Mineralogy and geochemistry of oil shale in Azerbaijan: classification, palaeoweathering and maturity features

Shamakhi-Gobustan and Absheron regions (Azerbaijan) are a part of the South Caspian Basin, which is a subsiding basin located between the colliding of Arabian and Eurasian plates. The intensive rate of sedimentation process creates a favorable condition for the formation of oil shale, hydrocarbon and as well as mud volcanoes in these regions.

The purpose of the article. The study of oil shale in Azerbaijan has been mainly devoted to their geological and organic-geochemical characteristics, etc. However, the chemical classifications, provenience, palaeoweathering and maturity characteristics have not been studied. This study is the first attempt to investigate noted issues.

The research methodology. 10 samples from the outcrops and eject of mud volcanoes were analyzed. The concentrations of major and trace elements and minerals were measured by “S8 TIGER Series 2 WDXRF”, “Agilent 7700 Series ICP-MS” mass spectrometers and XRD “MiniFlex 600”. The microscopes “Loupe Zoom Paralux XTL 745” and “MC-10” and a digital camera “OptixCam” were used to determine the age of the samples.

The major and trace elements in the composition of samples were compared with average shale, NASC, PAAS and average black shale as well as oil shale from the Green River Formation of USA, Kukersit of Estonia, etc. studied in the published literature. A diagram and index were used for the classifications and determination of maturity of rocks. The palaeoweathering characteristic was determined based on CIA versus ICV and some other plots and ratios.

Research results. The minerals found in oil shale were classified according to their classes. According to the used classification diagram, it was established that all studied samples correspond to shale. A superiority of clay minerals in the composition of oil shale compared to K-minerals, including K-feldspar was found.

The estimates based on geochemistry and some ratios of elements confirm the instability of oxides and minerals, and immaturity of the samples.

The values of the CIA, CIA versus ICV plot, etc. confirm moderate to high degree of weathering. The results confirm a conclusion that the original sediments were derived from mafic and intermediate source terrain.

The scientific novelty. The scientific analysis presented in the paper is based on several substantial theoretical conclusions, which related to the factual material of research conducted by the co-authors.

The mineralogy, classification features, stability characteristics of the major oxides and minerals as well as chemical maturity and palaeoweathering were studied based on the chemical composition of the samples.

The practical significance. The results of the current study can be used for the further utilization of oil shale in Azerbaijan and the selection of promising areas in terms of mineral raw materials.

