The sedimentary basins of the Crimean-Caucasian region, like all basins of folded regions, are characterized by a number of common features of oil and gas formation associated with both the emergence and the implementation of the oil and gas source potential due to the history of their geological development. The sharp contrasts of the relief and the relatively close position of the drift sources, the high rate of sediment accumulation, as well as the intensive supply of nutrients, including those from the deep strata, are ensured by high tectonic activity. All this determined a number of distinctive features of the formation of source potential of sedimentary rocks common for the area: a sharp predominance of silt-clay source rocks and a relatively small proportion of carbonates; large capacities of the main source strata and enormous amounts of the original bioproduction; and the ubiquitous presence of terrigenous OM. The influence of vascular plant vegetation is manifested even in relatively deep-water sediments. Hence the absence of true domanicites, notwithstanding the OM concentrations in a number of horizons being characteristic of domanicite. The elevated gas-generating potential is another consequence of the presence of type III kerogen; “… considerable depths of the sedimentation basin, which led to the prolonged presence of OM in the water column, promoted, on the one hand, biological and physicochemical degradation of OM, and, on the other hand, the formation of organomineral aggregates that retained the labile components of OM” [1]; water dynamics and the complex morphology of the basin floor led to the emergence of unstable redox conditions in the sediment, which resulted in large diagenetic losses of OM and the extremely uneven (patchy) nature of OM distribution and the source potential values within horizons of the same age.

The characteristics of the region’s geological development determined the geothermal regimen specifics in the sedimentary basins, which predetermined a number of features in the source potential implementation, the main features being the following: extended catagenic zonality and sharp differentiation in the position of the main zone of oil formation (MOFZ), the MOFZ being located at a considerable depth. Thus, the MOFZ roof is located at the following depths in the different districts (geographic districts): 4200 to 4300 m in the Kerch-Taman district, 4200 to 4250 m in the Dzhiginskaya-Varenikovskaya district, 3200 to 4000 m in the Levkino district, and 2600 to 3000 m in the Khadyzhensk district. The MOFZ roof is located at maximal depths (4200 to 4300 m) in areas characterized by well-developed mud volcanism and cryptodiapiric folds, that is, heat outflow from the deep part of the profile. Considerable thickness of the deposits in the oil and gas formation center; the predominance of montmorillonite in the clay mineral complex and, as a consequence, a pronounced matrix effect, that is, a partial return of the hydrocarbons generated; “fast” deflection compensated primarily by clay sedimentation provoked incomplete return of sedimentation water, which contributed to the emergence of abnormally high reservoir pressure and the formation of “closed” oil systems (the Maykopian series), and also caused the generation of liquid hydrocarbons in a range wider than the OFZ; the formation of authigenic oil systems with primary (granular) and secondary (clay) reservoir horizons.

METHODS

The study complex included the assessment of OM quantity, quality and degree of maturity in the rocks by the Rock-Eval pyrolysis method; organic carbon (Corg,%) quantitation in the rocks by dry incineration in the AN-7529 express analyzer after rock pretreatment with 10% hydrochloric acid; bitumoid isolation from the SOM (extraction with chloroform was carried out in a Soxhlet apparatus for 72 hours); the calculation of bitumoid coefficient (βCB,%); and carbon-petrographic studies (provided that representative vitrinite inclusions are present). Representative vitrinite inclusions were not found in the rock samples studied.

Eleven samples of the Khadumian deposits from 8 Western Ciscaucasia sites were studied in the laboratory of hydrocarbon geochemistry.

The values of Corg, S1, S2, and Tmax for the samples prepared according to the standard procedure [2] were measured on the Rock-Eval-6 instrument and the source potential of the rock (S1 + S2), the hydrogen index (HI) and the oil productivity index (PI) were calculated.

Considerable variation of organic matter concentration (Corg), from 0.85% (Yuzhno-Andreevskaya-5) to 3.22% (the Belaya River), at an average Corg value of 1.8% was characteristic of the samples studied.

