Do sedimentary basins in Russia have shale gas?

...First clay begets oil,

and in its turn it later begets shale gas

For quite a long time, since 2010, the author of this paper has come across multiple papers, which contain various — and apparently unproven —assessments of shale gas in the Russian subsoil... The concerns of the US, China, Argentina and some other countries are understandable: they have exhausted initial reserves and inferred resources of regular (coventional) free gas in normal gas-containing formations (of the gas, natural gas liquid, sodium hydrocarbonate, unconventional, etc. types), or the subsoil in these countries was initially poor with gas, so they had to explore, survey and develop resources of unconventional "dense gas" — in dense former reservoirs with low permeability (with the permeability below 0.1 mD), of coal gas — in coal-bearing formations and in coal, and of shale, gas-hydrate gas (dense gas, shale gas, combustible gases). But does Russia – the great gas-producing state – have to study and develop to large extent unconventional gas resources now and/or in the near future, before 2030? I guess, it has to. However, this will be with a long-range goal in mind, for the prospect after 2040, and the main concern will be the gas in dense low-permeability reservoirs.

Within Northern Eurasia, which is the land of Russia and the surrounding Arctic, Far East and inland seas, we know 30 sedimentary basins of sub-basins. Twelve of them are either large or the largest ones (mega-basins — West Siberian, Barents-Kara shelf basins), and they include oil- and gas-bearing mega-provinces, provinces and regions with the same names. For the initial and current reserves, initial and forecast resources of the conventional free gas, Russia is the global leader. The structure of initial potential conventional resources of free gas accumulated in the subsoil (by 01.01.2019) is as follows:

Initial reserves Resources

Unconventional gas resources = 23.6 + 49.3 + 23.7 + 49.6 + = 287.5/210

National Income A+B+C₁ B₂+C₂ I₁+I₂

*official/author's and corporate estimation

Without giving specifics to the size of free gas resources, we should emphasise the following: even with sober estimations, the real size of non-discovered gas resources in reservoirs exceeds 100 trillion m³. At present, Russia can produce not 725 billion m³ (national production in 2018), but significantly more — 900/950/100 billion m³, and the growth of gas production can happen quickly and easily if there are proper conditions in the world and regional gas markets (West + Central Europe, Asia-Pacific Region, etc.).

With the internal (national) consumption of 460-500 billion m³ (until 2030 and later on), all "surplus" gas recovered can be imported — to the Western and Eastern geostrategic directions. For example, in 2018 such an "excess" amounted to more than 250 billion m³. When the "second gas front" opens in the Far East (China, North Korea, South Korea, Japan) in 2020-2021, gas export from Russia will quickly exceed 300 billion m³ within a short period of time. This makes the US liquefied natural gas, which is mainly shale gas, stay far behind since within the near decade gas export from the US will hardly reach the amount of 100 billion m³ (in the natural, physical state): the country has high internal demand for gas even with export from Canada. However, let us come back to Russian shale gas.

For several decades the author has been working on the problem of oil and gas ontogenesis in sedimentary deposits. Subjects of his studies are classic bitumen-generating (oil-source) rocks in sedimentary deposits of Russia: the Bazhenov formation (the Volga Stage) in West Siberia, the Domanic formation (the Upper Devonian) in the Volga-Ural province, and to less extent the Kuonam formation in the East of East Siberia (the Cambrian) and the Kumsk formation in the Eastern Pre-Caucasian region (the Cenozoic). Research results are published here [1-9]. Along with the colleagues from Gazprom VNIIGAZ LLC, V.A. Kuzminov, V.A. Istomin, E.V. Perlova, L.S. Salina, V.S.Yakushev, all types of unconventional gas resources have been studied since late 80-s. It is beyond all doubts that the subsoil of a number of Russia sedimentary deposits is rich with shale oil, which estimated reserve only for West Siberia is within the range from 10 to 20 billion tons (however the reserves of regular Bazhenov-origin oil are still below 1 billion tons). In the central regions of the West-Siberian petroleum megaprovince (Khanty-Mansiysk Autonomous Okrug), production during the recent years has been at the level of 700-750 thousand tons/year which still cannot overcome the level of 1 million tons due to a number of reasons. Still it is the recovery of customary oil from a non-customary reservoir fractured in the volume of highly-bituminous rocks of the Bazhenov formation; production of shale oil is not present yet. By the way, according to the author's assessment, the ultimate potential resources of customary oil in the Bazhenov formation and its analogues in West Siberia are 2.5-3.0 billion tons, and this value must be considered along with the shale oil resources. According to the assessment by a number of researchers in the field of exploration and production, by 2040 the production of Bazhenov-origin oil can reach 35-50 million tons, and by 2050 it will be 80-100 million tons, due to the existing and future advanced technologies of "oil-bitumoids/bitumen oil" extraction from the Bazhenov formation. The latter values are over-optimistic, however, they are realistic basing on the large amount of organic oil-like substances dissipated in the Bazhenov formation volume over the area of 250-300 thousand km² (in central regions of the West-Siberian petroleum megaprovince). By the way, the USA produce not ultimately shale oil [2], but more likely combined oil in terms of its genesis and occurrence conditions. The real classic shale oil is known in Russia only — namely, in the Bazhenov formation in central-western regions of the West-Siberian petroleum megaprovince (Salym, Krasnoleninsk, Priob, etc.) [2, 10, 11, etc.]. So, let us be consistent. Let us begin with myths.