study is the first attempt to investigate the classifications, provenience, palaeoweathering and maturity characteristics of oil shale in Azerbaijan.
Geological settings. Shamakhi-Gobustan regionoccupies a significant part of the south-eastern plunge of the Greater Caucasus. The Meso-Cenozoic and Quaternary formations take part in the geological structure of the region [1; 11]. About 40 outcrops of oil shale associated with Upper Cretaceous-Miocene sediments were registered here [5; 6; 22; 41]. The layers of oil shale of the Upper Cretaceous are thin, and the content of organic matter is relatively low [1; 2]. The region is characterized by a change in the thickness of sediments of the Middle Eocene. The thickness of oil shale layers vary in intervals of up to 10 meters in arches of the anticline, synclinal and mould structures (Boyuk Siyaki, Kichik Siyaki, Jangichay, Jangidagh, etc.) [1; 9]. The layers, containing oil shale in sediments of the Upper Maikopian are associated with shale lithofacies of the "Riki horizon". The sediments of the Middle Miocene (Konkian) are characterized by light gray, brown-gray shale and carbonate rocks. In the northwest and southwest of Gobustan, these lithofacies alternate with layers of oil shale of various thicknesses. The layers of oil shale were also registered in the sections of the Upper Sarmatian sediments [6; 10].
The geological and tectonic positions of the region are very complex, and two microblocks (Bayanata and Toragay) were established here that bound to the Goradil-Masazir underthrust zone and Gujur-Gyzyldash thrust [11].
Absheron region -18 outcrops of oil shale were registered in the region [41]. In the geological structure of the region, the terrigenous and carbonate rocks of the Upper Cretaceous and Cenozoic deposits are noted [11]. Oil shale in the sections of the Eocene was registered only in the areas of Goytepe and Govundag [6]. The sediments containing oil shale in the region are associated mainly with the Upper Maikopian series and Upper Miocene [2; 5; 9]. The alternation of layers of black bituminous oil shale is registered on the northern slope of the mountains Uchtepe-Shorchala, as well as in the outcrops of the mountains Goytepe, Orjandagh and Fatmai. In the terrigenous and carbonate sediments of the Middle Miocene, the shale-bearing layers with a thickness of 30 m were observed within the Western Absheron in the Shorbulag and Garaheybat areas. The outcrops of laminated oil shale were recorded in Uchtepe-Ilkhidagh area, within the southern pericline of the Kecaldagh-Zigilpiri fold and further east of the village of Binagadi. The outcrops of oil shale were recorded in the Meotian sections that are characterized by a different thickness in the region. Due to tectonic and orographic settings, the region is a bottom of the south-eastern part of the Greater Caucasus. Most of the tectonic zones are gradually deepening in the direction of the expansion of the Greater Caucasusfrom the northwest to the south-east, and complicated with mud volcanoes (Keyreki, Lokbatan, Bozdagh-Qobu, etc.) [11]. The samples of oil shale were taken from the outcrops of Jangichay and Boyuk Siyaki of the central part of Shamakhi-Gobustan region, as well as from ejecta of mud volcanoes in the studied regions ( Fig. 1).