According to the recommendations of geologists of the French Institute of Petroleum, “… the oil source formations can be graded according to S2, the number of newly formed microoil molecules that were formed during catagenesis and attained a state that ensures cleavage from the kerogen during pyrolysis and release into the free phase” [2]. These molecules are a part of kerogen, that is, they are chemically bonded to the carbon skeleton of kerogen prior to thermolysis:

S2 < 2 mg HC/g rock–poor, 2 mg HC/g rock < S2 < 5 mg HC/g rock–medium, 5 mg HC/g rock < S2 < 20 mg HC/g rock–rich, S2 > 20 mg HC/g rock–very rich.

Pyrolytic analysis was carried out for samples from outcrops, emissions of mud volcanoes, and core samples from wells.

DISCUSSION

Interpretation of the results of pyrolytic studies of Cenozoic deposits of the Indolo-Kuban trough enabled us to determine the geochemical characteristics of the oil and gas source strata of the Cenozoic age of the Western Ciscaucasia.

The samples are very closely grouped in the field of the “transition zone” of type II/III kerogen in the modified Van Krevelen diagram (Fig. 1), and thus the substance can be characterized as mixed. Kerogen types III and IV, which mainly generate gas with insignificant amounts of dry gas, predominate. Kerogen quality for the mud volcano emissions varied over a wide range from type III to type II at nearly the same pyrolysis temperatures. Oxygen indices were high at up to 300 mg CO2/g rock and thus indicative of intensive oxidation of OM at the fossilization stage.

Fig. 1.
figure 1

Modified Van Krevelen diagrams for a) HI dependence on Tmax; b) HI dependence on OI.

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The degree of catagenetic OM transformation inferred from the values of the Tmax pyrolytic parameter, 410–450°С, corresponds to the initial and final stages of the MK1–MK3 “oil window”. The P3 deposits of the Western Ciscaucasia fall into the main zone of oil formation (Fig. 2a). A major part of the Tmax values for the mud volcanoes is within the range of 405–441°C (Fig. 2b) traditionally associated with the beginning of the oil window, but the OM falls into the immature zone due to the low degree of conversion (PI ranges from 0.02 to 0.31).

Fig. 2.
figure 2

Diagram of the relationship between the productivity index PI and the maximum pyrolysis temperature Tmax: a) for the Cenozoic deposits; b) with samples from mud volcanoes included.

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The characteristics of hydrocarbon-generating potential of the source rock OM samples from the Cenozoic deposits fall into a wide range, from satisfactory to excellent. Source rocks of high and very high (excellent) quality prevail (Fig. 3).

Fig. 3.
figure 3

Diagram of the dependence of the full generation potential (S1 + S2) on the total organic carbon (TOC) level: a) for the Cenozoic deposits; b) for the mud volcanoes.

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The virtually direct relationship between Сorg and S1 + S2 points at the syngenetic nature of the hydrocarbons. The pyrolytic parameters were converted into atomic ratios and the results of pyrolytic analyses were plotted on the Van Krevelen diagram directly to ensure reliable determination of kerogen type. H/C = 0.0014HI + 0.5753, O/C = 0.004OI + 0.0392.

Cross-diagram analysis also enabled sample classification as type II and III kerogen (Fig. 4). The MGU mud volcano distinctly falls into the main zone of oil formation on the cross-diagram.

Fig. 4.
figure 4

A graph that illustrates axis transformation for a Van Krevelen diagram.

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Samples of the Indolo-Kuban trough deposits were assigned to type II and III kerogen on the basis of the HI/Tmax diagram, since almost all samples fell into the evolution region of this type of kerogen. Most points were also grouped on the evolution curve of type II and III kerogen after the conversion factors were used to construct the original Van Krevelen diagram (Fig. 5), and thus the OM can be regarded as mixed. Samples from the MGU mud volcano were originally assigned to type II and III kerogen on the basis of the HI/Tmax diagram, since almost all measurement points fell into the evolution zone of this kerogen type, but most points were grouped on the evolution curve of type II kerogen after the conversion factors were applied.

Fig. 5.
figure 5

The original Van Krevelen diagram illustrating the main kerogen types and evolutionary curves of type I, II, and III kerogens.