Myths about shale gas in Russia

This is the feature of petroleum geology as a science. It has plenty of scientific myths in all areas, such as the oil and gas genesis, and the shale gas issue.

Myth No.1: shale gas is widely spread in the subsoil of Northern Eurasia sedimentary basins [12].

Myth No.2: shale gas resources in Russia are vast.

Myth No.3: commercial shale gas production is possible in the foreseeable future. It is criticised in a number of papers [2, 13, etc.].

Let us consider the genetic conditions favourable for the formation of vast geological resources of shale gas fields in Northern Eurasia.

Ubiquity and overall distribution of various gases are evident, including hydrocarbon gases and oil-like substances of various density, geochemical types, genesis and level of maturity in The Earth's crust [2]. From its surface (wetlands, rivers, lakes, seas, from the solid surface of sedimentation basins) to very low depths (7-9 km and more), organic movable and non-movable compounds (OMC) are distributed in all organo-fluid-mineral systems in dissipated and concentrated forms (reserves of hydrocarbons, coal, combustible shale and more rare carbon shale, apart from red-color rocks where gas and oil quickly disappear within the scale of geological time, as they get oxidised).

"Shale hydrocarbons" is not a fully correct term, though in a number of cases bounding clay-siliceous-carbonate impermeable rocks can be enriched with organic compounds, reach a combustible/bituminous shale condition (with the content of sapropelic organic matter as dispersed organic matter from 12-15% to 22-25%, and even 30% in certain interlayers).

Among other "conventional" gases, shale gas is the leader in what concerns the amount of conducted scientific, experimental, technical and technological studies, and modern industrial importance in a number of countries [12, 14, and many others], notwithstanding the fact that it is a genetic "heir" of shale oil (oil-bitumoids) in the volume of producing strata with sea and lake genesis. The point is that on the whole shale gas is more frequent in many sedimentary basins and all over the world than shale oil.

Comparison of genetically various types of unconventional gas in highly-transformed terrigenous, including coal-bearing strata (with R⁰ over 1.2%) is shown in Figure 1.

Ontogenetic types of unconventional resources

Phase state	Фазовое состояние
Initial dissipated (microconcentrated) state in the matrix of bituminous/kerogenic shale	Изначально рассеянное (микроконцентрированное) в матрице битуминозных/горючих сланцев
Free and "semi-associated" gas	Свободный и «полусвязанный» газ
Water-dissolved	Водорастворенное
Free phase	Свободная фаза
Solid	Твердое
Shale oil and gas	Сланцевые нефть и газ
Coal gas of open-type coal-bearing basins (coal, adjacent formation)	Угольный газ угленосных бассейнов открытого типа (уголь, вмещающие породы)
Water-dissolved gas (methane) of geopressure areas — (hydrogas)	Водорастворенный газ (метан) геопрессурных зон – (гидрогаз)
Ultra-heavy oil, gas and oil of dense low-permeability natural reservoirs	Сверхтяжелая нефть, газ и нефть плотных низкопроницаемых коллекторов природных резервуаров
Methane gas-hydrate	Газогидраты метана
Generation	Генерационный
Emigration	Эмиграционный
Emigration-diffusion	Эмиграционно-диффузионный
Migration-evolution	Миграционно-эволюционный
Generation-migration	Генерационно-миграционный
GENETIC TYPES	ГЕНЕТИЧЕСКИЕ ТИПЫ

Figure 1 — Types of unconventional sources for natural gas obtaining/resources (as per phase state and genesis/ontogenesis)

Thus, hydrocarbon gases, and primarily methane, is generationally ubiquitous; oil is catagenetically and spatially restricted by the "oil window", both in the concentrated form (deposits in reservoir rocks), and in dissipated form ("oil-bitumoids/bitumen oil" and shale oil in especially favourable geothermal and geochemical, i.e. in generational conditions).

