gradients, and potential formation mechanisms of adamantanes described above suggest the presence of deeper buried source rocks (deeper than 8 km in the case of the cold South Caspian Basin) to support temperatures enough to form mature source rocks that generate hydrocarbon fluids accumulated in the study area.