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The Kumian sequence (Eocene) and the Maykop Formation (Oligocene to Lower Miocene) classified as the regional oil and gas source strata of the Cenozoic age and the local oil and gas source strata of Khadyzhensk and Belaya Glina, as well as diatom and Chokrakian stage source strata, and others, were identified in the Western Ciscaucasia as a result of the present study. “… Deposits of the Kumian Sequence, which were formed in reducing and strongly reducing environments, are represented by dissimilar strata in the western and eastern parts of the side of the West Kuban trough. The western part of the Kumian Formation section to the west of the Kholmskaya settlement is composed of gray clays with silty, less often calcareous admixtures and argillite interbeds. Argillites gradually become rarer in the profile further east, and the Kumian Formation to the east of the Psekups River is composed of brown and dark brown bituminous marls mixed with admixed aleurite material” [3].

As reported in [4], Сorg content in the individual rock types ranges from 0.6% to 11.3% (average 3.3%). Сorg content in the series of rocks increases from the clayey argillites (2%) to carbonate clays (2.6%) and marls (4.6%). The organic matter is enriched in bitumoids (βCB = 4–16%). Сorg content at the depth of 1907 m determined in a Kumian Formation sample at the Utash site (southern side of the trough) was 6.44%. CBA content was low at 0.21%, p–3.23%. The element composition of CBA was as follows (%): carbon (C) 79.02, hydrogen (H) 10.4, and heteroelements (S + O + N) 10.6. CBA included the following components, %: oils 28.9, resins 38, asphaltenes 35. As shown by component analysis, the average hydrocarbon content was 0.45 kg/t. «…The Kumian Formation deposits within the Khadyzhensk raion located in the eastern part of the Western trough are characterized by high Corg content over the entire area under consideration: 1.7–6.3% in the 2375–3556 m depth range, with the average Corg value of 3.12% maintained over the entire block area for depths of 2475–3075 m. The CBA content in the individual rocks is in the range of 0.05–0.71% (average value 0.38%). The bitumoid coefficient (βCB) varies over a wide range of 2.9–26.3% and increases with depth (average value 11.7%). Carbon (C) content in the CBA does not change much and remains in the range of 83.5–86.4% (average value 84.7%), thus being higher than in the Levkino block. The average hydrogen (H) content is 11.3%. The average content of CBA components is as follows: 48.7% oil fractions and 49.6% resinous-asphaltene fractions. HC content in the clays and argillites ranges from 0.1 to 0.7 kg/t rock (Novodmitrievskaya site), and that in marls amounts to 4 kg/t rock (Novodmitrievskaya, Martan, and North Kutaisi sites)” [3].

According to comprehensive geochemical studies, the Khadumian deposits of the Maykop series of the Western Ciscaucasia are characterized by relatively high Corg content and generation potential values.

The highest Corg content in the Khadumian deposits within the Western Kuban trough was registered in the central part of the trough (Severskaya, Severo-Krymskaya, Chernomorskaya, and others), where the average Corg concentration was 1.77%. The Corg concentration decreased in the western, Taman part of the trough, where the average Corg content was 1.25% (Taman). The average Corg content in the eastern part of the trough was 1.39% (Martan, Baku, Dysh, and Ilich). The highest average (S1 + S2) values were also registered in the central part of the trough (2.02 mg HC/g rock), whereas the values in the Taman part of the trough dropped to 0.95 mg HC/g rock. The Tmax values for the majority of samples analyzed by the Rock-Eval method were in the range of 410–430°C, whereas the HI values were close to 200 mg HC/g Corg, and therefore the original OM can be considered immature and “low-quality” (type III–II kerogen).

Clay deposits of the Khadumian Formation were studied in the Cheburgol and Yuzhno-Andreevskaya sites at the northern platform side of the West Kuban Trough. Khadumian deposits are located at depths of 3–4 km in this area. Corg values for the clay samples studied ranged from 0.46 to 0.89% per rock type (0.73% on average) and were thus lower than the Clarke values. CBA content was generally elevated at 0.5%, and βCB was 8.4%.

The pyrolysis data showed that the source potential at the Cheburgol site was 1.1 mg HC/g rock at a depth of 2930 m, the peak S1 value was 0.34 mg HC/g rock, and Tmax was 437°С. The hydrogen index (HI) remained low at 98 mg HC/g Corg, the productivity index (PI) was 0.31, and Corg was 0.77%.

The parameters listed above point at the low generation capacity of the Khadumian deposits in this zone. This is explained by the Khadumian deposits at the Cheburgol site not having entered the main oil formation zone (MOFZ): The degree of catagenesis for these deposits corresponds to the end of PC3, that is, the deposits within the platform side of the deflection mainly lie in the protocatagenesis zone.