Fig. 1. Location map of the studied oil shale from outcrops and mud volcanoes
The accuracy of the age of oil shale from the outcrops is beyond doubt. Thus, the geological structure of the studied areas has been studied accurately. The geological age of oil shale found in the ejecta of mud volcanoes was determined according to the genus and species of fauna contained in the rocks assemblages. The Globigerina bulloides (Orbigny), Cibicides sp., Globigerina triloculinoides (Plummer), etc. planktonic and benthic foraminifera belonged to Eocene were determined in the studied rocks.
Chemical composition of rocksanalysis of major element oxides were performed on the "S8 TIGER Series 2 WDXRF" spectrometer and trace elements on the "Agilent 7700 Series ICP-MS" mass spectrometer at the Institute of Geology and Geophysics, Azerbaijan National Academy of Sciences.
Mineralogical composition of rocksstudied using the XRD "MiniFlex 600" at the same Institute.
Age of rocks -the microscopes "Loupe Zoom Paralux XTL 745" and "MБC-10" and a digital camera "OptixCam" were used. The study was carried out at the Integrated Engineering Exploration Production, Department of Geophysics and Geology, SOCAR.
Methodology. The published literature was used to obtain information about average shale [47], average black shale [49], North American shale composite [25], post-Archean Australian Shale [46] as well as oil shale from the Green River Formation of USA [45], Gurun basin and Hatildag deposit of Turkey [38; 44], Sultani deposit of Jordan [8], Baltic deposit of Russia [28] and Estonia deposit of Estonia [15] for comparative studies based on the distribution of major oxides and trace elements. A diagram and index proposed by [27] and [24] were used for the classification and determination of maturity of samples. The palaeoweathering characteristics were studied based on two indexes -Index of Compositional Variability [18] and Chemical Index of Alteration [35], and also plots proposed by [18; 31; 35; 43].
Results and discussions. Mineralogy -11 minerals belonging to 5 classes: mainly carbonates, sulphates and silicates were found in the samples.
Only one sample contains 1% of halite (halogen class), and hematite was found in three samples (Table 1).  The analyze of mineral assemblages in the shale samples showed the presence of clay minerals illite, chlorite and montmorillonite. The value of montmorillonite (mean = 15.1 %) and chlorite (mean = 16.2 %) higher than illite (mean = 10.9 %) ( Table 1).
Illite is widely distributed in clay and shale associated with the marine origin as a result of weathering of feldspar and muscovite. In the process of converting muscovite to illite, some part of K +1 is replaced by Ca +2 , Mg +2 and H3O + [7]. On the other hand, in a process of metamorphism of clay, its crys-tallization provides an indicator between diagenesis and low-temperature metamorphism [23].
Chlorites are formed in a result of physical and chemical change in the minerals of a mafic type such as pyroxene, amphibole, biotite, talc, and pyrophyllite. In coastal areas, where the degree of metamorphism is low, iron-enriched chlorite is formed. This type of chlorite is more unstable. At low temperatures, chlorite converts to montmorillonite, while and at high temperatures to talc, cordierite and garnet [7].
Montmorillonite belonging to the group of smectite forms as a result of hydrothermal changes or weathering of aluminum-rich minerals, in particular, bentonitesvolcanic ash and tuff. Formation of this mineral often occurs together with other clay minerals, including illite. In the process of formation, the role of weathering and transformation of aluminosilicate rocks, especially, feldspar as well as rivers and streams are not excluded [29]. This mineral is an authigenic, and its origin related to an accumulation of shale and a change in detrital materials in a relatively calm condition.
The mean value of quartz in samples is 25.77%, feldspar -8.2 % (Table 1). Quartz is a wide-spread accessory mineral associated with siliceous eruptions, such as volcanic rhyolite and plutonic granite [33]. Taking part in all degrees of metamorphism, and having a high resistance to chemical weathering, this mineral is widely distributed in sedimentary rocks including metamorphosed oil shale.
Anorthite ( Fig. 2a and 2c) and albite ( Fig. 2b and 2d) are present in the studied samples. Anorthite is the calcium-rich endmember of the plagioclase solid solution series, the other endmember being albite (the sodium endmember). Albite twinning is characteristic for plagioclase associated with igneous rocks, such as basalt, andesite, dacite, rhyolite, etc. Along with the metamorphic rocks, they are found in the composition of detrital sedimentary rocks. At shallow depths, close to the Earth's surface, they are relatively less stable in comparison with alkaline feldspar (orthoclase, anorthoclase, etc.) and quartz, and decompose to clay mineral during weathering [33]. Quartz and feldspar are associated with coastal zones [29]. During diagenesis, various processes occur in sediments, such as activation of silicate source for quartz cementation, as well as a replacement of quartz and feldspar with calcite or clay transformation [26; 42].