The experience of studying ontogenetic processes in subsoils of various-age sedimentary deposits around the world shows the specific nature of oil and gas shale fields, depending on the generational, emigrational, and evolutionary conditions inside clay-sapropelic formations/generators, with their progressive submerging to moderate and big depths in more and more severe thermal-catagenetic conditions [2, 6, 8, 15, 16].

Issues of oil and gas ontogenesis in various geological, geochronothermabaric and geochemical conditions are considered in detail in other author's papers [3, 4, 6, 9, etc.].

The author's calculations and conclusions on gas and bitumen generation in rocks of various types and age containing the humus organic matter — dispersed organic matter, semi-concentrated (carbonaceous shale), and concentrated (coal with the organic carbon content, Corg, over 50% of weight), sapropelic dispersed organic matter and pyroclastic organic matter, and mixed — humus-sapropelic type — clay shale/shale gas, including various admixtures of liptinite component (natural resines, flower dust, wax, etc.) are given in [2, 3, 6,etc.]. Figure 2 shows the author's calculations of hydrocarbon gas and bitumoid generation within the range of mature and late catagenesis over the oleum scale (with R⁰ from 0.85 to 3.40%). Figure 2 also shows catagenetic ranges of shale oil and shale gas formation in clay strata of sea and lake genesis with significantly sapropelic organic matter.

R0	Organic matter
	Liptinite-humus dispersed organic matter	Sapropelic dispersed organic matter in terrigenous subsoils
0.85	200/4.7	45/19.2	Shale oil Interval of intense fracture of rock and oil bitumoids in reservoirs Shale gas generation inside shale strata
0.90	204/5.0	60/22.0
1.00	210/6.0	70/24.0
1.05	214/6.5	80/26.0
1.10	217/7.0* (start of bitumoid and oil destruction in reservoirs)	90/28.0
1.15	220/5.0	100/30.0
1.20	228/3.0	250/20.0
1.25	237/2.5	320/15.0
1.30	245/2.0	370/10.0
1.35	258/1.0	400/7.0
1.50	300/1.0	450/3.0
2.00	320/0.5	500/2.0
2.50	330/-	550/0.5
3.00	335/-	570/-
3.40	340/-	620/-
*) adopted on the basis of natural factors, calculations and experimental data

Figure 2 — hydrocarbon gas and bitumoid generation within the range of "mature" and late organic matter catagenesis (m³, % of weight per 1 t)

It is significant, that at the phase of common bituminous coals (gradation MC³₃/ MC₄), organic matters of all types promptly switch to generation (R⁰ 1.20-1.25%). At that in regular reservoir, the oil phase (in the from of accumulations-reservoirs) disappears within the range 1.30-1.35% [6, 15, 16], however, it remains within the volume of Bazhenov formation (up to 1.4%). At the phase of lean coal (R⁰=2.00%) sapropelic organic matter significantly overruns humus organic matter in terms of hydrocarbon gas generation (500 and 320 m³ respectively per one ton of "residual organic matter" at this phase). This is what was at the base of the statement on the main gas formation phase (S.G. Neruchev, E.A. Rogozina, et al.), which is, however, referred to sapropelic organic matter only (gas accumulation in reservoirs which is secondary in terms of genesis).

The conditions which support the realisation of all links in the ontogenetic chain of processes and phenomena must be present for the oil and gas hydrocarbon shale formation in traditional reservoirs:

— generation (G) — emigration (Em) (preliminary migration from generator to the reservoir rocks) — secondary migration (via permeable rocks) of real phase-specific oil and gas — accumulation+conservation — evolution of hydrocarbon shale in traps — fall (partial/full) of deposits...Thus, shale oil and shale gas are the forms (components) of OMC, generated within mother rocks — generators, which did not migrate [2]. Poor migration conditions or their absence make the majority of OMC stay within mother strata (clays, coals, claystones, polymineral complexes of strata like those in the Bazhenov formation in West Siberia).