Only isolated values of geochemical parameters are available for the Khadumian deposits from different tectonic zones of the western part of the Scythian Plate, this being due to both the absence of these deposits in the reduced section of the sedimentary cover and sabularisation of the Maykop section. Corg content in the Khadumian deposits of the Margaritovskaya-1 well located within the Rostov ledge is 0.9%, CBA concentration is 0.009%, and βCB is 1.0%. Corg content in the Khadumian deposits at the Velikaya site near the junction of the Kanev-Berezan and Dinskoy swells is 1.72%, CBA content, 0.039%, and βCB content, 6.0%.

The wells in the Sochi-Adler Depression within the Northwest Caucasus encountered Maykop deposits represented by dark-colored clays and clay aleurolites with sandstone interbeds; the total thickness of the deposits exceeded 2 km. The pyrolysis data for the clays of the Matsesta and Khosta Sequences (Khadumian Sequence analogues of the same age) were as follows: Corg content 0.38–1.87% (average 0.85%) and (S1 + S2) 1.6 kg HC/t rock [5–8]. Luminescence bituminological analysis showed that the chloroform bitumen content in the rocks ranged 0.08 to 0.156%.

Corg content in the profiles along the Mzymta River was in the range of 0.67–1.81%, (S1 + S2) = 0.7–5.3 HC/t rock, and HI = 76–282 mg HC/g Corg. The rock kerogen in this section has a mixed composition and belongs to type II–III [9].

Khadumian Formation rocks in the outcrops of the Loo and Mamaika rivers are represented by alternating dark-colored bituminous carbonate clays (marls) with sandstone interbeds. Corg content is 0.08–2.27%, and the source potential (S1 + S2) ranges from 0.08 to 7.45 kg HC/t rock, with an average of (S1 + S2) = 3 kg HC/t rock for both profiles, and therefore they can be regarded as source rocks with moderate source potential. The relatively low initial potential of OM from the deposits is due to the kerogen type. Mixed kerogen of types II and III with a hydrogen index of HI = 180–272 (average 230 mg HC/g Corg) prevails in the rocks from this area, with only one clay sample from the Loo River characterized by a value of 326 mg HC/g Corg, which falls in the range for the actual oil source type II kerogen. Bitumoids are represented by heavy, alcohol-benzene fractions, with alcohol-benzene bitumoid (ABB = 0.002–0.06%) prevailing over chloroform bitumoid (CB = 0.0002–0.01%) in almost all samples; the CBA/ABB ratio is 0.1–0.67% (average value 0.22%). This bitumoid composition corresponds to immature OM.

The degree of rock conversion is low, with a Tmax of 402–433°C (average value 425°C), that is, they are on the verge of PC catagenesis and have hardly begun to implement their potential yet. The productivity index PI = S1/(S1 + S2), which characterizes the degree of kerogen development, is very low at 0.01 to 0.17 (average 0.02) for most samples. However, the weak thermolysis processes have begun already, as evident from the emergence of paratochthonous HCs. Relatively low Tmax values (402–406°C), and elevated PI (0.16–0.55) and hydrocarbon coefficient βS1, (S1 × 10/Corg) = 1.25–2.5% values distinguish them from the background (the background βS1 values are 0.08–0.57%, with an average of 0.34%). Thus, the Khadumian Formation rocks in the Loo and Mamaika river area can be considered as oil and gas source rocks with satisfactory and good potential.

