Catagenesis of Organic Matter and Petroleum Potential of Proterozoic and Phanerozoic Deposits of the Anabar–Khatanga Saddle

Catagenesis of organic matter (OM) determines the beginning, maximum intensity, and attenuation of the generation of oil and gas hydrocarbons and the location of oil and gas windows in the section. For this reason, information about the degree of OM maturity is one of the most crucial criteria for assessing the petroleum potential of sedimentary basins. In the study area, sparse research was performed mainly on Mesozoic deposits (Sorokov, 1974; Danyushevskaya, 1975; Voitsekhovskaya, 1977; Bogorodskaya and Soboleva, 1983; Afanasenkov, 1984, 1987). Some information about the catagenesis of OM in the upper Permian deposits (data by E.I. Soboleva) is given by Stepanenko (1985). Many years ago, OM catagenesis schemes were constructed for some Mesozoic, Paleozoic, and Vendian stratigraphic horizons, based on the available materials (Kontorovich et al., 1971, 1974, 1981). These authors assessed the degree of maturity of OM based mainly on its elemental composition and the vitrinite reflectance in kerogen. Note that different researchers reported contradictory data on these characteristics. Later, Mazitov et al. (2017), based on 91 measurements of vitrinite reflectance and results of 507 pyrolytic analyses, constructed a scheme of OM catagenesis in the Permian Lower Kozhevnikova Formation and schemes of the hydrogen index distribution in the supposed oil and gas source strata: Early Riphean Billyakh Series, Kuonamka and Lower Kozhevnikova formations, and Upper Devonian and Jurassic deposits. In this work, the measured reflectance of macerals (mainly vitrinite) of OM from Proterozoic and Phanerozoic deposits (>400 analyses) and its pyrolytic characteristics were compared with the data of other researchers. The pyrolytic index of the degree of OM maturity, T_max (temperature of the maximum rate of hydrocarbon release, corresponding to the peak S2 in the pyrogram), was used only when there were no more accurate coal-petrographic data. The objects of study were cores from deep wells or, more seldom, samples from outcrops (Fig. 1). New results significantly supplement the available information about the OM catagenesis in the Proterozoic and Phanerozoic deposits of the Anabar–Khatanga saddle.

Today, coal-petrographic methods, mainly determination of vitrinite reflectance $(RVto)$⁠, are the most accurate for estimating the degree of OM catagenesis. In marine and ancient (pre-Devonian) deposits lacking vitrinite, the maturity level of OM was determined from pseudovitrinite $(RAlgo)$⁠, a maceral of aquagene OM (it is an alga resembling vitrinite), or from bituminite $(RBTo)$ and sorbomixtinite $(RSMo)$⁠. There are no approved catagenesis scales based on these components, and the values of the reflectance (R^o) of alginite, bituminite, and sorbomixtinite are compared with $RVto$⁠; therefore, the degree of OM catagenesis can be estimated from the three macerals only approximately. The macerals that reached mesocatagenesis grades differ from vitrinite in slightly lower values of reflectance (R^o). This difference gradually disappears for the macerals that reached apocatagenesis. Reflectance was measured on an MSFP-2 microscope–spectrophotometer at a wavelength of 546 nm, using an immersion medium. Polished ground rock samples were examined without crushing and subsequent treatment with strong acids, which change the optical parameters of OM.