Efficiency and the scale of spatially and temporarily conjugated generation and emigration processes determine the weights and the volumes of OMC which stay in a non-associated condition within strata-generators and compose the so-called "shale oil", or to be more exact — "oil-bitumoids/bitumen oil" and shale gas, sequentially replacing each other within the range of mesocatagenesis MC₂-MC₃ (with R⁰ 0.65-1.25%). At that, in the frames of hydrocarbon ontogenesis, "shale oil" is essentially a transitional substance — oil-bitumoid (bitumen oil). The more the initial content of sapropelic organic matter was, the more "noble" its content was, the more isolated the internal areas from the reservoir horizons were (preliminary sand rock reservoirs, etc.), the longer shale oil stays within the clay-shale rocks (for the absolute weight and fraction of the generation mass). The same is referred to shale gas.

From oil-source clays (argillites) 100 m thick,for example, migration of a significant amount of the most mobile components of bitumoids will take place from 10-15 m zones adjacent to the covering and underlying reservoir horizons (fractureless variant), while the central areas of strata 70-80 m thick will be weakly involved in the emigration. In a gas-source bed (dispersed organic matter) of similar thickness, the hydrocarbon gas and part of bitumoids withdrawal in gas-dissolved state (within the range of "oil window" for humus organic matter) will be realised rather actively and relatively completely even for central areas at the distance from the nearest reservoir top of 30-40 m. However, in the oil-source bed in the middle and the end of mezacatagenesis (MC₃-MC₄), along with gradual change of the oil-like substance emigration (free phase-specific into gas-dissolved one), its relative scales and distances increase, that is "yield capacity" of source strata.

Thus, within the range of the "oil window" (R⁰ from 0.45-0.55 to 1.2-1.35%) with the significant generation of bitumoids in clay rocks with average and high content of dispersed organic matters of significantly sapropelic type (from 2-3% to 10% and more) to relatively thick beds, not all bitumoids pass the "preliminary migration cleaning phase" and turn into the migration-capable oil at the bed boundaries - reservoir/cover=generator interface. The remaining/nonmigrating oil (its part in the form of parautochtonous bitomioids) forms so-called "shale oil" which later transforms into shale gas. Thus, shale gas is a secondary-sapropelic gas that is spatially dispersed within generating impermeable strata of argillite-like dense clays and clay-siliceous-carbonate rocks.

As the global experience suggests, the formation of shale gas fields requires more strict ontogenetic conditions than the shale oil formation. Namely, the level of catagenesis of sapropelic/humus-sapropelic organic matter must correspond to the start of gradation MC₄ (at least R⁰ 1.20-1.25%), the total thickness of a clay bed is 40-50 m or more at various current content of Corg., but at least 3% even in apocatagenesis (R⁰>2.0%, and in the middle and the end of mesocatagenesis — at least 5%), minimum disjunctive disruption of generator strata.

In the end, the formation of efficient shale-oil fields requires the following:

• rather thick clay-bituminous "shales" (at least 20 m);

• intense bitumen generation within the organic matter catagenesis range from 0.6 to 1.1% R⁰;

• minimum, or no emigration, including via fractures.

The maximum favorable conditions for the formation of shale gas fields are as follows:

• intense secondary thermal-destructive gas generation (R⁰1.2%);

• sufficiently thick strata-generators, as well as isolating clays on the reservoir boundary (the thicker, the better, but at least 20-30 m);

• with any thickness of generating strata — low disjunctive disruption of the "generation system" (low development of strata-degasing fractures — and low areal "density" of medium- and especially high-amplitude ones).

The higher the level of disruption of generation-emigration system of sapropelic-shale strata at all phases of their development under soil, the less the amount of oil-bitumoids and shale gas that remain in them with all other conditions being equal.

The main difference between conventional and unconventional gas and oil resources is the hydrocarbon concentration level which depends on the realisation of emigration and migration potentials. Regular (=normal) gas and oil accumulations pass through long-term phases of migrations, accumulation + conservation, and finally — evolution of their accumulations inside traps [6, 9]. Dissipated forms of hydrocarbons "avoided" preliminary migration and "evolutionised" within source strata-generators.

Two forms of unconventional gas — shale gas and "dense" gas are a kind of genetic antipodes: the first one comes from bitumoids inside highly-transformed clay strata ("shale") with significantly sapropelic dispersed organic matter/pyroclastic organic matter; the second one is a result of descending (with submersion) evolution of gas (gas condensate) accumulations in reservoirs which loose permeability as they become denser.

Genetically and often spatially they "have contact" at the end of mesocatagenesis (with R⁰ 1.3-2.0%) in terrigenous strata when classical reservoirs loose their permeability; and vice versa, in clay-sapropelic strata, pore-crack systems of the evolution-generation type start to form (the Bazhenov formation in West Siberia, etc.).

