Geochemical Trends Reflecting Hydrocarbon Generation, Migration

geosciences

Article Geochemical Trends Reﬂecting Hydrocarbon Generation, Migration and Accumulation in Unconventional Reservoirs Based on Pyrolysis Data (on the Example of the Bazhenov Formation)

Mikhail Spasennykh 1, Polina Maglevannaia 1,* , Elena Kozlova 1, Timur Bulatov 1 , Evgeniya Leushina 1 and Nikita Morozov 2

1 Center for Hydrocarbon Recovery, Skolkovo Institute of Science and Technology (Skoltech), Skolkovo Innovation Center, 30, Build. 1, Bolshoi Boulevard, 121205 Moscow, Russia; [email protected] (M.S.); [email protected] (E.K.); [email protected] (T.B.); [email protected] (E.L.) 2 Gazprom Neft Science & Technology Centre, 75-79 Liter D Moika River emb., 190000 St Petersburg, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-9859511638

Abstract: The current study is devoted to the determination and interpretation of geochemical trends reflecting hydrocarbon generation, migration and accumulation in unconventional reservoirs; the Citation: Spasennykh, M.; study is performed on the Bazhenov shale rock formation (Western Siberia, Russia). Results are Maglevannaia, P.; Kozlova, E.; based on more than 3000 Rock-Eval analyses of the samples from 34 wells drilled in the central Bulatov, T.; Leushina, E.; Morozov, N. part of the West Siberian petroleum basin, which is characterized by common marine sedimentation Geochemical Trends Reflecting environments. Pyrolysis studies were carried out before and after the extraction of rocks by organic Hydrocarbon Generation, Migration solvent. As a result, we have improved the accuracy of kerogen content and maturity determination and Accumulation in Unconventional and complemented the standard set of pyrolysis parameters with the content of heavy fraction of Reservoirs Based on Pyrolysis Data hydrocarbons. The data obtained for the wells from areas of different organic matter maturity was (on the Example of the Bazhenov summarized in the form of cross-plots and diagrams reflecting geochemical evolution of the source Formation). Geosciences 2021, 11, 307. rocks from the beginning to the end of the oil window. Interpretation of the obtained results revealed https://doi.org/10.3390/ geosciences11080307 quantitative trends in the changes of generation potential, amount, and composition of generated hydrocarbons in rocks at different stages of oil generation process. The analysis of geochemical trends Academic Editors: Vladimir allowed us to improve approaches for the productivity evaluation of the formation and study the A. Cheverda and Jesus Martinez-Frias effect of organic matter maturity on distribution of productive intervals of different types.

Received: 11 May 2021 Keywords: Rock-Eval pyrolysis; oil generation potential; maturity; hydrocarbons; unconventional Accepted: 21 July 2021 formation; generative organic carbon; the Bazhenov Formation Published: 24 July 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- Source rocks with high organic matter content and related processes of hydrocarbon iations. formation in petroleum basins have been studied for many decades [1–5]. The shale revolution of the past two decades has demonstrated that these sedimentary rocks can be a promising target for exploration. However, a number of problems associated with the choice and application of methods for prospecting and exploration as well as production of Copyright: © 2021 by the authors. hydrocarbons from these unconventional reservoirs remain unresolved because they differ Licensee MDPI, Basel, Switzerland. significantly from the methods and technologies used for traditional oil and gas fields. This article is an open access article Geochemical studies of organic matter are commonly used to establish the most impor- distributed under the terms and tant characteristics that reflect hydrocarbon forming processes and determine the current conditions of the Creative Commons hydrocarbon potential of rocks [6–10]. The list of geochemical methods includes pyrolysis Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ and coal petrographic analysis, element, molecular, and isotopic composition studies of 4.0/). rocks and fluids, and other analytical methods. One of the most widely used geochemical

Geosciences 2021, 11, 307. https://doi.org/10.3390/geosciences11080307 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 307 2 of 17

methods for the analysis of organic matter in unconventional reservoirs is Rock-Eval pyrolysis [6,7,9,11–14]. This highly informative method is applied for determination of the organic matter content, its source (kerogen type), maturity, oil and gas generation potential, mobile hydrocarbon content, and other characteristics which are necessary to assess the quality of source rock in exploration of conventional petroleum-geological systems, to estimate oil and gas reserves of unconventional fields, and to choose the methods for hydrocarbon recovery. The estimation of thermal maturity of rocks is especially important in this case, and the common approach is based on vitrinite reflection measurements. In case of the absence of vitrinite, the combination of pyrolysis and biomarker analysis of oils and rock extracts allows one to determine maturity using the data on kerogen and mobile hydrocarbons composition [7,15]. Bulk rock pyrolysis data collected from the areas of different maturity of the same source rock formation provide an opportunity to analyze the geochemical evolution of rocks from the immature kerogen to the end of hydrocarbon generation process and to determine regional and local criteria for oil and gas reserves assessment. The current study is devoted to the analysis of generation, migration and accumulation of hydrocarbons based on pyrolysis data obtained from a representative collection of rock samples from one of the largest oil shale formations in the world, the Bazhenov Formation. In the study, we consider the deposits of the central part of the West Siberian basin, which have been formed in the similar marine environment. Such deposits are the major constituent of the whole Bazhenov Formation. Analysis of more than 3000 rock samples was performed according to the unified procedure and using the same instrument. The level of maturity of the studied rocks covers the entire range established for the Bazhenov Formation—from immature to the end of the oil window—and thus allowed us to identify the main patterns associated with the hydrocarbons generation and migration. Based on the results of the analysis, we have determined the criteria for the identification of the productive intervals.