There are many catagenesis scales to determine the degree of OM catagenesis, but they were developed in different countries and thus have different names, which makes their comparison difficult. In addition, translation of the names of catagenesis grades accepted in different countries is often wrong. Neruchev et al. (1976) published the draft International Catagenesis Scale (Table 1), which was discussed by M.K. Kalinko, A.A. Kartsev, A.E. Kontorovich, V.I. Ruchnov, A.A. Trofimuk, etc. The authors took into account the already existing OM catagenesis scales compiled by I.I. Ammosov, V.I. Gorshkov, N.P. Grechishnikov, G.S. Kalmykov, I.E. Val’ts, I.B. Volkova, A.I. Ginzburg, N.M. Krylova, L.I. Sarbeeva, etc. They used the term “catagenesis” as the basis for a unified system of terms for the stage division of sedimentary rocks and proposed new terms and indexing for the substages and grades of OM catagenesis. To designate the early, middle, and late substages of catagenesis, the authors used the ancient Greek prefixes “proto-”, “meso-”, and “apo-”. The identified catagenesis substages and grades (abbreviated as PC_1–3, MC_1–5, and AC_1–3) nearly coincide with the Coal Grade Scale: Protocatagenesis corresponds to brown coals; mesocatagenesis, to stone coals; and apocatagenesis, to meager coal, semi-anthracite, and anthracite. Kontorovich and Trofimuk (1976) proposed a more detailed division of the mesocatagenesis substage into early (⁠$MC11–2$ grades), middle (MC₂), and late (⁠$MC31–2$⁠), with proper indexing. The above two catagenesis grade systems are widely used by Russian geologists. This work follows the index scale introduced by A.E. Kontorovich.

Organic matter of these strata was studied in the Khorudalakhskaya-1 (depth interval 2694–2867 m) and Kostrominskaya-1 (1587–1595 m) wells. The rocks contain only highly anisotropic graphitized particles of unknown genetic nature with R^o = 12–13%. Such high R^o values are typical of OM that has reached the metamorphism stage. The scarce available material, however, does not give an insight whether the discovered graphitized particles are syngenetic to the host strata or redeposited. Judging from the pyrolytic parameter T_max (478–581 °C), these strata have already exited the oil window and have almost completely exhausted the oil potential. Similar pyrolysis data were obtained by Lezhnin et al. (2021).

The Cambrian. The maturity level of OM in terrigenous–carbonate rocks of this age has been determined in the Vostochnaya, Kostrominskaya, Rybinskaya, and Yuzhno-Suolemskaya areas. The obtained data indicate mainly a high degree of transformation of these strata. The undivided Cambrian deposits in the Vostochnaya-1 well were studied in the depth interval 3001–3280 m. Judging from the reflectance of alginite $(RAlgo=2.97–3.04%)$⁠, sorbomixtinite $(RSMo=3.3–4.8%)$⁠, and bituminite $(RBTo=5.7%)$⁠, these strata transformed to deep apocatagenesis (AC_3–4 grades). There are also highly anisotropic graphitized particles of unknown genesis. In the Kostrominskaya-1 well, only graphitized particles were found in the middle Cambrian terrigenous deposits at a depth of 1106 m. Similar particles with R^o = 8% were also discovered in the middle Cambrian limestones from the Rybinskaya-1 well (depth of 2345 m). This indicates that the OM has reached the metagenesis stage. In the lower and middle Cambrian limestones from the Yuzhno-Suolemskaya-10 well (depth interval 2505–3135 m), the OM transformed to deep apocatagenesis (AC_3–4 grades), as follows from the R^o value of sorbomixtinite (3.0–4.8%). Graphitized particles were also found here.

The Devonian. The maturity level of OM in rocks of this age has been determined in outcrops on the Yuryung-Tumus Peninsula (Nordvik Bay) and in the Ulakhanskaya-2 well. The obtained data indicate mainly a high degree of OM catagenesis in these strata. In most of the studied Middle Devonian carbonate rocks of the peninsula, the reflectance of alginite $(RAlgo=3.32%)$⁠, bituminite $(RBTo=2.50–4.18%)$⁠, and sorbomixtinite $(RBTo=2.83–3.93%)$ indicates that the OM transformed to the middle of apocatagenesis (AC_2–3 grades). Organic matter of the early MC₂ grade $(RVto=0.85%)$ was found only in one outcrop of the Yuryung-Tumus Peninsula. Judging from the reflectance of alginate $(RAlgo=2.66–3.47%)$ and vitrinite $(RVto=4.26%)$⁠, the OM of Devonian terrigenous rocks at a depth of 3010 m in the Ulakhanskaya-2 well reached the middle or end of apocatagenesis (AC_2–3, mainly AC₃). The high degree of OM catagenesis is confirmed by the well-pronounced anisotropy of plant remains, which is usually observed at the apocatagenesis stages and is related to high dynamic loads on the rocks.