Evolutionary development of various types of gas accumulations in terrigenous strata is shown in Figure 3.

Geological time	Геологическое время
Organic matter catagenesis scale	Шкапа катагенеза OB
0.5	0,5
1.0	1,0
1.3	1,3
1.5	1,5
Bitumen	Битум
Oil	Нефть
Gas	Газ
Clay-shale bituminous section	Глинисто-сланцевая битуминозная толща
Chromobarothermodegrading of organic matter, dispersed bitumoids, oil microaccumulations within clay-sapropelic strata	Хромобаротермодеградация ОВ, рассеянных битумоидов. микроскоплений нефти внутри глинисто-сапропелевых толщ
Kper — 10-300 mD	Кпр - 10-300 мД
H	Н
Kper 0.5-10 mD	Кпр 0,5 -10 мД
Light gas-saturated bitumoid	Легкий газонасыщенный битумоид
Natural reservoir inside a trap in sandy-clay grey nonmarine strata	Природный резервуар внутри ловушки в песчано-глинистых сероцветных неморских толщах
Kper 0.1-0.5 mD	Кпр 0,1-0,5 мД
Free gas	СГ
Gas in dense low-permeability reservoirs	ГПНК
Kper <0.1 mD	Кпр <0,1 мД

Figure 3 — Evolution of shale gas accumulations compared with the evolution of gas deposits in dense reservoirs with low permeability in severe thermal-depth and catagenetic conditions

Comparison of various types of unconventional gas in highly-transformed terrigenous, including coal-bearing strata (with R⁰ over 1.2%) is shown in Figure 4.

Coal gas	Угольный газ
Shale gas (organic matter like carbon, clay shale, free gas)	Сланцевый газ (ОВ типа С, ГС, СГ)
Gas in reservoirs with low permeabiity	Газ в низкопроницаемых коллекторах
Gas in coal	Газ в угле
Micro-accumulations of free gas related to gas-bearing strata 5	Микроскопления свободного газа, ассоциированные с угленосной а толщей 5
Separately, micro-accumulations of "normally" recoverable gas (<0.1 billion m3) in any formations and thermal-depth conditions of sedimentary basins. *probable "boundary" which separates unconventional gas resources from conventional gas resources 0.1 mD.	Отдельно микроскопления “нормально” извлекаемого газа (< 0,1 млрд м3) в любых формациях и термоглубинных условиях осадочных бассейнов. *вероятный “рубеж”, отделяющий НТРГ от традиционных ресурсов газа 0,1 мД.
Emigration	Эмиграция
Main condition: no fractures, low gas emigration into the above and underlying reservoirs	Главное условие: отсутствие разломов, малые масштабы эмиграции газа в выше- и нижележащие коллектора
Corg > 3% R0> 1% Н > 50 m	Сорг > 3% R0> 1% Н > 50м
Kper<0.1 mD* (dense gas-saturated sandstone, siltstone)	Кпр<0,1мД* (плотные газонасыщенные песчаники, алевролиты)

Figure 4 — Forms/types of unconventional gas in terrigenic sandy-clay strata, including coal-bearing, shale strata, etc.

According to VNIGRI experts (O.M. Prishepa et al., 2013), optimum stratum parameters for shale gas fields formation are as follows:

• content of Corg more than 1%;

• sapropelic-type organic matter (II);

• "gas window", R⁰ over 1.4 %;

• silicon content in the rock must be over 30% with a little carbonate;

• porosity is 4-7%, permeability is less than 0.1mD;

• thickness of clay-siliceous strata is over 45 m (for the Bazhenov formation this is a very rare phenomenon...);

• areas and sections are far from fractures and complications.

By the way, the geological-otnogenetic "necessities" for oil-bitumoid areas in the Bazhenov formation, West Siberia, with yielding reservoirs were determined by the author along with V.I. Ermolayev and S.G. Krasnov far back in 1978-1986 [3, 5, 8, etc.].

As for the formation of shale gas fields, the author's point of view in this paper is close to the specified one, apart from the values of Corg content and the level of catagenesis R⁰=1.4% (must be more than 3-4% and not less than 1.25% respectively), while 1% is a too low content of dispersed organic matter, and the Bazhenov formation thickness is not surely 45 m.
25-30 m are sufficient.