2. Geological Setting The Bazhenov Formation is one of the largest source rock formations worldwide in terms of area and hydrocarbon resources [11,16–21]. The deposits of the Bazhenov Formation together with its stratigraphy analogs J3—K1 with similar characteristics (named the Bazhenov horizon) cover an area of more than 1 million km2 in the West Siberian Basin. The Bazhenov horizon is located at a depth of 2200 ÷ 3500 m; it is underlain by the Upper Jurassic terrigenous deposits and is overlain by Lower Cretaceous mudstones. The thickness of the formation varies from 20 to 80 m with an average of 25 to 30 m. According to various assessments [19,20,22,23], the amount of oil resources ranges from 50 billion to 1–3.5 trillion barrels, while recoverable reserves are estimated from 5 to 400 billion barrels. The considerable uncertainty of estimations comes from the insufficient investigation of the deposits over the large area of distribution, as well as from the poor adaptation of the resource assessment methods for the development of shale formations with extremely low permeability. The initial production rates of wells are usually low (8 to 12 t/day) and reach values up to 300 t/day only in rare cases, such as the Salym field [24]. At the same time, a large number of wells turn out to be “dry”, even after the application of hydraulic fracturing [24]. Low production rates are caused by extremely low permeability (from 0.001 to 0.1 mD) of the Bazhenov Formation rocks [25]. Oil and hydrocarbon gases of the Bazhenov Formation occupy void space of the rocks, including intergranular space and pores in kerogen. Intergranular porosity varies from 1% to 4%, and in some intervals could reach 8% [25]; organic porosity depends on thermal maturity and could reach 50% of the kerogen volume for the mature rocks [26,27]. The Bazhenov Formation is characterized by water-wet and neutral wettability [28]. The sediments of the Bazhenov Formation have been accumulated mostly (except paleoshelf areas) in marine conditions corresponding to the maximum of sea-level trans- gression period [29], which explains its wide spread. The Bazhenov Formation deposits significantly differ from the underlying and overlying rocks by increased radioactivity Geosciences 2021, 11, 307 3 of 17

and high organic matter content. It is composed mainly of organic-rich clayey-siliceous, organic-rich clayey-carbonate-siliceous and organic-rich siliceous rocks [17–19,21,29–33]. According to the results of lithological and geochemical studies, the Bazhenov Formation is usually subdivided into Lower Bazhenov and Upper Bazhenov. The Upper Bazhenov is represented by organic-rich clayey-siliceous and organic-rich carbonate-clayey-siliceous rocks with increased content of pyrite in most of the studied areas. The Lower Bazhenov is composed mainly of siliceous rocks; carbonates are almost absent, except for the secondary dolomites and limestones, which replace the siliceous rocks. By lithological composition, the section is subdivided into six members that differ by the content of the main rock-forming components: silica, organic matter, clay minerals, carbonates, and pyrite [29]. The lower member 1 is predominantly siliceous, with a low clay minerals content (up to 15%) and relatively low content of organic matter (TOC up to 5%). In the member 2, the TOC content increases compared to member 1 (up to 10%); it is characterized by a significant amount of thin radiolarite interlayers composed of radiolarian shell fragments and remains of bivalves and fish. The member 3 is composed of siliceous and clayey-siliceous rocks (up to 20% of clays), and a large amount of radiolarites. In this member, TOC values usually do not exceed 5 wt.%. Radiolarite layers are often productive due to improved reservoir properties by comparison with other layers. The radiolarite member 3 is the uppermost member of the Lower Bazhenov. The sediments of the Upper Bazhenov contain an increased amount of organic matter (TOC up to 30%), which usually reaches its maximum in the member 4 which is represented by clayey-siliceous rocks. The organic-rich carbonate-clayey-siliceous member 5 also contains a high amount of organic matter, framboidal pyrite and biogenic carbonates. The member 6, composed of siliceous claystone and clayey-siliceous rocks and often also contain an increased amount of pyrite, indicating reducing depositional environment. The Bazhenov Formation is overlaid by the Frolov Formation and underlaid by the Abalak Formation. These deposits are predominantly terrigenous clayey with a significantly lower organic matter content compared to the Bazhenov. The described sequence [29] is quite typical for the central part of the West Siberian Basin, with some variations in the amount of rock-forming components and the absence of some elements in the Bazhenov Formation areas that were subjected to erosion. The composition of the formation forming members is very similar, and the main differences come from the quantitative fluctuations of the main components, the appearance/disappearance of pyrite framboids, and the variations in the organic matter content. The simplified lithostratigraphic column of the Bazhenov Formation with underlying and overlying formations based on typical section from existing studies [29,34] is presented below (Figure1). TOC values vary along the section from 2 to 30 wt.%. For most regions of the Bazhenov Formation, the average TOC values are 9–12 wt.%, while the Upper part is characterized by higher values [35–39]. Kerogen is one of the main rock-forming components, its content varies from 5–50 vol.% [19,35]. Most of kerogen of the Bazhenov Formation is classified as type II [40–42], although some thin layers (usually 10–20 cm) are identified as type I kerogen [40,43]. The Bazhenov Formation contains kerogen and hydrocarbons in different forms including heavy oil fractions, light oil and hydrocarbon gases. The composition of organic matter in the rocks depends on the level of the catagenetic transformation. Despite the formation having quite uniform depth [35,44], the organic matter maturity varies significantly: from immature to the end of the oil window (stages PC3, MC1, MC2, MC3 according to Vassoevich’s scale [35,45]). Maturity of the Bazhenov Formation organic matter changes by area, from field to field, while from bottom to top of the formation in the particular area, the maturity is approximately constant. The maturity depends on a number of factors, including heat flow, tectonic conditions such as the proximity of faults and intrusions, and often varies considerably within relatively small areas. Geosciences 2021, 11, 307 4 of 17 Geosciences 2021, 11, x FOR PEER REVIEW 4 of 18