The Carboniferous. Deposits of this age were studied in the Severo-Suolemskaya and Rybinskaya areas and in outcrops of the Kiryaka-Tas Ridge in the southeast of the Taimyr Peninsula. The obtained data testify to significant transformation of these strata. For example, at a depth of 2048 m in the lower Carboniferous mudstones in the Rybinskaya-1 well, OM reached a high degree of apocatagenesis (AC_3–4 grades, $(RVto=4.43–6.33%)$⁠. The OM in the middle–upper Carboniferous terrigenous rocks from the Severo-Suolemskaya-1 well (depth of 2554 m) has transformed to approximately the same grade $(RVto=4.8%)$⁠. The high degree of OM transformation is confirmed by the well-pronounced anisotropy of vitrinite fragments. In the middle–upper Carboniferous limestones of the Yuzhno-Suolemskaya-10 well, highly anisotropic graphitized particles of unknown genesis were found at a depth of 2435 m. The high degree of OM catagenesis in these deposits (AC grade) was also reported by Afanasenkov (2019). The OM in the middle Carboniferous Makarov Formation of the Kiryaka-Tas Ridge reached apocatagenesis (AC₄, $(RVto=5.2–5.6%)$⁠.

The Permian. Comprehensive coal-petrographic research in these strata has been carried out by the Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk. According to the data obtained, the degree of OM transformation in the Permian rocks varies along the entire catagenesis scale, from $MC11$ to AC₄. Similar conclusions were earlier drawn by Mazitov et al. (2017) and Afanasenkov (2019). According to our data, the level of OM maturity in the lower Permian terrigenous rocks of the Tustakh Formation in the Vostochnaya, Gurimisskaya, Severo-Suolemskaya, Yuzhno-Suolemskaya, Ulakhanskaya, Rybinskaya, Yuzhno-Tigyanskaya, and Nordvikskaya areas corresponds to MC₂–AC₃. In the Vostochnaya-1 well, deposits of this age were studied in the depth interval 2238–2608 m. Judging from the vitrinite reflectance $(RVto=3.47–3.74%)$⁠, the OM reached the beginning of AC₃ at a depth of ca. 2600 m. Upsection, the degree of catagenesis slightly decreases and corresponds to the end of AC₂$(RVto=3.21–3.47%)$⁠. In the terrigenous rocks of the Gurimisskaya-2 well, the degree of OM catagenesis is higher, reaching the end of AC₃/the beginning of AC₄$(RVto=4.43–5.40%)$ in the interval 1940–2248 m. The high degree of OM catagenesis is also evidenced by the well-pronounced anisotropy of vitrinite fragments. Upsection (1748 m), the level of OM maturity decreases and corresponds to the middle/end of AC₂$(RVto=2.76–3.32%)$⁠. A serious transformation of OM in these deposits has been established in the Gurimisskaya-1 well: In the interval 1515–2660 m, $RVto$ (3.54–4.52%) marks AC₃ grade. The vitrinite fragments also show high anisotropy. Some samples (taken from a depth of 2545 m) contain graphitized particles with R^o ≈ 10%, which is typical of deep metagenesis. It is still unclear whether these particles are syngenetic to the host rocks or not because of the scarcity of the studied material.