Comprehensive analysis of the organic matter of Khadumian rocks by bituminology, pyrolysis, chromatography, and mass spectrometry showed that all types of kerogen are developed in them, however, mixed OM with moderate oil source potential predominates. The hydrogen index (HI) values range from 10 to 420 mg HC/g Corg, and the average value is close to 100 mg HC/g Corg. The low values of the hydrogen index are in some contradiction with the chemical, petrographic, and facies characteristics of the OM of the Maykop (Khadumian) rocks, originally formed from planktonic algae (mostly dinoflagellates, blue-green, and green algae, less often diatoms). The admixture of terrigenous organic material is quite noticeable in the OM of the entire trough, but the maximal influence of land was noted only in sections of the Kerch and Taman Peninsulas and in the northern regions of the Azov-Kuban OGB. Such a discrepancy in the interpretation of the OM type is explained by the significant adsorbing effect of the mineral matrix composed by montmorillonite and illite, which can reduce the hydrogen index value 2–3 times; the developers of the Rock-Eval pyrolytic method mentioned this. This effect is especially pronounced under weak catagenesis conditions (at the beginning of the MOFZ). Separate pyrolysis of rock samples and pure kerogenes performed by researchers from the Moscow State University [10], confirmed a similar effect and revealed the presence of OM with a high oil source potential (HI = 650–750 mg HC/g Corg) in the Khadumian deposits The Khadumian Formation rocks are completely or partly located outside the focus of oil formation in most of the west of the Western Ciscaucasia.

The location of the PC-MK1 boundary that corresponds to the beginning of MOFZ has a differentiated character in the Cenozoic sediments.

Maximal depths of the MOFZ roof in the West Kuban trough (4.2–4.3 km, current temperature 130–140°C) are noted in the most curved, western part of the trough (Kerch-Taman and Dzhiginskaya-Varenikovo districts), characterized by well-developed mud volcanism and cryptodiapir structures. Heat outflow from the deep part of the profile is the most intensive in this zone. The position of the upper MOFZ boundary moves upwards in the eastern direction and the highest hypsometric position of the focus roof (2.5–2.6 km, temperature 100–105°C) is registered in the warmest part of the trough (the Khadyzhensk district) and on the platform side, where a general decrease in the thickness of the Maykop deposits occurs. MOFZ completion with regard to any geochemical parameter is not observed in the Maykop series (and, accordingly, the Khadumian Sequence) rocks of the Western Ciscaucasia located at depths of 5.3 km and less, that is, the oil formation focus in the Cenozoic rocks stretches over approximately 3 km, and this can be regarded as a positive factor in view of HC generation. “… Studies of the nature of geochemical parameter distribution over the area and sections of the trough showed that, despite the general similarity of the lithological composition, there is quite strong variation of Corg content and source potential in the Maykop deposits (the scatter of the values exceeds one order of magnitude)” [3]. Maxima of Corg concentration and source potential are registered within the Western Kuban trough (especially in its central part) and in the strip of outcrops near the Adygea ledge (the Belaya, Fars, and Kuban River basins). The improvement of geochemical indices in the central and eastern parts of the deflection is explained by the terrigenous component in the initial OM decreasing in the same direction [11–16].

The S2 values for the samples studied are in the range of 0.80–8.56 mg HC/g rock, but most of the samples fall into the “poor” and “medium” categories. The only rocks that can be assigned to the class of “rich” oil source formations are the clays of the Morozkino ravine horizon at the Belaya and Kuban rivers. Given the strong dependence of S2 on the degree of catagenesis, this gradation requires a differential approach. Strictly speaking, it can be used to characterize only the rocks that have not yet entered OFZ, namely, the samples collected in the Belaya River basin (Adygea ledge) and in the eastern regions of the West Kuban trough (North Kutaisi and North Khadyzhensk). Some of the samples studied are already in the main oil formation zone (Aleksandrovsk, Seversk, and Taman), since the Tmax exceeds 430°С; therefore, only residual S2 values can be discussed in this case.

The values of the total generation potential (which are in the range of 1.14–8.87 kg HC/t rock) show that most of the samples studied can be assigned to the “satisfactory” and “high” classes. Again, the clays of the Morozkino ravine in the outcrop areas of the Northwest Caucasus are classified as “very high”. “… Various combined graphs that enable the assessment of the genetic and catagenetic characteristics of kerogen are the most common in practical geochemical studies that use Rock Eval data. The graph that characterizes the relationship of HI and OI for different kerogen types is of primary importance among all these graphs.” The meaning of this diagram is similar to that of the famous diagram proposed by Van Krevelen, which clearly reveals three sets of points corresponding to three kerogen types in the H/Cat–O/Cat coordinate system” [17].

Type I kerogen is represented mainly by alkane structures with a small admixture of cyclomethylene and aromatic compounds. Microalgae and bacteria are the predominant starting material for type I kerogen. It is characteristic of bituminous and oil shales.