Figure 1. The Bazhenov Formation map with marked location of the studied areas and simplified lithostratigraphic col- Figure 1. The Bazhenov Formation map with marked location of the studied areas and simplified lithostratigraphic umn. Legend: 1—Bazhenov Horizon boundary; 2—Bazhenov Formation boundary; 3—studied field; 4—city; 5—river; 6— column.claystone; Legend: 7—limestone; 1—Bazhenov 8—siliceous Horizon claystone; boundary; 9—organic-rich 2—Bazhenov siliceous Formation rocks boundary; with low 3—studiedclay content; field; 10—radiolarite; 4—city; 5—river; 11— 6—claystone;organic-rich siliceous 7—limestone; rocks 8—siliceous; 12—organic-rich claystone; carbonate-cl 9—organic-richayey-siliceous siliceous rocks; rocks 13—organi with low clayc-rich content; clayey-siliceous 10—radiolarite; rocks 11—organic-richwith pyrite (modified siliceous after rocks; Panchenko 12—organic-rich et al., 2021 carbonate-clayey-siliceous[34]). rocks; 13—organic-rich clayey-siliceous rocks with pyrite (modified after Panchenko et al., 2021 [34]). TOC values vary along the section from 2 to 30 wt.%. For most regions of the Ba- zhenovThe Formation, Bazhenov Formationthe average deposits TOC values have ar ae high 9–12 initial wt.%, oil while generation the Upper potential, part is which char- acterizedcan reach by 150 higher÷ 180 values mg HC/g [35–39]. rock, Kerogen in rare casesis one up of tothe 250 main mg rock-forming HC/g rock [35 components,,39,41]. The ÷ itsinitial content hydrogen varies index from (HI5–500) vol.% is in the [19,35]. range Most of 650 of kerogen750 mg of HC/g the Bazhenov TOC [35]. Formation is classifiedIn general, as type the II [40–42], Bazhenov although Formation some canthin be layers considered (usually as 10–20 an entire cm) are petroleum identified sys- as typetem coveringI kerogen the [40,43]. central part of the West Siberian Basin. The studied central area of the Bazhenov Formation was formed under similar reducing marine sedimentation condi- The Bazhenov Formation contains kerogen and hydrocarbons in different forms in- tions. The genesis and initial characteristics of organic matter are common. The currently cluding heavy oil fractions, light oil and hydrocarbon gases. The composition of organic observed differences in the organic matter content, composition and thermal maturity in matter in the rocks depends on the level of the catagenetic transformation. Despite the rocks result from variations in the amount of initial organic matter, thermal history of formation having quite uniform depth [35,44], the organic matter maturity varies signifi- the region, mineral matrix composition and structure, and other factors, which determine cantly: from immature to the end of the oil window (stages PC3, MC1, MC2, MC3 accord- kerogen transformation to hydrocarbons. ing to Vassoevich’s scale [35,45]). Maturity of the Bazhenov Formation organic matter changes3. Samples by area, from field to field, while from bottom to top of the formation in the particular area, the maturity is approximately constant. The maturity depends on a number In the current study, we have analyzed core samples from 34 wells located within of factors, including heat flow, tectonic conditions such as the proximity of faults and in- the central part of the West Siberian Basin (Figure1). The studied Bazhenov Formation trusions, and often varies considerably within relatively small areas. intervals from different wells and oilfields are similar to each other in terms of structure and The Bazhenov Formation deposits have a high initial oil generation potential, which lithological composition. Considerable variations in the organic matter thermal maturity canover reach the studied 150 ÷ 180 Bazhenov mg HC/g Formation rock, in area rare allow cases us up to to identify 250 mg and HC/g analyze rock the [35,39,41]. evolution The of initialthe oil hydrogen forming process index (HI during0) is in the the geological range of history650 ÷ 750 of mg the HC/g basin. TOC Samples [35]. were collected fromIn the general, core at the equal Bazhenov intervals Formation (on average, can be 2 samples considered per as 1 m),an entire from intervalspetroleum with system the coveringknown lithological the central composition part of the West and correspondingSiberian Basin. toThe lithostratigraphic studied central members.area of the TheBa- zhenovoverlying Formation and underlying was formed terrigenous under similar rocks were reducing not involved marine sedimentation in the analysis. conditions. A total of

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1909 rock samples were subjected to Rock-Eval pyrolysis before extraction with an organic solvent and 1100 of them were analyzed after extraction.

4. Methods Pyrolysis studies were carried out on HAWK pyrolyzer (Wildcat Technologies LLC, Humble, TX, USA) according to the bulk-rock temperature program. The basis of Rock-Eval pyrolysis technique has been repeatedly described in previous studies [8,14,42,46]. The bulk-rock program includes two stages of heating (in an inert atmosphere and the presence of air), with continuous recording of released hydrocarbons using a flame ionization detector (FID), and carbon oxides CO and CO2 using IR cells. The crushed sample in the crucible is transferred to the oven, where stepwise heating takes place according to a specified temperature program. At the first stage of pyrolysis, gaseous hydrocarbons are released at temperatures up to 90 ◦C, giving S0. With a further increase of temperature to 300 ◦C, liquid hydrocarbons (S1) are thermally evaporated from the rock. High-boiling point hydrocarbons and hydrocarbon products of kerogen pyrolysis are detected at higher temperatures up to 650 ◦C (S2). The output pyrolysis parameters are S0 (gaseous hydrocarbons), S1 (light petroleum hydrocarbons), and S2 (products of kerogen pyrolysis plus heavy oil fractions) measured in mg HC/g rock, as well as parameters S3 (the amount of CO2 released during the pyrolysis), S4 (the amount of CO2 released during the oxidation stage) and S5 (the amount of CO2 released during the carbonate decomposition) measure and reported as mg CO2/g rock. These parameters are used for the calculation of the total organic carbon content TOC in wt.%. The temperature of the maximum of peak S2 is Tmax, which is an indicator of the maturity, it was reported to correlate with the vitrinite reflectance [47,48]. The sum of (S0 + S1 + S2) is attributed to the petroleum generation potential, and for immature organic matter is equal to the initial generation potential of the rock. The parameter S2 characterizes the residual generation potential of the rock. Heavy hydrocarbon fractions (resins and asphaltenes) are registered as part of the peak S2 due to high boiling temperatures. For a more reliable determination of the heavy hydrocarbons content, pyrolysis was carried out before and after the extraction of powdered rock with chloroform in Soxhlet apparatus for 1-2 months. The proportion of soluble resins and asphaltenes was determined as the difference in S2 value before and after extraction (∆S2 = S2 − S2ex). Here and after pyrolysis parameters and indices after extraction are labeled by “ex”. The processing of the obtained data also included calculation of hydrogen index HI (mg HC/g TOC) = S2/TOC × 100 (or HIex = S2ex/TOCex × 100), oxygen index OI (mg CO2/g TOC) = S3/TOC × 100, productivity index PI = (S0 + S1)/(S0 + S1 + S2), generative organic carbon content GOC (wt.%) = (S0 + S1 + S2) × 0.085 + S3 × 12/440 + (S3CO + S30CO) × 12/280) [12,49]. Along with the mentioned indices, we propose the use of the coefficient Kgoc, which characterizes the proportion of kerogen that could be transformed to hydrocarbons under elevated temperatures. It is calculated as the ratio of the carbon in generative kerogen (GOC—generative organic carbon) to the total organic carbon TOC for the samples extracted by chloroform. The calculation was performed according to the formula Kgoc = (GOCex/TOCex) × 100, %. In the current study, the coefficient Kgoc is used for the quantitative representation of the organic matter transformation to petroleum as the result of the gradual realization of the generation potential. Unlike Tmax and HI, Kgoc characterizes the organic matter maturity directly as the proportion of kerogen remaining: it decreases from the highest value at the initial stage of catagenetic transformation to zero when the generation potential is fully realized.

5. Results and Discussion Rock-Eval pyrolysis data interpretation before and after extraction is described above (Section4). The obtained results for the selected Bazhenov Formation rock samples are Geosciences 2021, 11, 307 6 of 17

summarized in Table1. Because we sampled the rock at equal intervals, the number of samples reﬂects the distribution of intervals with the speciﬁed characteristics within the Bazhenov Formation. The results of the analyses are represented as the diagrams and plots.