Organic matter in the rocks of the Tustakh Formation in the Severo-Suolemskaya area transformed to approximately the same degree. At depths of 2478–2554 m in the Severo-Suolemskaya-2 well here, the level of OM maturity reaches the end of AC₃ (⁠$RVto=3.94–4.80%)$⁠. Upsection, the degree of catagenesis decreases; at a depth of 2090 m, OM corresponds to the end of AC₂ (⁠$RVto=3.25%)$⁠. The end of AC₃ (⁠$RVt o=4.18–4.70%)$ has been established in the Severo-Suolemskaya-2 well (depth interval 1944–2169 m). The high degree of OM catagenesis in these strata is also evidenced from the well-pronounced anisotropy of vitrinite fragments. In addition, graphitized particles with R^o = 12% were found in this well at a depth of 2293 m. The scarce material does not permit us to determine whether these particles are syngenetic or redeposited. A high level of OM maturity in this formation has been established in the Ulakhanskaya area. In the Ulakhanskaya-1 well here, only highly anisotropic graphitized particles with R^o ≈ 10% were found at a depth of 3041 m. In the depth interval 2768–2857 m, OM transformed to the beginning of AC₃ (⁠$RVto=3.73–4.09%)$⁠. Upsection, the degree of catagenesis significantly decreases; at depths of 2420–2594 m it corresponds to the end of AC₁$(RVto=2.10–2.45%)$⁠. Organic matter in the Ulakhanskaya-2 transformed with approximately the same intensity; in the depth interval 2041–2889 m, it reached AC_1–3$(RVto=2.27–4.09%)$⁠. The thick section of the Tustakh Formation (2768–3303 m) was penetrated by the Yuzhno-Tigyanskaya-1 well. Here, OM has undergone serious transformations at a depth of ca. 3300 m (AC_4–5, $(RVto=5.90–6.33%)$⁠. Upsection, the degree of OM catagenesis significantly decreases. In the depth interval 2768–3221 m, OM corresponds to the end of AC₂$(RVto=2.91–3.18%)$⁠, and there are also graphitized particles. Organic matter in the Rybinskaya-1 well is less transformed; at a depth of 1915 m, it corresponds to the beginning of $MC32$$(RVto=1.66%)$⁠.

The OM of the Tustakh Formation was studied in detail in the AKh-1 (depth interval 775–1220 m) and AKh-2 (1191–1539 m) wells. In AKh-1, $RVto$ is 2.97–5.40%, and in AKh-2 it varies from 4.8 to 5.3%. These data indicate that the OM has transformed to AC₂–AC₄ grades. Moreover, no regular increase in the level of OM maturity with depth is observed in both wells. The reason is that the section is rich in dolerite dikes, which caused local heating of sedimentary strata and, thus, more intense catagenesis of OM in certain depth intervals. Apparently, in this case, the additional heating of the strata by traps influenced the regional catagenesis of OM. The data obtained indicate that the OM reached the middle/end of apocatagenesis. This is also confirmed by the well-pronounced anisotropy of vitrinite fragments, typical of strata that have undergone strong dynamic loads.

A completely different situation is observed in the Yuzhno-Suolemskaya-10 well (depth interval 1518–1789 m), where OM reached only the middle of mesocatagenesis (MC₂, $(RVto=0.98–1.09%)$⁠. Organic matter in the Nordvikskaya-41 and Nordvikskaya-42 wells (depths of 711–768 m) transformed nearly with the same intensity $(RVto=0.96–1.00%)$⁠. Hence, the lower Permian deposits in these areas have not yet exited the oil window and may be of interest for search for hydrocarbon pools.