The mineral calcite (carbonate class) was found in eight rock samples (mean = 7.0 %) and siderite in only two samples (mean = 2.9 %) ( Table 1). The ma-  (9) in components of carbonate minerals are limestone and dolomite. The main element structure center of the carbonate minerals consists of an equilateral triangle in which a C +4 ion is located, and oxygen ions in its hills. Each Ca +2 ion is surrounded by six oxygen ions. The carbonate minerals, found as a result of analytical studies, belong to the calcite group. Calcite is the most common carbonate mineral, mainly consist of limestones that their fine-grained powder as a cementing agent plays a key role in the formation of sedimentary rocks, oil shale and skeletal structures of living organisms. This group of mineral is associated with rare products of magmatic eruptions called carbonatites. They form as a veinlet associated with sulphide ores after precipitation from hydrothermal fluids. Unlike calcite, aragonite forms as a result of high pressure and temperature related to depths close to the surface. According to this feature, the presence of the aragonite mineral in metamorphic oil shale can be considered as an indicator of metamorphism, reflecting a condition of low temperature and high pressure. In terms of temperature, a relatively hot paleosource is characterized by aragonite, and cold by calcite [33]. The common genesis of calcite mineral identified in the samples is bound to cold sources. Another calciumcontaining mineral is diagenetic siderite, often presents in the composition of oil shale and sandstone. Forming in shallow water and taking part in the formation of concretion, this mineral gives a reason to take this factor into account during the study of the genesis of oil shale. A widespread mineral of sulphate class in the content of samples is jarosite, with an average value of 11.85%. In addition, 5% of gypsum was found in sample No. 6. Jarosite is an alunite group mineral, which is the main potassium and iron hydrosulfite. The mineral is considered as a secondary mineral associated with the oxidized part of sulphidecontaining rocks. The genetic feature of this mineral related to a weathering occurring in an arid climate. This mineral often forms due to weathering of iron sulphide. Gypsum is an aqueous sulfate of calcium. As a less dense mineral, genetically forms in a lake, sea and geothermal water as well as also in a veinlet that is rich in sulphate precipitation.
Unlike magnetite, hematite is more prone to oxidation and can be considered a changing product of magnetite. In many igneous rocks, hematite is a solid solution in the composition of ilmenite. Larger sediments of hematite are found in strip-like iron formations. The mineral origin is associated with geothermal waters, as well as with stagnant water environments at the bottom of the lakes, sometimes anhydrous conditions and volcanic activity [33].
Major oxides. The SiO2 value in the samples ranges from 45.72 to 55.55 % (mean = 49.81 %). The values of this major oxide in the samples taken from the Boyuk Siyaki and Jangichay outcrops differ from oil shale sampled from ejecta of mud volcanoes in Shamakhi-Gobustan region. A tendency on decreasing of the value for this oxide was recorded towards the south-east from North Gobustan. In addition to Si, the second major oxide in the chemical composition of samples is Al. It is nearly impossible to give statistical distributions for this element on the studied areas and regions. The Al2O3 values in the samples range from 12.69 to 16  The major oxides in the composition of samples were compared with average shale [47], NASC [25] and average black shale [49]. The SiO2 and Al2O3 values corresponds to the average shale, while the Fe2O3 value proper to NASC and the CaO, Na2O, K2O, MgO, TiO2. The P2O5 values coincide with both shales. Only for the Al2O3 value and alkaline compounds were established a correspondence compared with the average black shale ( Table 3).
The major oxides in the samples were compared with the cognominal rocks of some countries that have extensive experience in the oil shale industry. The studied oil shale demonstrates similarity with the oil shale of the Estonian kukersite deposit [15] and Green River Formation of USA [45]. A associative similarity was recoded in the concentration of SiO2, Al2O3 and CaO. The studied samples prevail on the oil shale of the Gurun basin of Turkey [38], Baltic basin of Russia [28] and Sultani deposit of Jordan [8] on the mean SiO 2 value. Referred to the CaO content, these three countries have a significant advantage (Table 4). High concentrations of CaO characterize a relationship with biochemical source, including cementing in the center of the basin [52]. The CaO value is significantly higher in the samples taken from the mud volcanic areas in Absheron region (excluding Bozdagh-Qobu) than in Shamakhi-Gobustan region (excluding Jangichay) ( Table 2). This factor may be related to a depositional environment, in which the samples formed; instabilities at the level of lake, lagoon, etc., as well as intensity of transport of dissolved chemical components.