Table 1. Characteristic ranges of pyrolysis parameters for the Bazhenov Formation (BF) samples at different maturity stages.

Maturity Catagenesis Number of Share in the BF Tmax HI Tmax HI K Stage Stage 1 Samples Section, % ex ex goc Immature PC3-MC1 422 22 425 ÷ 438 500 ÷ 700 430 ÷ 440 400 ÷ 750 45 ÷ 62 Early oil MC1–2 464 24 436 ÷ 441 425 ÷ 550 434 ÷ 444 350 ÷ 550 35 ÷ 45 window Middle oil MC2 874 46 439 ÷ 445 200 ÷ 450 436 ÷ 450 100 ÷ 450 15 ÷ 35 window Late oil MC3 149 8 440 ÷ 455 <200 444 ÷ 466 50 ÷ 150 3 ÷ 15 window 1 According to Vassoevich et al., 1975 [45] and Kozlova et al., 2015 [35]. Geosciences 2021, 11, x FOR PEER REVIEW 7 of 18

5.1. Generation Potential of the Bazhenov Formation Rocks The generation potential of rocks can be estimated using the data on the balance thecharacteristics studied samples of the belong hydrocarbon to rocks composition with “excellent” (S0 + generation S1 + S2) and potential, TOC, the and diagram most of is theseshown samples in Figure are2 .characterized This type of diagram by Kgoc above and the 35%. classiﬁcation About 15% of of rocks the samples by their fall generation into the rangepotential of “good” value was and initially “very good” reported potential, in [7,15 with]. The a givenproportion diagram of pyrolyzable is complemented carbon by K thegoc

Figure 2. Characteristics of the generation potential of the Bazhenov Formation deposits. The color Figure 2. Characteristics of the generation potential of the Bazhenov Formation deposits. The color indicates the proportion of generative kerogen Kgoc = (GOCex/TOCex) × 100, %. indicates the proportion of generative kerogen Kgoc = (GOCex/TOCex) × 100, %.

5.2. KerogenThe obtained Type data provide the statistically reliable characterization of the Bazhenov FormationInformation deposits about in the the central kerogen part type of thecan Westernbe obtained Siberian from Basin the HI-Tmax and do not diagram, contradict of- tenthe called results the of individualmodified van studies Krevelen for different diagram. geological It was used objects in inmany this region.previous As studies it follows to analyzefrom the the diagram, kerogen the type, rocks for of example the Bazhenov in [3,9]. formation In Figure are 3, characterizedwe report a modified by TOC version content ofin van the rangeKrevelen from diagram 0.5 to 30.5 for wt.%,the studied with an sample average collection. of 9.6 wt.%. In addition The generation to the previous potential studies,of rocks we (S0 have + S1 made + S2) variesa diagram in a wideusing range,the HI andex and more Tmax thanex values 90% of determined the samples for show the extractedvalues from samples 15 to 105in order mg HC/g to characterize rock. According kerogen to theitself classiﬁcation without the from consideration [7,15], 75% of of the the soluble hydrocarbon fractions in S2 and Tmax. Another feature is the addition of Kgoc to the diagram (shown in color), which reflects maturation as the proportion of pyrolyzable organic matter.

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studied samples belong to rocks with “excellent” generation potential, and most of these samples are characterized by Kgoc above 35%. About 15% of the samples fall into the range of “good” and “very good” potential, with a proportion of pyrolyzable carbon Kgoc < 35%.

5.2. Kerogen Type Information about the kerogen type can be obtained from the HI-Tmax diagram, often called the modified van Krevelen diagram. It was used in many previous studies to analyze the kerogen type, for example in [3,9]. In Figure3, we report a modified version of van Krevelen diagram for the studied sample collection. In addition to the previous studies, we have made a diagram using the HIex and Tmaxex values determined for the extracted samples in order to characterize kerogen itself without the consideration of the Geosciences 2021, 11, x FOR PEER REVIEW 8 of 18 soluble hydrocarbon fractions in S2 and Tmax. Another feature is the addition of Kgoc to the diagram (shown in color), which reflects maturation as the proportion of pyrolyzable organic matter.

Figure 3. Modified van Krevelen diagram for the studied sample collection. Values of HIex and Figure 3. Modified van Krevelen diagram for the studied sample collection. Values of HIex and Tmaxex were determined for samples after extraction with chloroform. The color indicates the pro- Tmax were determined for samples after extraction with chloroform. The color indicates the portionex of generative kerogen Kgoc = (GOCex/TOCex) × 100, %. Lines divide samples into: I—type I proportionkerogen, II—type of generative II keroge kerogenn, III—type Kgoc III= (GOCkerogen.ex/TOC ex) × 100, %. Lines divide samples into: I—type I kerogen, II—type II kerogen, III—type III kerogen. Figure 3 is useful for reliable determination of the organic matter source and analysis of organicFigure3 matter is useful geochemical for reliable evolution determination during of maturation. the organic matterAccording source to the and position analysis on of organic matter geochemical evolution during maturation. According to the position the diagram, more than 95% of kerogen samples can be classified as type II kerogen with on the diagram, more than 95% of kerogen samples can be classified as type II kerogen different thermal maturity, which is consistent with previous studies based on the char- with different thermal maturity, which is consistent with previous studies based on the acterization of the Bazhenov Formation kerogen by elemental analysis, petrography, and characterization of the Bazhenov Formation kerogen by elemental analysis, petrography, pyrolysis [40,41,50]. and pyrolysis [40,41,50]. About 1.5% of the samples are characterized by increased values of the hydrogen About 1.5% of the samples are characterized by increased values of the hydrogen index index HIex > 750 mg HC/g TOC, which is typical for type I kerogen. Type I kerogen is HI > 750 mg HC/g TOC, which is typical for type I kerogen. Type I kerogen is commonly commonlyex associated with lacustrine settings. However, type I kerogen can also be associated with lacustrine settings. However, type I kerogen can also be formed from formed from concentrated marine algae or from organic matter that has undergone exten- concentrated marine algae or from organic matter that has undergone extensive bacterial sive bacterial reworking [7,51]. In the Upper Bazhenov Formation, type I kerogen was reworking [7,51]. In the Upper Bazhenov Formation, type I kerogen was previously previously identified based on organic petrography, elemental analysis, Rock-Eval pyrol- identified based on organic petrography, elemental analysis, Rock-Eval pyrolysis, and molecularysis, and molecular parameters parameters [43,52]. About [43,52]. 3.5% About of the3.5% studied of the studied samples samples may be may incorrectly be incor- referredrectly referred to type to III type kerogen. III kerogen. Taking Taking into account into account the general the general trend and trend the and proportion the propor- of tion of converted kerogen, we conclude that these samples contain type II kerogen of very high maturity, where organic matter is transformed to the maximal extent. Further discussion relates only to the samples containing type II kerogen. Individual samples with enormously high HI probably refer to type I kerogen (red markers on Figure 3), and they were excluded from analysis to avoid incorrect interpretations. Thus, for the further studies of maturity trends we have carefully selected the samples, representing the rocks formed under similar depositional environment, containing the same kerogen type and collected from different areas of the central part of the Ba- zhenov Formation. The conclusions made are reliable for rocks with marine type II organic matter accumulated under reducing environments, which is characteristic for the most of the Bazhenov deposits.