The level of OM maturity in the terrigenous rocks of the Lower Kozhevnikova Formation (P₁) has been determined in the Gurimisskaya, Severo-Suolemskaya, Yuzhno-Tigyanskaya, Ulakhanskaya, Rybinskaya, and Vostochnaya areas; it varies from MC₂ to AC₂. In the Gurimisskaya-2 well, the beginning of AC₁$(RVto=2.15–2.21%)$ has been established at a depth of 1660 m. The same degree of OM catagenesis $(RVto=2.27%)$ is in the Gurimisskaya-1 well (1151 m). Downsection, the degree of catagenesis increases and reaches AC₂$(RVto=2.64–3.32%)$ at a depth of 1309 m. A similar level of OM maturity is observed in the Severo-Suolemskaya-1 well (depth interval 1476–1517 m, $(RVto=2.57–3.32%)$ and in the Yuzhno-Tigyanskaya-1 well (1542–1596 m, $(RVto=2.90−3.62%)$⁠. In the depth interval 1873–2033 m in the Ulakhanskaya-1 well, OM transformed to the end of AC₁/the beginning of AC₂$(RVto=2.33–2.57%)$⁠. A similar level of OM maturity has been established in the Ulakhanskaya-2 well at depths of 1927–1964 m (the beginning of AC₁, $RVto=2.04–2.27%$⁠). Upsection, the degree of OM catagenesis decreases; in the interval 1652–1818 m it corresponds to the end of $MC31$/the beginning of $MC32$$(RVto=1.42–1.61%)$⁠, and at a depth of 1364 m it reaches the middle of MC₂$(RVto=0.97%)$⁠. Highly transformed OM was found in the AKh-1 and AKh-2 wells. In the former (depth interval 262–323 m), the vitrinite reflectance is within 2.24–2.64% and corresponds to the middle of AC₁/the beginning of AC₂. In the latter well (depths of 584–792 m), the OM is more transformed $(RVto=3.0–5.1%)$ and corresponds to the middle of AC₂/the end of AC₃. As noted above, the sections of the two wells are rich in dolerite dikes, which is reflected in the anomalous increase in $RVto$ near them.

A significantly lower and almost the same level of OM maturity in the rocks of the Lower Kozhevnikova Formation has been established in the Rybinskaya-1 and Vostochnaya-1 wells. In the former (depth of 1855 m), the vitrinite reflectance (0.97%) indicates that the OM reached the middle of MC₂ grade. Similar values (0.97–1.05%) were determined in the Vostochnaya-1 well (depths of 1787–1817 m). The lowest level of OM maturity is observed at a depth of 478 m in the Nordvikskaya-41 well, where OM reached the end of $MC12$$(RVto=0.80%)$⁠. Hence, the formation rocks in these wells have not yet exited the oil window and may be of interest for searching for hydrocarbon pools. A very high degree of OM catagenesis has been established in the lower Permian Sokolinaya Formation in the outcrops of the Kiryaka-Tas Ridge in the southeast of the Taimyr Peninsula. Here, the vitrinite reflectance $(RVto=4.8–5.9%)$ corresponds to AC_3–4 grades.

The level of OM maturity in the terrigenous rocks of the Upper Kozhevnikova Formation (P₂) has been determined in the Gurimisskaya, Rybinskaya, Ulakhanskaya, and Volochansksya areas; it varies from $MC11$ to AC₁. In the Gurimisskaya-2 well, the end of MC₁ (R_Vt = 0.80–0.81%) is observed at depths of 1231–1258 m. In the Rybinskaya-1 well (depth of 1337 m), OM is less transformed and corresponds to the transition from MC₁ to MC₁ (R_Vt = 0.65%). Upsection (depth of 1047 m), the degree of catagenesis decreases; in the rocks of the Misailap Formation (P₂) it reaches only the end of $MC11$$(RVto=0.62%)$⁠. The OM of the Upper Kozhevnikova Formation in the Ulakhanskaya area is slightly more transformed. At depths of 1640–1778 m in the Ulakhanskaya-1 well, OM reached the middle/end of $MC31$ grade $(RVto=1.32–1.47%)$⁠. Upsection (depth of 1369 m), the degree of catagenesis decreases; in the Misailap Formation, OM transformed to the middle of MC₂ (R_Vt = 0.97–0.99%). In the Upper Kozhevnikova Formation in the Ulakhanskaya-2 well (depth of 1364 m), OM transformed to the same degree $(RVto=0.97%)$⁠. Hence, the rocks of the Upper Kozhevnikova and Misailap formations in these wells are localized in the oil window.