Trace elements. The mean Zr value in oil shale is 207 ppm, and there is a coincidence for this trace element compared to NASC [25] and PAAS [46] (Table 3). This indication differs sharply from some of the world's oil shale deposits and basins, including the Hatildag deposit of Turkey (14 ppm) [44] and Sultan deposit in Jordan (46 ppm) [8]. The mean Sr and Ni values for the oil shale are 355 and 68 ppm (Table 3). Relatively, the Sr content in average shale is 300 ppm [47], which roughly corresponds to oil shale in the present study (Table 3). However, the values of the same trace element in the NASC [25], average black shale [49] and PAAS [46] considerably less (142, 200 and 200 ppm) ( Table 3). The mean Sr value is 430 ppm in the Hatildag oil shale deposit [44], Sultani -707 ppm [8].
High Sr value in sedimentary rocks is associated with nano-calcareous planktons [32] and also aragonite sources. The mean Ni value is distributed in approximate concentrations in the noted shales ( Table 3). However, the content of this trace element in the oil shale of the Hatildag deposit is 103 ppm [44], Sultani -139 ppm [8], Baltic basin -10 ppm [30], Kukersit of Estonia -17 ppm [48] and Green River -1-10 ppm [17]. The origin of Ni in the studied oil shale is most likely associated with organic sources. The Zn and Cu values in oil shale are 135 and 47 ppm ( Table 3). The Zn value is relatively high in the average shale and in PAAS (95 and 85 ppm) ( Table  3). The mean Zn value for average black shale is 300 ppm. The average Zn value in the oil shale deposit in Turkey is 15 ppm [44], Sultani -649 ppm [8], Baltic basin -75 ppm [30], Kukersit deposit -49 ppm [48] and Green River formation -10 ppm [17]. The mean value of Cu does not differ much from the average shale and PAAS (45 and 50 ppm).
Geochemical classifications and maturity. Based on the chemical classification diagram of log (Fe2O3/K2O) versus log (SiO2/Al2O3) [27], it was determined that most of samples correspond to shale, and only one sample with a rich Fe2O3 content correlate with Fe-shale (Table 2, Fig. 3). Shale is clastic sedimentary rock consisted of clay minerals, as well as quartz and calcite. The biological, chemical and coastal-mechanical erosions, as well as the shelf, lagoon, deep but relatively stagnant water environments, are characteristic for its genesis.
A ratio K2O/Al2O3 differs sharply in clay min- This feature provides an association between sodium and illite. The determined ratio for samples varries from 0.27 to 0.60 (mean = 0.41), corresponds to a low degree of maturity. A result of the correlation between Na2O and Al2O3 is negative (r = -0.37), which can be explained by the fact that sodium does not bind to plagioclase in the samples, and the main bond is associated with illite. The K2O/Na2O ratio is 1.66-3.72 (mean = 2.56), which is characteristic for K-bearing minerals.
Al2O3/(CaO + MgO + Na2O + K2O) ratio indicates the mobility of major oxides that form rocks [24]. The oxides present in the content of samples are unstable since the determined values of this ratio are lower (0.80-2.05).
The ratios of SiO2/Al2O3 and Fe2O3/K2O are very informative as an indicator of mineralogical stability in sedimentary rocks. Lower SiO 2 /Al 2 O 3 and higher Fe2O3/K2O values are considered mineralogically less stable and more prone to reactivity under supercritical CO2 exposure [20; 39]. The ratio of SiO2/Al2O3 is lower than that of Fe2O3/K2O, and this result leads to the fact that the studied samples are classified as mineralogically unstable.
A ratio of SiO2/Al2O3 is used to determine the maturity properties of sedimentary rocks, as well as to assess the presence of quartz associated with clay minerals or feldspar [16; 19]. In this regard, high ratios characterize mineralogical mature (quartzose, rounded) and while low ratios represent chemically immature rocks [40]. Since the sizes of fine-grained rocks have a positive effect on maturity, but rocks, mainly consist of clay minerals, have a negative im-pact on this feature. Thus, maturity can be recorded in rocks, which rich with quartz, fine-grained sands, and feldspar. If the SiO2/Al2O3 ratio is characterized by high values, up to 5, it indicates progressive maturity for quartz and feldspar. The value of this ratio for present study range from 3.22 to 3.66, the mean value is 3.4. Such values point to the superiority of clay minerals in the studied rocks. The Al2O3, Fe2O3 and TiO2 values for the composition of samples shows a tendency like Al2O3 > Fe2O3 > TiO2 ( Table  2). The obtained result corresponds to clay minerals [37]. A positive result of the correlation between TiO2 and Al2O3 (r = 0.57) also confirms this conclusion.
Unstable and recrystallization features of clay minerals lead to acute mineralogical changes during diagenesis and low-temperature metamorphism [50]. Isochemical features for such processes is characteristic. Index of Compositional Variability (ICV), which indicates a richness of alumina relative to the cations of some major oxides in rocks, is determined by the formula [ICV = (Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2)/Al2O3 (%)] [18]. The index also allows detecting detrital mineralogical properties of rocks. The calculated ICV value for samples is 0.91-1.78 (mean = 1.32). Since rocks characterized by low ICV values are associated with the cratonic envoriments and clay minerals. Most of the samples have a higher value than 1, indicating the presence of fewer clay minerals and more rock-forming minerals, such as plagioclase, alkali-feldspar, etc.