5.3. Relations between Thermal Maturity and Organic Matter Content in Rocks Analysis of the inter-relations between TOC and the degree of catagenesis was carried out using S2ex-TOCex diagram (Figure 4), illustrating the organic matter content and

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converted kerogen, we conclude that these samples contain type II kerogen of very high maturity, where organic matter is transformed to the maximal extent. Further discussion relates only to the samples containing type II kerogen. Individ- ual samples with enormously high HI probably refer to type I kerogen (red markers on Figure3), and they were excluded from analysis to avoid incorrect interpretations. Thus, for the further studies of maturity trends we have carefully selected the samples, representing the rocks formed under similar depositional environment, containing the same kerogen type and collected from different areas of the central part of the Bazhenov Formation. The conclusions made are reliable for rocks with marine type II organic matter accumulated under reducing environments, which is characteristic for the most of the Bazhenov deposits.

Geosciences 2021, 11, x FOR PEER REVIEW 9 of 18 5.3. Relations between Thermal Maturity and Organic Matter Content in Rocks Analysis of the inter-relations between TOC and the degree of catagenesis was carried out using S2ex-TOCex diagram (Figure4), illustrating the organic matter content and the residualthe residual generation generation potential potential of rocks, of rocks, see s foree for example example [42 ].[42]. In In the the current current study, study, S2- S2- TOCTOC diagram diagram was was built built for for the the extracted extracted rock rock samples samples to to evaluate evaluate the the content content of of kerogen kerogen andand generation generation potential potential withoutwithout contributioncontribution of extractable heavy heavy hydrocarbon hydrocarbon to to S2. S2. In goc InFigure Figure 4,4 ,points points are are colored colored according according to toK K values,goc values, which which reflect reﬂect the content the content of genera- of generativetive kerogen kerogen in rock in rockat the at current the current maturity maturity stage. stage. As Asit follows it follows from from the the diagram, diagram, the themaximum maximum of ofS2 S2 and and TOC TOC values values correspond correspond to to organic organic matter matter with with the the lowest lowest degree degree of oftransformation transformation (K (Kgocgoc > >55%, 55%, yellow yellow points). points). A Adecrease decrease in in K Kgocgoc leadsleads to tothe the decrease decrease in inthe thehighest highest S2 S2 and and TOC TOC values values at at each each maturity level, whichwhich isis thethe result result of of hydrocarbons hydrocarbons generationgeneration at at the the previous previous stages stages of of catagenesis. catagenesis.

Figure 4. S2ex-TOCex diagram for the studied sample collection. The color indicates the proportion Figure 4. S2ex-TOCex diagram for the studied sample collection. The color indicates the proportion of generative kerogen Kgoc. The calculated maturity trends are drawn as solid and dashed lines. of generative kerogen Kgoc. The calculated maturity trends are drawn as solid and dashed lines. Straight solid lines reflect different values of kerogen conversion GOC/TOC0: 1—65%; 2—55%; 3— Straight solid lines reﬂect different values of kerogen conversion GOC/TOC : 1—65%; 2—55%; 45%; 4—35%; 5—25%; 6—15%, 7—5%. Dashed curves reflect maturity trends0 for the samples with 3—45%; 4—35%; 5—25%; 6—15%, 7—5%. Dashed curves reﬂect maturity trends for the samples initial values of TOC0 equal to a—15, b—20, c—25, d—30, e—35, f—40 wt.%. with initial values of TOC0 equal to a—15, b—20, c—25, d—30, e—35, f —40 wt.%. The difference in the location of points, corresponding to different kerogen conver- The difference in the location of points, corresponding to different kerogen conversion, sion, is explained by the solid lines 1–7. These lines have been calculated using the equa- is explained by the solid lines 1–7. These lines have been calculated using the equation S2 tion S2 (mg HC/g rock) = 10 × Kgoc × TOC (wt.%)/CHC, where CHC is the average mass frac- (mg HC/g rock) = 10 × K × TOC (wt.%)/C , where C is the average mass fraction tion of carbon in hydrocarbongoc products, equalHC to 0.85 [12].HC The line 1 (in red) is located above the area of all experimentally measured values and corresponds to the generative kerogen proportion Kgoc of 65%. From this fact, we conclude that the initial Kgoc did not exceed 65% and this line reflects the initial state of the system before the start of hydrocarbon generation. With increasing maturity, the line slope decreases and tends to the horizontal line by the end of the system evolution, which corresponds to the complete exhaustion of the generation potential and the very end of hydrocarbons generation (S2 = 0). The calculated values are in good agreement with the experimentally determined Kgoc. The dashed lines a–f reflect the expected maturity trends for points with different values of TOC0. For example, line c (in red) corresponds to TOC0 = 25 wt.%. Because this line c restricts the highest TOC values in the diagram, we conclude that TOC0 generally did not exceed 25 wt.% in the studied area. For the latest maturity stage, this line comes to the point TOC = 8.75 wt.%, which corresponds to the non-pyrolyzable carbon fraction of 35%. This estimation is in accordance with the earlier made conclusion that the initial GOC0/NGOC0 ratio for the BF samples is 65:35. We have observed only a few exceptions