A higher degree of OM catagenesis in the Upper Kozhevnikova Formation (the beginning of AC₁, $RVto=2.04%$⁠) has been established in the deep-seated strata of the Volochanskaya-1 well (depth of 3342 m). Upsection, the degree of catagenesis significantly decreases; in the depth interval 2909–3164 m it corresponds to the beginning of $MC32$$(RVto=1.63–1.66%)$⁠. A similar level of OM maturity $(RVto=1.66–1.68%)$ in this formation has been established in the Volochanskaya-2 well at a depth of 3102 m. The OM of undifferentiated Permian deposits is highly transformed in the Balakhninskaya-3 well: At a depth of 4488 m, it reached the middle of AC₁ grade $(RVto=2.36%)$⁠. Thus, in these areas, the Permian strata have already exited the oil window and are located in the deep-seated gas formation zone.

The predominantly high degree of OM catagenesis in the Proterozoic and Paleozoic deposits is also confirmed by pyrolytic studies. Figure 2, taking into account the coal-petrographic and pyrolysis data, shows the deep-seated boundaries of the meso- and apocatagenesis zones in some wells of the study area (⁠$RVto>2.0%$⁠, T_max > 500 ºC). As seen from the figure, the mesocatagenesis boundary in the Permian terrigenous strata in different wells is traced in the depth interval ca. 1500–2200 m. Hence, the deposits located below have already exhausted their petroleum potential and cannot preserve significant amounts of hydrocarbons.

The Triassic. Catagenesis of OM in these strata has been established in the Volochanskaya, Vladimirskaya, Ulakhanskaya, Logatskaya, and Balakhninskaya areas. In the Volochanskaya-1 well, the Middle–Upper Triassic terrigenous deposits were studied in the depth interval 1786–2305 m. In the lower horizons of the section, the degree of OM catagenesis corresponds to the middle/end of MC₂$(RVto=1.05%)$⁠. Upsection, it decreases and corresponds to the beginning of this grade $(RVto=0.86%)$⁠. Organic matter in the Middle-Upper Triassic strata of the Ulakhanskaya-1 well is still less transformed: At a depth of 1206 m, its maturity level corresponds to the end of $MC12$$(RVto=0.81–0.83%)$⁠. The OM in the Logatskaya-361 well (depths of 3265–3330 m) is of the same degree of catagenesis. In the coeval terrigenous rocks of Cape Tsvetkov, OM transformed to the middle/end of MC₂$(RVto=1.03–1.12%)$⁠. The beginning of this grade $(RVto=0.92%)$ is observed in the Lower Triassic terrigenous deposits of the Vladimirovskaya-21 well at a depth of 2650 m. A high degree of OM catagenesis has been established in the deep-seated undifferentiated Triassic deposits of the Balakhninskaya-3 well. Here, in the depth interval 3833–4188 m, it reaches the end of MC₃/the beginning of AC₁$(RVto=1.80–2.10%)$⁠.

The Jurassic. The OM catagenesis of these strata has been studied only in a few wells and outcrops because of the lack of samples, the low content of OM in the rocks, and the poor preservation of plant fragments. Organic matter in the Lower Jurassic deposits transformed to $MC11–MC2$⁠. In the Volochanskaya-1 well, the level of OM maturity at a depth of 1475 m corresponds to the end of $MC11$$(RVto=0.63%)$⁠. In the Logatskaya-361 well (depth of 3205 m), OM transformed to the same grade $(RVto=0.59%)$⁠. Organic matter in the Kubalakhskaya-1 well shows a much higher degree of catagenesis, reaching the end of MC₂$(RVto=1.12%)$ at a depth of 3590 m and the middle of this grade $(RVto=1.02%)$ at a depth of 3495 m. A similar level of OM maturity $(RVto=1.10%)$ has been established at a depth of 3459 m in the Novaya-1 well. In the Lower Jurassic terrigenous rocks from the Cape Tsvetkov outcrops, OM transformed to the middle of MC₂$(RVto=0.97%)$⁠. The degree of OM catagenesis in the Middle Jurassic (and Lower Jurassic) deposits is within $MC11–MC2$⁠. In the Balakhninskaya-1 well (at depths of 1250–1743 m, Vym’ and Laida formations), the vitrinite reflectance $(RVto=0.50–0.56%)$ indicates that the OM reached the beginning of $MC11$⁠. The beginning and end of this grade $(RVto=0.54–0.65%)$ are observed in the Novaya-2 well (depths of 1817–2457 m, Malyshevka and Vym’ formations). The degree of OM catagenesis is significantly higher in the Middle Jurassic strata (Leont’evka and Vym’ formations) in the Kubalakhskaya-1 well: In the depth interval 2842–3220 m it corresponds to $MC12$ grade $(RVto=0.70–0.81%)$⁠. Organic matter of the Malyshevka Formation in the Zapadno-Kubalakhskaya-359 well (depth of 3160 m) transformed to approximately the same degree $(RVto=0.67–0.73%)$⁠. Upsection (depths of 2907–3052 m), the degree of OM catagenesis decreases to the end of $MC11$$(RVto=0.62–0.64%)$⁠.