Palaeoweathering. Intensity of chemical weathering is directly related to a number of factors, such as the composition of the parent rock, climatic condition, the duration of weathering, the degree of uplift tectonic movement, etc. The chemical weathering has a certain effect on the composition of silicate rocks. This is characterized by more stable properties of large cations (Ba, Rb, etc.) compared to small ones (Na, Ca, etc.) in weathered residues [34]. During weathering, an element with a smaller ionic radius is separated from feldspar and the abundance of such an element compared to a stable element in the content of rocks indicates the degree of chemical weathering. A comparison of arid and humid conditions indicates that arid is characterized by relatively poor chemical weathering. A climatic condition accompanied by heavy rainfall leads to the loss of unstable minerals. In addition, CO2 in the atmosphere and temperature condition has a positive effect on the intensity of weathering. Thus, the high temperature increases the decomposition of plagioclase and potassium feldspars, as well as hydrolysis. The lack of plants, the intensification of temperature and rainfall can lead to strong weathering. Some methods are used for the determination of weathering based on the abundance of mobile or immobile major oxides in the rock. Chemical Index of Alteration ((CIA), [35]) is an index indicates of the conversion of plagioclase and potassium feldspar into clay minerals. The computational formula of the coefficient is: CIA = 100 x [Al2O3/(Al2O3 + CaO * + Na2O + K2O)]. A value of CaO * indicates only CaO in silicates. Since this value is different from CaO, the following formula [CaO * = CaOcalcitedolomiteapatite] was used for the index calculation. The calculation of the index was performed within the molar proportions of the oxides. For the studied rocks, the CIA values are between 72 and 80 (the mean value is 75), which indicate moderate to high degree of weathering. CIA versus Al/Na plot for the samples is almost identical to the PAAS (Fig. 4). Such a trend characterizes N + diagenetic loss.
According to the ICV-CIA plot, the samples were subjected to intensive weathering in the source area and only after the post-sedimentation processes; there were changes in their composition (Fig. 5). Up to 50 CIA values indicate unchanged plagioclase, as well as K-feldspar. The minimum values of CIA for all samples > 70. Since CIA values approach 100 indicate the conversion of feldspar to clay minerals, such as gibbsite, kaolinite [21]. The CIA values and silicate clay minerals in the shale samples (Table 1) also confirm moderate to high degree of weathering. The obtained values may be related either of the original sources or during transport before deposition and may reflect lowrelief and arid climatic state in the source area [51].
The results of the ternary diagram of Al2O3, CaO * + Na2O and K2O (A-CN-K) [36] was used to determine the chemical weathering properties, show that all the samples plot parallel to the A-CN join. The normal weathering tendency is focused toward illite, but in general, the effect of K-metasomatism is not excluded. The obtained result supports a conclusion that the protolith of oil shales coincides with mafic and intermediate types of igneous rocks (Fig. 6).
The plot based on the amount of Al2O3 and K2O of samples indicates a relationship with illite (Fig. 7). Such a conclusion is most likely based on the conversion of K-feldspar and muscovite into clay minerals as a result of weathering in arid/semiarid climatic conditions. In addition, the relationship between potassium and illite is confirmed by the result of correlation Ka2O versus Al2O3 (r = 0.65).
Conclusions. 11 minerals belonging to 5 classes were determined in the samples which silicates predominate. The coincidence between samples and shale was established using the classification plot based on log (SiO2 /Al2O3) versus log (Fe2O3/K2O).
The K2O/Al2O3 ratio confirms a superiority of clay minerals in the composition of oil shale compared to K-minerals, including K-feldspar. The estimates based on the Al2O3/(CaO + MgO + Na2O + K2O) and Fe2O3/K2O, SiO2/Al2O3 ratios confirm the instability of oxides and minerals in the composition of samples. In addition, the ratios of SiO2/Al2O3 and Al2O3 > Fe2O3 > TiO2 support an idea that the samples are immature and associated with clay minerals.
The values of the CIA, and the plots for CIA versus ICV, CIA versus Al/Na ratio and the ternary diagram A-CN-K, as well as the study related to major oxide confirm moderate to high degree of weathering. The result of A-CN-K ternary diagram support that the weathering tendency of the samples plots close to the illite. The effect of Kmetasomatism in this process is also not excluded. The role of post-sedimentation processes were most likely involved in the conversion. The protolith of oil shale samples is associated with mafic and intermediate igneous sources.