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of carbon in hydrocarbon products, equal to 0.85 [12]. The line 1 (in red) is located above the area of all experimentally measured values and corresponds to the generative kerogen proportion Kgoc of 65%. From this fact, we conclude that the initial Kgoc did not exceed 65% and this line reﬂects the initial state of the system before the start of hydrocarbon generation. With increasing maturity, the line slope decreases and tends to the horizontal line by the end of the system evolution, which corresponds to the complete exhaustion of the generation potential and the very end of hydrocarbons generation (S2 = 0). The calculated values are in good agreement with the experimentally determined Kgoc. The dashed lines a–f reﬂect the expected maturity trends for points with different values of TOC0. For example, line c (in red) corresponds to TOC0 = 25 wt.%. Because this line c restricts the highest TOC values in the diagram, we conclude that TOC0 generally Geosciences 2021, 11, x FOR PEER REVIEWdid not exceed 25 wt.% in the studied area. For the latest maturity stage, this line comes10 of 18 to the point TOC = 8.75 wt.%, which corresponds to the non-pyrolyzable carbon fraction of 35%. This estimation is in accordance with the earlier made conclusion that the initial GOC0/NGOC0 ratio for the BF samples is 65:35. We have observed only a few exceptions toto thisthis rulerule forfor thethe pointspoints locatedlocated inin thethe lower lower partpart ofof thethe diagram, diagram, theythey reachreach thethe line line corresponding to TOC0 of 30 wt.%. corresponding to TOC0 of 30 wt.%. To sum up, the estimation of the initial TOC0 content and the degree of its transfor- To sum up, the estimation of the initial TOC0 content and the degree of its transforma- tionmation to hydrocarbons to hydrocarbons during during catagenesis catagenesis are availableare available for eachfor each particular particular point, point, according accord- toing the to trends the trends illustrated illustrated in Figure in Figure4. The 4. predictionsThe predictions are reliable are reliable under under the assumptionthe assumption of 0 0 theof the same same kerogen kerogen type type and and the the same same initial initial GOC GOC0/NGOC/NGOC0 ratio. ratio.

5.4.5.4. RelationsRelations betweenbetween ThermalThermal MaturityMaturity and and Oil Oil Saturation Saturation of of the the Rocks Rocks AnalysisAnalysis of of the the relation relation between between the the maturity maturity and and oil oil saturation saturation of of rocks rocks was was carried carried outout using using the the diagram diagram (S0 (S0 + + S1 S1 + +∆ ΔS2)S2)− − S2S2exex (Figure5 5).). SimilarSimilar to to the the previous previous diagrams, diagrams, thethe color color reﬂects reflects the the K Kgocgoc value characterizing bothboth thethe maturitymaturity andand thethe degreedegree of of kerogen kerogen conversionconversion to to hydrocarbons. hydrocarbons.

Figure 5. S0 + S1 + ΔS2 − S2ex diagram for the studied samples. The color indicates the proportion of Figure 5. S0 + S1 + ∆S2 − S2ex diagram for the studied samples. The color indicates the proportion of generative kerogen Kgoc. The calculated maturity trends are drawn as solid and dashed lines. Solid generative kerogen K . The calculated maturity trends are drawn as solid and dashed lines. Solid lines illustrate the differentgoc degrees of realization of the kerogen generation potential (1 − S2ex/S20): lines1—0.1; illustrate 2—0.2; the 3—0.3; different 4—0.4; degrees 5—0.6; of 6—0. realization8. Dashed of the lines kerogen limit generationthe different potential percentage (1 − ofS2 fluidex/S2 loss0): 1—0.1;Kloss: a—0%; 2—0.2; b—52%, 3—0.3; 4—0.4;c—67%, 5—0.6; d—75%, 6—0.8. e—85%, Dashed f—92%. lines limit the different percentage of ﬂuid loss Kloss: a—0%; b—52%, c—67%, d—75%, e—85%, f —92%. The solid lines starting from the origin are calculated for various degrees of realization of the kerogen generation potential, determined by the ratio (S20 − S2ex)/S20, where S20 is the value of S2 for a given TOC at the initial stage of catagenesis. The horizontal axis (S0 + S1 + ΔS2) = 0 is the line reflecting the initial state of the system, and the vertical axis (S2 = 0) corresponds to the final state. The intermediate lines correspond to values of (S20 − S2ex)/S20 ranging from 0 to 1. The dashed lines with a negative slope a–f represent both maturity trend and hydrocarbons loss. In the diagram (Figure 5), these lines reflect the evolution of the rock with TOC0 = 25 wt.% and S20 = 191.2 mg HC/g rock. The value of TOC0 was chosen based on the analysis of the diagram shown in Figure 4, where 25 wt.% is the maximum for the majority of the samples. The value of S20 was calculated by the formula discussed above S20 (mg HC/g rock) = 10 × Kgoc × TOC0 (wt.%)/CHC. Before the beginning of hydrocarbon generation, the point reflecting the state of the system is located on the horizontal axis; and after the complete generation, it is located on