Across the West-Siberian petroleum megaprovince, the catagenesis conditions in the Jurassic cover are severe (MC₄- MC₅), as it has been established in papers [3,7]. They are developed on around 10% of the area: at the west of Salym and the east of Krasnoleninsk oil and gas region, in certain highly-heated areas in lows and deflections of the northern part of Nadym-Pur-Taz region, in the Kharasavey area on the land and in the Southern-Kara regions within the Priyamal shelf. However, even in the Salym oil and gas region, no inflow of free gas was obtained from wells of the Bazhenov formation intervals (medium or light gravity oil+dissolved gas). This means that even in the regions where catagenesis is very high (MC₃¹-MC₄), the Bazhenov formation is insufficiently mature to transform bitumoids and free gas into shale gas.

Sources (areas) of shale gas in the Bazhenov formation are possible in highly-heated areas only (125-140 C or higher), where bitumoids and micro-oil start transforming into a mixture of hydrocarbone gases. Their presence is probable at the west of the Salym and at the east of the Krasnoleninsk area, where thermal anomalies have been registered. The scope of published estimations of resources in West Siberia is prominent. It should be mentioned, that shale gas resources are unlikely to be significant here. The explanation is very simple: the major formation for shale — the Bazhenov formation — was catagenetically transformed up to gradation MC₁, MC₂-start of MC₃ at a large part of the megaprovince, which means that it has not yet "switched" to large-scale secondary generation of gas. Out of the three areas with thermal anomalies — Salym, East-Krasnoleninsk and Kharasvey-Kruzenshtern (the size of shallow invasion is over 1.20% R⁰) — only at the west of the Salym region the conditions for shale gas accumulation within the formation exist: in some sections, average temperature varies from 125 to 140 C (R⁰ is up to 1.3-1.4%); however, inflows of condensate-like highly-saturated oil were obtained here, full degradation (disintegration) of which is hindered by abnormally high formation pressure (in the fluidal fractured-porous system). The major part of gas may have emigrated to upper and lower half-spaces (with the 10-15 m cover thickness under and above the Bazhenov formation) even without low-amplitude fractures.

It is untimely to speak about shale oil resources in the rocks of the Upper Jurrassic – Lower Neocomian age in the South-Kara region (shelf); moreover, it is incorrect from the genetic point of view due to low percentage of dissipated organic matter (less than 3%) and its mixed content. In the north-west of the West-Siberian petroleum megaprovince, there is a remote chance of developing shale gas fields within the Bazhenov formation interval with small-thickness (8-12 m) grey clays.

However, during testing of the Bazhenov formation interval, a gas inflow with condensate was obtained from one of the wells at the Kharasavey site. There were no comments on that, as the mid-Jurassic horizons Ju₂ and Ju₃ contain GC — the deposits with abnormal (exceeding) stratum pressure up to 2.00-2.03, and there was a possibility of a gas breakthrough up the column. Along with that, the interval, where the Bazhenov formation correlate is located, is separated from the horizon Ju₂ by a thick layer of the Abalak formation grey clays (60 m), while the geothermic conditions "allow" to generate shale gas in the interlayers, rich with dissipated organic matter (2-3% or more). Such interlayers can be found in the mid-Jurassic strata in the Bazhenov formation interval.

In the future, it may become relevant to study and search for shale gas inside rather thick clay strata of the mid- and low-Jurassic age (up to 40-70 m and more each) in the north of the megaprovince, where organic matter is highly transformed, in the South-Kara region in particular (up to catagenesis gradation MC₄-MC₅ and higher) [4].

Table 1 shows the results of probability assessment of shale hydrocarbons formation in sedimentary rocks of the West-Siberian petroleum megaprovince.

Table 1 — Expert probability assessment of formation and areal extent of shale hydrocarbons in various-age strata of West Siberia regions

Lithostratigraphic formations	Oil and gas bearing region
	Middle Ob (south-west, north-west)		Frolov (west/east)		Nadym-Pur-Taz region		Yamal and South-Kazakhstan Region
	Shale oil	Free gas	Shale oil	Free gas	Shale oil	Free gas	Shale oil	Free gas
Senonian* (clay-siliceous formation)	-	-	-	-	-	++*	-	-
Apt-alb-Cenomanian	-	-	-	-	-	-	-	-
Neocomian (lower)	+	-	+/++	-	-	-	-	-
Upper Jurassic (the Bazhenov formation)	+++	+	++	+	+ (south)	-	-	+
Mid-Jurassic	-	-	+	+	++ (west)	-	-	+
Lower Jurassic (the Togura formation)	+	+	+	++/+	+	+ (west)	-	+
+++ high, ++ medium, + low, - utterly no

*) secondary migration gas (from the Cenomanian age)

Thus, the major role of the Bazhenov formation for the assessment and industrial development of customary oil accumulations in a non-customary reservoir (conventional oil reserves and resources), as well as of shale oil in West Siberia, remains steady, which cannot be said about shale gas [2].