MINERALOGY AND GEOCHEMISTRY OF OIL SHALES IN AZERBAIJAN: CLASSIFICATIONS, PALAEOWEATHERING AND MATURITY FEATURES
Shamakhi-Gobustan and Absheron regions (Azerbaijan) are a part of the South Caspian Basin, which is a subsiding basin located between the colliding of Arabian and Eurasian plates. The intensive rate of sedimentation process creates a favorable condition for the formation of oil shale, hydrocarbon and as well as mud volcanoes in these regions.
The purpose of the article. The study of oil shale in Azerbaijan has been mainly devoted to their geological and organic-geochemical characteristics, etc. However, the chemical classifications, provenience, palaeoweathering and maturity characteristics have not been studied. This study is the first attempt to investigate noted issues.
The research methodology. 10 samples from the outcrops and eject of mud volcanoes were analyzed. The concentrations of major and trace elements and minerals were measured by "S8 TIGER Series 2 WDXRF", "Agilent 7700 Series ICP-MS" mass spectrometers and XRD "MiniFlex 600". The microscopes "Loupe Zoom Paralux XTL 745" and "MC-10" and a digital camera "OptixCam" were used to determine the age of the samples.
The major and trace elements in the composition of samples were compared with average shale, NASC, PAAS and average black shale as well as oil shale from the Green River Formation of USA, Kukersit of Estonia, etc. studied in the published literature. A diagram and index were used for the classifications and determination of maturity of rocks. The palaeoweathering characteristic was determined based on CIA versus ICV and some other plots and ratios.
Research results. The minerals found in oil shale were classified according to their classes. According to the used classification diagram, it was established that all studied samples correspond to shale. A superiority of clay minerals in the composition of oil shale compared to K-minerals, including K-feldspar was found.
The estimates based on geochemistry and some ratios of elements confirm the instability of oxides and minerals, and immaturity of the samples.
The values of the CIA, CIA versus ICV plot, etc. confirm moderate to high degree of weathering. The results confirm a conclusion that the original sediments were derived from mafic and intermediate source terrain.
The scientific novelty. The scientific analysis presented in the paper is based on several substantial theoretical conclusions, which related to the factual material of research conducted by the co-authors.
The mineralogy, classification features, stability characteristics of the major oxides and minerals as well as chemical maturity and palaeoweathering were studied based on the chemical composition of the samples.
The practical significance. The results of the current study can be used for the further utilization of oil shale in Azerbaijan and the selection of promising areas in terms of mineral raw materials.