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The solid lines starting from the origin are calculated for various degrees of realization of the kerogen generation potential, determined by the ratio (S20 − S2ex)/S20, where S20 is the value of S2 for a given TOC at the initial stage of catagenesis. The horizontal axis (S0 + S1 + ∆S2) = 0 is the line reflecting the initial state of the system, and the vertical axis (S2 = 0) corresponds to the final state. The intermediate lines correspond to values of (S20 − S2ex)/S20 ranging from 0 to 1. The dashed lines with a negative slope a–f represent both maturity trend and hydrocarbons loss. In the diagram (Figure5), these lines reflect the evolution of the rock with TOC0 = 25 wt.% and S20 = 191.2 mg HC/g rock. The value of TOC0 was chosen based on the analysis of the diagram shown in Figure4, where 25 wt.% is the maximum for the majority of the samples. The value of S20 was calculated by the formula discussed above S20 (mg HC/g rock) = 10 × Kgoc × TOC0 (wt.%)/CHC. Before the beginning of hydrocarbon generation, the point reflecting the state of the system is located on the horizontal axis; and after the complete generation, it is located on the vertical axis. The intermediate position of points in the diagram depends on the transformation degree and the amount of migrated fluids loss. In the case of a closed system, the decrease in S2ex is accompanied by the equal increase of the sum (S0 + S1 + ∆S2), thus leading to the shift of the point with the initial value S20 along the line a, according to the equation (S0 + S1 + ∆S2) = S20 − S2ex. If the system loses mobile hydrocarbons due to migration, the trajectory will show a smaller slope to the horizontal axis following the equation (S0 + S1 + ∆S2) = (100% − Kloss) × (S20 − S2ex), where Kloss (%) is the proportion of the generated fluid that was lost. The slope of the dashed lines a–f starting from the point S2 = 191.2 mg HC/g rock is determined by the maximum values of (S0 + S1 + ∆S2) for the samples at each maturity stage with corresponding Kgoc (the sets are shown in different colors, Figure5). Therefore, the loss factor Kloss can be estimated. For the set of samples with the maximum percentage of generative kerogen (in yellow, 55% < Kgoc < 62%), the obtained straight line corresponds to the total hydrocarbon preservation in the rock. For the other sets of points, the fraction of the fluid loss increases according to (S0 + S1 + ∆S2) = (100% − Kloss) × (S20 − S2ex): Kloss = 52%—for 45 < Kgoc < 55 %, Kloss = 67% for 35 < Kgoc < 45%, Kloss = 75% for 23 < Kgoc < 35%, Kloss = 85% for 15 < Kgoc < 23%, and Kloss = 92% for 0 < Kgoc < 15%. We observe that decreasing Kgoc leads to a decrease in the hydrocarbon preservation in rock. It is easy to show that the maximum hydrocarbon content in the rock corresponds to the group of points with minimal thermal maturity and maximal fluid preservation (45 < Kgoc < 62%). For rocks with a higher degree of kerogen conversion, the fluid is rapidly lost, and the volume of generated hydrocarbons does not exceed the rate of migration. At a low degree of organic matter transformation, we also expect low hydrocarbon volume in the rock due to a lower amount of already generated hydrocarbons; however, no such points are present in the analyzed sample set. It should be emphasized that the present analysis was carried out using the sum of generated hydrocarbons, including gaseous (S0), liquid (S1), and heavy (∆S2) hydrocarbons. From oil production point of view, it is more important to analyze the relationship between the catagenesis stage and the liquid hydrocarbon content (S1), the S1-Tmax plot is shown in Figure6. As it follows from Figure6, the optimal values of Tmax, at which the maximum fluid saturation is maintained (up to 14 mg HC/g rock) for the Bazhenov Formation, are in the range of 438 ÷ 448 ◦C. At Tmax less than 438 ◦C, a sufficient amount of hydrocarbons is not yet formed in the rocks, and at Tmax above 448 ◦C, a significant loss in oil saturation is observed. On the conversion scale, the maximum content of liquid hydrocarbons corresponds to the range 23% < Kgoc < 45%. The difference from the estimation made above for the generated hydrocarbons content appears when we took into account ∆S2, which are heavy hydrocarbons that can be transformed into mobile liquid hydrocarbons at later stages of the system evolution. Geosciences 2021, 11, x FOR PEER REVIEW 11 of 18

the vertical axis. The intermediate position of points in the diagram depends on the transformation degree and the amount of migrated fluids loss. In the case of a closed system, the decrease in S2ex is accompanied by the equal increase of the sum (S0 + S1 + ΔS2), thus leading to the shift of the point with the initial value S20 along the line a, according to the equation (S0 + S1 + ΔS2) = S20 − S2ex. If the system loses mobile hydrocarbons due to migration, the trajectory will show a smaller slope to the horizontal axis following the equation (S0 + S1 + ΔS2) = (100% − Kloss) × (S20 − S2ex), where Kloss (%) is the proportion of the generated fluid that was lost. The slope of the dashed lines a–f starting from the point S2 = 191.2 mg HC/g rock is determined by the maximum values of (S0 + S1 + ΔS2) for the samples at each maturity stage with corresponding Kgoc (the sets are shown in different colors, Figure 5). Therefore, the loss factor Kloss can be estimated. For the set of samples with the maximum percentage of generative kerogen (in yellow, 55% < Kgoc < 62%), the obtained straight line corresponds to the total hydrocarbon preservation in the rock. For the other sets of points, the fraction of the fluid loss increases according to (S0 + S1 + ΔS2) = (100% − Kloss) × (S20 − S2ex): Kloss = 52%—for 45 < Kgoc < 55 %, Kloss = 67% for 35 < Kgoc < 45%, Kloss = 75% for 23 < Kgoc < 35%, Kloss = 85% for 15 < Kgoc < 23%, and Kloss = 92% for 0 < Kgoc < 15%. We observe that decreasing Kgoc leads to a decrease in the hydrocarbon preservation in rock. It is easy to show that the maximum hydrocarbon content in the rock corresponds to the group of points with minimal thermal maturity and maximal fluid preservation (45 < Kgoc < 62%). For rocks with a higher degree of kerogen conversion, the fluid is rapidly lost, and the volume of generated hydrocarbons does not exceed the rate of migration. At a low degree of organic matter transformation, we also expect low hydrocarbon volume in the rock due to a lower amount of already generated hydrocarbons; however, no such points are present in the analyzed sample set. It should be emphasized that the present analysis was carried out using the sum of generated hydrocarbons, including gaseous (S0), liquid (S1), and heavy (ΔS2) hydrocarbons. From oil production point of view, it is more important to analyze the relationship between the catagenesis stage and the liquid hydrocarbon content (S1), the S1-Tmax plot Geosciences 2021, 11, 307 11 of 17 is shown in Figure 6.

Figure 6. Cross-plot of the amount of generated hydrocarbon fluid (S1) and temperature Tmax il- Figure 6. Cross-plot of the amount of generated hydrocarbon fluid (S1) and temperature Tmax lustrating the relations between maturity and fluid saturation of the rocks. The color indicates the illustrating the relations between maturity and fluid saturation of the rocks. The color indicates the proportion of generative kerogen Kgoc. proportion of generative kerogen Kgoc. 5.5. Identification of Potentially Productive Intervals According to Pyrolysis Studies For conventional reservoirs, the identification of oil saturated intervals is performed using S1. The higher S1 is, the more promising in terms of production the interval is considered. For oil shale reservoirs, this criterion is not optimal because, in addition to the presence of mobile fluids, the interval should be also characterized by the increased reservoir properties. Lithological studies and production data for different oil shales including the data on the Bazhenov Formation oilfields show that the reservoir properties strongly depend on TOC content and kerogen maturity [13,24,53,54]. Reservoir properties are better developed for intervals with low TOC content due to increased primary intergranular porosity and increased maturity of organic matter caused by the development of secondary porosity during kerogen transformation [26,55–57]. Because the Bazhenov Formation rocks generally are low-permeable, the migration process predominantly involves nonpolar hydrocarbons, which are accumulated in intervals with better reservoir properties (including radiolarian intervals). Heavy hydrocarbons are less prone to migrate due to their higher viscosity and large molecular size; therefore, such hydrocarbons remain in the void space of the rock and further decrease permeability. Based on analysis of data on the Bazhenov Formation (Figures2–6) we suggested the criteria for identification of potentially oil-productive intervals based on pyrolysis parameters. Intervals with low TOC content (<5 wt.%) are usually characterized by high oil saturation S1 (>7 mg HC/g rock) and low content of heavy oil fractions (∆S2 < 20 mg HC/g rock). On the contrary, at high TOC values (>5 wt.%) especially in the case of low kerogen maturity (Kgoc > 35%), the rocks of unconventional reservoirs usually have ultra-low permeability because the pores existing in the mineral matrix are “sealed” by a high content of heavy oil fractions (∆S2 > 20 mg HC/g rock). These intervals are not interesting from the point of view of mobile hydrocarbons production even in the case of high oil saturation S1 (>7 mg HC/g rock). However, they can be considered as promising for oil production using thermal enhanced oil recovery methods (EOR), which induces in situ conversion of kerogen (S2ex) and heavy oil fractions (∆S2) into mobile hydrocarbons (S1). Thus, to assess the interval prospects, it is necessary to take into account both S1, ∆S2, TOC values, and kerogen maturity (Kgoc, HI, PI). The suggested pyrolysis criteria are listed in Table2. Geosciences 2021, 11, 307 12 of 17