The Jurassic–Cretaceous (Gol’chikha Formation, К₁–J₂). The level of OM maturity in this formation was determined in the Zapadno-Kubalakhskaya, Vostochno-Kubalakhskaya, Massonovskaya, and Logatskaya areas. At depths of 2786–2877 m in the Zapadno-Kubalakhskaya-359 well, OM transformed to the end of $MC11$ grade $(RVto=0.59%)$⁠. Organic matter in the Vostochno-Kubalakhskaya-357 well (depth of 2471 m) transformed to the same degree $(RVto=0.62%)$⁠. In the Logatskaya-361 well (depth of 2318 m), the degree of OM catagenesis reached the beginning of $MC12$ grade $(RVto=0.70%)$⁠. In the Massonovskaya-363 well, these deposits occur at depths of 4198–4346 m; therefore, the degree of OM catagenesis is significantly higher and reaches the middle of MC₂ grade $(RVto=0.90–0.97%)$⁠.

The Cretaceous. These strata, like the Jurassic ones, were studied with a limited material. In the basal horizons of the Lower Cretaceous Lower Kheta Formation (depths of 3505–3591 m) in the Massonovskaya-363 well, OM reached the beginning of MC₂ grade $(RVto=0.87–0.90%)$⁠. Upsection, the degree of catagenesis decreases; at a depth of 2623 m, it reached only the end of $MC11$$(RVto=0.62%)$⁠. In the Vostochno-Kubalakhskaya-357 well (depth of 1604 m), the level of OM maturity in the Sukhaya Dudinka Formation (C₁) corresponds to the beginning of $MC11$$(RVto=0.52%)$⁠; upsection (depth of 1245 m), it decreases to the middle of PC₃$(RVto=0.46%)$⁠. In the Logatskaya-361 well (depth of 1711 m), the degree of OM catagenesis reached the middle of $MC11$ grade $(RVto=0.57%)$⁠. In an outcropped coal seam (C₁, Tigyan Formation) on the Yuryung-Tumus Peninsula, the degree of OM catagenesis reached the beginning of $MC12$$(RVto=0.71%)$⁠. In the Upper Cretaceous deposits of the bank outcrops of the Kheta, Ledyanaya, Boyarka, Romanikha, Maimecha, and Kotui rivers, the level of OM maturity corresponds to PC₃$(RVto=0.43–0.49%)$ (Afanasenkov, 2019).

To assess the petroleum potential of sedimentary strata, it is necessary to take into account many factors. This paper discusses only the catagenesis of OM, i.e., whether the studied deposits experienced thermobaric conditions that ensured the generation of hydrocarbons and the preservation of their pools. In the Paleozoic, there were probably prerequisites for the formation of deposits with a high petroleum potential in the Anabar–Khatanga saddle. The level of OM catagenesis in these deposits was sufficient for the main ore generation phase and deep-seated gas generation phase. This is confirmed by numerous shows of solid and soluble bitumens in the Paleozoic rocks, by the hydrocarbon smell from the core from certain intervals, and by the presence of autochthonous, parautochthonous, and allochthonous bitumens. Hence, the moderate catagenesis of OM in the deposits was accompanied by intense oil and gas generation, but the latter cannot be precisely assessed because of the recent transformation of these strata. At present, only a few wells (Yuzhno-Suolemskaya-10, Vostochnaya-1, Rybinskaya-1, and Gurimisskaya-2) have depth intervals with a low level of OM maturity $(MC11–MC2)$ in the Lower and Upper Permian deposits (Lower Kozhevnikova, Upper Kozhevnikova, and Misailap formations). These intervals might be petroleum-promising. At the same time, the predominantly terrigenous (humic) OM of these strata is unfavorable for the generation of oil hydrocarbons but does not rule out the possible formation of gas hydrocarbons.