The estimations of recovered shale gas resources are extremely contradictory for Russian basins.

Beyond the West-Siberian petroleum megaprovince in other sedimentary basins of Northern Eurasia, the conditions for the formation of shale gas fields are still not so favorable (as of the current moment); due to this fact the assessment reliability for shale gas resources for Russia is low — significantly lower than those for shale oil [1, 2].

In Russia, the second important bitumen-generating (oil-source) formation of marine-genesis is the Domanic deposit of the Volga-Ural province of the Late Devonian-Early Carboniferous age (D₃fr-С₁t), which consists of alternating carbon and clay-siliceous rocks with rare interlayers of terrigenous sandy-clay formations.

The formations in the south of the province have variable thickness (10-90 m), locate at the depths of 2-3 km and include sapropelic (rarely mixed) dispersed organic matter in the amount from 2-5 to 20% per rock. Many researchers of the Domanic age consider the Eagle Ford formation (USA) as an analogue, where large-scale industrial shale oil production takes place.

Along with that, the prospects of forming shale gas fields within the Volga-Ural province are estimated as minimum, and in the Caspian Depression — as undetermined. The same is true for the analogues of the Kuonam formation in the Lena-Vilyuy depression and for the north Caucasus Region.

The papers by the author et al. [1, 2, 6] show that the classic shale gas has a secondary origin. It is formed in high-carbon clay (clay-carbonate) bitumen-generating, initially oil-source formations of the Bazhenov formation in West Siberia at high stages of catagenesis (MC₄-AC₁) which correspond to coaxing, forge, non-baking coals up to semi-anthracites (with R⁰ from 1.35-1.40 to 2.6-2.8% - over the vitrinite reflection parameter - catagenesis indicator), when source rocks switch to gas generation due to continuing thermal destruction of sapropelic organic matter and earlier generated dissipated bitumoids (bitumen oil=shell oil), i.e. there is genetic connection between shale oil and shale gas. All classical oil-source formations in Northern Eurasia: Bazhenov, Domanic, Kumsk, Kuonam formations are at small and medium depths (1.0-3.5 km), they have not left the "oil window" range – they are immature in terms of gas generation. However, there was shale oil formation and they have reliable resources of bitumen oil in dissipated and micro-concentrated state (at the areas of fractures development). They contain fat gas dissolved in shale oil, but its amount is very small - the first m³ in 1 m³ of rock with shale oil content up to 50-70 l/m³(significant gas amount has already emigrated)

Shale gas fields can be connected with deep sediments of Middle and Lower Jurassic age (4-5 km and more) in the West Pre-Caucasian region. However severe thermal and depth conditions of their deposit make it difficult to study the possibilities of shale gas generation at the North Caucasus, including discovery and development of the probable shale gas location areas from the economical point of view.

Shale gas resources estimations in Russia vary from 8 to 20 trillion m³. Note the following. Assessments of geological and recoverable resources of shale hydrocarbons carried out within the recent decade (2009-2018) on Russian sedimentary deposits, including assessments made by non-specialists unfamiliar with the sites of West Siberia, Volga-Ural province and Pre-Caucasian region [17-20], are non-reliable: this is simply a "homage to fashion" and an ambition to "establish a foothold" to make other researchers refer to such assessments. Many authors of "popular scientific" papers and monographs, without proper knowledge of how to estimate potential or even conventional oil and gas resources, easily assess the quantity of unconventional gas and oil resources including such complicated types as shale oil and especially shale gas in Russian subsoil. What is the value of such assessments? The same applies to the deductions on unconventional hydrocarbon resources of sedimentary basins in Northern Eurasia made by some foreign incompetent authors, who are unfamiliar with the structure, and have not calculated anything, but weighed up expertly.

The shale gas status in the world is as follows. Table 2 shows the estimations of shale gas resources in ten countries with the richest gas reserves and in the world in general. Average gas-recovery ratio is 0.25, though in reality it can be significantly lower - down to 0.10-0.15. Thus, the global recovered free gas resources in 2014 were 207-221 trillion m3. This value is significantly lower than the estimations of the ultimate potential resources of customary (conventional) natural gas for the recent years, 703-720 trillion m³ [14].