Cyclic (naphthenic and aromatic) structures predominate in type II kerogen. The primary OM that forms type II kerogen consists of a mixture of algal, grass, and wood residues. It is characteristic, in particular, of the lower Toarian of the Parisian OGB, the Upper Paleozoic of Svalbard Island, and the Silurian of Libya. Type III kerogen mainly contains acyclic and aromatic structures and oxygen-containing functional groups, as well as high-molecular-weight alkanes. Wood-inertinite organic matter is the source material for this type. Kerogen of this type is found in the Upper Cretaceous schists of the Douala OGB (Cameroon).

The graph [18] allows for confident identification of the kerogen type at the stages of protocatagenesis and MK1-MK2 gradations. The HI value decreases along with generation potential depletion at higher gradations, type I and II kerogens become difficult to distinguish, and graph construction and analysis becomes meaningless.

Graphs of hydrogen index (HI) dependence on Tmax have a higher information content, since they enable the tracing of catagenetic evolution of the kerogens, in addition to demarcation of the fields of different kerogen types. The correlation of Tmax values with R° enabled the French researchers to identify the Tmax < 430°C range as the immature kerogen zone (PC), to put the 430–465°C interval into correspondence with the “oil window” (MK1–MK3), and to assign the Tmax > 465°C interval to the gas zone. Unfortunately, it is not possible to obtain such dependences for the samples studied due to the lack of vitrinite reflectivity assessment data for these samples (importantly, this is a characteristic feature of the Maykop section).

Some of the samples fall into the main zone of oil formation (Tmax = 427–451°С) on the modified Van-Crevelen diagram (HI dependence on Tmax). The kerogen is assigned to types II and III (marine and mixed substance) because of considerable variation of the hydrogen index (54 to 314 mg HC/kg Corg).

CONCLUSIONS

Consolidation of the materials on the geochemistry of organic matter of the Cenozoic sedimentary formations of the Caucasus-Scythian region resulted in the identification of the following oil and gas source (OGS) strata:

Eocene (Kumian Formation): marls and calcareous clays, Corg content 0.6 to 11.5%, average value 3.3%), type II kerogen, catagenesis gradations PC2–MK3.

Oligocene to Lower Miocene (Maykop series): a thick stratum of dark-colored clayey aleurolites with interbeds of sandstones and olistostrome horizons with a thickness of 0.1 to 4.0 km. Catagenesis gradations PC2–MK4, type II and III kerogen, Corg content 0.1 to 18.0%. Source potential values (S1+S2) range from 0.7 to 127.3 mg HC/g rock. The hydrogen index (HI) ranges from 110 to 680 mg HC/g Corg.

This list can be expanded if the local sources, such as Khadyzhensk, Belaya Glina, and diatom series, Chokrakian rocks, and others, are taken into account.

The geochemical characterization of OM from the Kumian rocks revealed the high oil source potential of these rocks. Qualitative and quantitative changes reflect both the formation and the emigration of hydrocarbons; the sapropelic OM in the Kumian Formation generates large amounts of hydrocarbons, which increase from top to bottom along the profile. Para-autochthonous bitumoid is widespread in the rocks located at the greatest depth, and low-boiling HCs (HC7-C17) are common in it. Migration and paraautochthonous bitumoids are registered on the PC3 gradation already, but are extremely widely developed on the MK2 gradation (depth 4.6–5 km). Almost all bitumoids in this zone are paraautochthonous (β = 18%, 67% oils, HC content in the rock is up to 4 kg/t or higher).

Comprehensive analysis of the organic matter from Khadumian rocks by bituminology, pyrolysis, and chromatography showed that all kerogen types were developed in them, but OM mixed with moderate oil source potential predominated. The HI (hydrogen index) values varied in the range of 10–420 mg HC/g Corg, with an average of approximately 100 mg HC/g Corg. Only single rock samples from the southern regions of the study area (the Belaya and Kuban River basins) were identified as “rich” source rocks.

Maxima of Corg concentrations and genetic potential were registered within the Western Kuban trough (especially in its central part), as well as in the strip of outcrops in the Adygea ledge area (Belaya, Fars, and Kuban River basins). The improvement of geochemical parameters in these areas is explained by the terrigenous component content in the initial OM decreasing in the same direction