In the study area, however, small oil fields were discovered in the Yuzhno-Tigyanskaya, Kozhevnikovskaya, Il’inskaya, Chaidakhskaya, and Nordvikskaya structures. The main oil pools are localized in Permian deposits, but few oil clusters and shows were also found in Triassic rocks and in Jurassic sandstones exposed along the periphery of the Yuryung-Tumus Peninsula (Neftyanoi Brook). There are different viewpoints of the origin of these oils. In the 1950–1970s, many researchers (Gedroits N.G., Gramberg I.S., Emel’yantsev T.M., Kalinko M.K., etc.) assumed a Permian source of oils in the Yuzhno-Tigyanskaya and Nordvikskaya areas. According to Krinin (2011), oil hydrocarbons of the northwestern Siberian Platform (including the Anabar–Khatanga saddle) were generated from the lower Cambrian and Vendian OM localized at the beginning of the oil window. However, our above-presented data indicate that these strata transformed to deep apocatagenesis (sometimes even metagenesis) and exited the oil and gas windows. Other researchers (Kashirtsev et al., 1999; Kontorovich et al., 1999), based on geochemical materials, believed that oil pools formed mainly from OM of Devonian oil source rocks.

Organic matter that reached the end of mesocatagenesis/the beginning of apocatagenesis $(MC31–AC1)$ is also present in the deposits that have already exited the oil window and occur in the deep-seated gas formation zone. These deposits might have preserved clusters of dry and condensate gas. Organic matter of the above catagenesis grade has been revealed in the Upper Kozhevnikova Formation in the Volochanskaya-1 well and in the lower horizons of this formation in the Ulakhanskaya-1 well. Syngenetic hydrocarbon pools are virtually unlikely in strata with OM of AC_2–3 grades and higher. In the study area, such deposits prevail. The abundance of intrusions and faults there makes the upper Paleozoic strata of low interest for petroleum exploration.

In the study area, Paleozoic deposits have undergone mainly significant thermobaric transformations unfavorable for the preservation of large syngenetic hydrocarbon pools. These conclusions are confirmed by the pyrolysis data. We have established that the highly mature OM has virtually exhausted its residual petroleum potential because of its high maturity level; only the OM with the degree of catagenesis not higher than MC₂ is still capable of generating oil hydrocarbons. However, the discovery of large oil pools in Paleozoic sedimentary complexes with more transformed OM seems unlikely. Nevertheless, we cannot rule out the future discovery of sedimentary strata with moderately transformed OM in which hydrocarbon accumulations were not completely destroyed under paleothermobaric conditions.

The Triassic, Jurassic, and Lower Cretaceous sedimentary deposits have not yet exited the oil window (⁠$MC11–MC2$ grades) and might be of interest for search for pools of liquid hydrocarbons. The Upper Cretaceous deposits have reached the end of protocatagenesis (PC₃ grade) and are localized in the upper gas formation zone. However, the predominantly terrigenous (humic) composition of OM in the Mesozoic strata is unfavorable for the generation of oil hydrocarbons, although the formation of gas hydrocarbons is not ruled out. These theoretical prerequisites for the petroleum potential of the studied strata have not yet been confirmed by the results of geological prospecting.

We thank the staff of the Trofimuk Institute of Petroleum Geology and Geophysics SB RAS for providing rock samples from outcrops for research.

The work was supported by project FWZZ-2022-0011 from the state program for basic research.