Table 2 – Countries with the largest technically recoverable gas resources from shale formations (as of January 01, 2014)

No.	Countries	Amount of recovered gas, trillion m3
1	China	31.6
2	Argentina	22.7
3	Algeria	20
4	USA*(**)	18.8 (32.9)
5	Canada	16.2
6	Mexico	15.4
7	Australia	12.4
8	Republic of South Africa	11
9	Russia	8.1
10	Brazil	6.9
Total for 10 countries 163.1* - 177.2**
Total in the world 206.7* (220.7**)
* EIA, ** ARI, *** the value is not proved by certain calculations

Averaged data on the TOP10 gas-bearing shales in the USA and Canada are as follows:

1. Modern average depth is 1.2-2.7 km, rarely - down to 4.0-4.1 km.

2. Thickness: from 60-70 to 300 m with efficient gas saturation from 50 to 90%.

3. Dispersed organic matter content, % is 2.0-5.3 (up to10-12)

4. Organic matter catagenesis level R⁰ from 1.3-1.6 to 2.0-4.0% and higher (coals of the grades K, OS, T, PA) – "gas window" for all types of organic matter).

5. Gas content 1.7-10.0 m³/7 (including Free gas 50-55%).

The parameters of the Marcelius shales (USA) are given as an example:

This gas is in fact the residual one which did not emigrate. With the thickness more than 100 m the content of free gas would exceed 6-7 m³/t.

Apparently, conditions for the formation of shale gas fields in the majority of North America sedimentary deposits are quite favorable, in contrast, to those in Russia. As for the conventional gas, the picture is counter-narrative.

Due to genetically caused ubiquity of natural gas the conditions for the formation of shale gas fields are fulfilled for many sedimentary basins on all continents.

The countries - leaders in unconventional gas resources are as follows:

Gas in dense reservoirs - Russia, USA, China, Canada;

Coal gas - China, USA, Russia, Australia;

Shale gas - China, Argentina, Algeria, USA;

Combustible gas - Russia, Japan, Canada.

Four countries are the world leaders in terms of estimations of all types of unconventional resources: Russia, USA, China, Argentina.

Sedimentary basins which are most rich with shale gas: Preappalachian oil and gas basin, Williston (USA), Neuquén (Argentina), Sìchuān (PRC); shale oil: West Siberian, Williston, Neuquén.

Absolute monopolist and record-holder in industrial shale gas production is the USA: in 2014 they extracted 377.8 billion m³ of such gas, in 2018 it was 500 (!) billion m3. The same is true for shale oil (195.5/230 mln.t). With the total gas recovery volume in 2019 over 800 billion m³ up to 70% were for shale gas. All this takes place not out of bare necessities of life: the reserves and the resources of normal gas are almost exhausted.

Three main conditions for forming measurable accumulations of shale gas: development of marine clays with high content of highly transformed sapropelic organic matter with big thickness (at least 40-50 m) not damaged with fractures (no fractures with the amplitude more than 10-15 m). These conditions are not met in any of Russian sedimentary deposits, at least, down to the depths of 4.5-5.0 km where shale gas utilisation is unreal in the foreseeable future, as well as in the regular natural gas accumulations, at least, in terrigenous reservoirs (possibly, out of carbonates...).

Thus, Russia does not have the "big" free gas in its subsoil due to genetic reasons. Often the recovered dissipated resources are estimated by experts as 3-4 trillion m³, but it is obscure which formations they are exactly referred to, in contrast to the real shale oil resources, which can be calculated (they are calculated in papers [1, 2]), and the production is already on (though not so intense - less than 1 million t/year) in the central-east regions of West-Siberian petroleum megaprovince (Salym, Krasnoleninsk, etc.). Thus, the provided estimation of shale gas unconventional resources of 9-10 trillion m³in a number of publications is inconsistent.

General conclusion from the provided research is the answer to the question raised in this paper: in the volume of Russian sedimentary deposits, there are no generation-conservation conditions required for the formation of vast and lengthy shale gas fields (though, there are such local conditions...), but the volume-mass "geological" shale gas resources are insignificant, the recovered ones remain non-assessed, at least they will hardly exceed 4-5 trillion m³ and at least 4 times less than the real reserves of shale oil (with the nominal ratio of 1000 m³=1 t).

It should be noted that the Company considers it impractical to extract shale gas over the long term, according to the results of the PJSC Gazprom Board Meeting, November 18, 2019 (RIA Novosti November 19, 2019). Similar conclusions can be found in papers [1, 13 etc.].

This is rare consensus of science and business, especially on difficult issues, in particular on shale gas in Russia.

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