Table 2. Pyrolysis characteristics of productive intervals in the Bazhenov formation.

Productive Intervals Possible Method of TOC S1 S2 ∆S2 Kgoc Features Development wt.% mg HC/g Rock % Increased oil saturation, Hydraulic Low High Low Low Low Increased reservoir fracturing <5% >7 <35 <20 <35 properties

GeosciencesHigh generation 2021, 11, x FOR PEER REVIEW 13 of 18 potential of kerogen, Thermal High High High High High Increased heavy oil EOR >5% >7 >35 >20 >35 fractions content An example of the application of the criteria to the analysis of the studied Bazhenov FormationAn example samples of the set application is given in of Figure the criteria 7, where to the the analysis color of of the the points studied indicates Bazhenov the Formationproportion samples of generative set is given kerogen in Figure Kgoc, 7and, where the size the colorof the of points the points reflects indicates the heavy the hydro- pro- carbon content (ΔS2). portion of generative kerogen Kgoc, and the size of the points reﬂects the heavy hydrocarbon content (∆S2).

Figure 7. The S0 + S1 − S2 diagram. The color indicates the proportion of generative kerogen Kgoc, the size of the points Figure 7. The S0 + S1 − S2 diagram. The color indicates the proportion of generative kerogen Kgoc, the size of the points reflects the content of ΔS2, and the areas indicate the development prospects. Lines divide plot into four zones: I—zone I, reflects the content of ∆S2, and the areas indicate the development prospects. Lines divide plot into four zones: I—zone I, II—zone II, III—zone III, IV—zone IV. II—zone II, III—zone III, IV—zone IV. We attribute the intervals falling into zone I to the group of accumulating intervals, We attribute the intervals falling into zone I to the group of accumulating intervals, characterizedcharacterized by by increased increased oil oil saturation saturation and and better better reservoir reservoir properties. properties. In terms In terms of produc- of pro- tion,duction, these these intervals intervals are similar are similar to tight to rocktight reservoirs, rock reservoirs, for which for which hydraulic hydraulic fracturing fracturing and multistageand multistage hydraulic hydraulic fracturing fracturing technologies technologies can be efficientlycan be efficiently applied. applied. The prospect The prospect of the of the intervals increases with decreasing Kgoc (shown in color) and ΔS2 (shown by point intervals increases with decreasing Kgoc (shown in color) and ∆S2 (shown by point size). size).The points in zone II are classified as conditionally productive. They differ from the intervalsThe in points zone Iin by zone lower II are oil saturationclassified as (S1) conditionally which may productive. be associated They with differ the partialfrom the lossintervals of fluid in duringzone I by core lower recovery oil saturation and storage. (S1) which The promising may be associated intervals inwith zone the IIpartial are loss of fluid during core recovery and storage. The promising intervals in zone II are char- characterized by higher values of S1, Kgoc and lower S2. As it follows from Figure7, such pointsacterized are located by higher in thevalues upper-left of S1, K partgoc and of zonelower II. S2. We As suppose it follows that from up Figure to 50% 7, of such samples points fallingare located into zone in the II upper-left represent part the intervalsof zone II. with We suppose reservoir that properties up to 50% and of oilsamples saturation falling similarinto zone to those II represent of intervals the intervals in zone I, with and lowerreserv S1.oir properties and oil saturation similar to those of intervals in zone I, and lower S1. The points in zone III are classified as oil saturated source rocks and promising for the use of thermal EOR, promoting the conversion of kerogen and high-viscosity hydrocarbons into mobile hydrocarbons. The closer to the right and higher the point is, the more promising the interval is considered when using thermal EOR. The prospects of the interval increase with an increase in Kgoc (shown by color) and an increase in ΔS2 (shown by point size). The points in zone IV belong to the category of nonproductive rocks, although these may include the intervals with a very low organic matter transformation degree, which may be promising for the use of thermal EOR in case of high TOC content.

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The points in zone III are classiﬁed as oil saturated source rocks and promising for the use of thermal EOR, promoting the conversion of kerogen and high-viscosity hydrocarbons into mobile hydrocarbons. The closer to the right and higher the point is, the more promising the interval is considered when using thermal EOR. The prospects of the interval increase with an increase in Kgoc (shown by color) and an increase in ∆S2 (shown by point size). The points in zone IV belong to the category of nonproductive rocks, although these may include the intervals with a very low organic matter transformation degree, which may be promising for the use of thermal EOR in case of high TOC content. To evaluate the distribution of intervals with different properties, we calculated the number of points falling into zones I–IV. The proportions of intervals for the whole Bazhenov and for the areas with different maturity are given in Table3.

Table 3. Distribution of the studied samples over the identiﬁed production approaches zones. The estimations are given for each maturity stage.

Interval Occurrence Estimated from Number of Samples Oil Generation Stage Kgoc,% Number of Samples Zone I Zone II Zone III Zone IV Beginning of 45 ÷ 62 422 2.5% 18% 70.5% 9% generation Early oil 35 ÷ 45 464 5% 19.5% 61.5% 14% window Middle oil 15 ÷ 35 874 14.5% 52% 12.5% 21% window Late oil 0 ÷ 15 149 36% 64% 0% 0% window Total for BF 1909 11% 38% 36% 15%

As it follows from Table3, the percentage of oil saturated intervals with increased reservoir properties, which are promising for development by hydraulic fracturing technology, is 11% (zone I shown in Figure7). Thirty-eight percent of intervals falling into zone II have the same reservoir properties as those in zone I, but lower oil saturation (S1). Taking into account possible losses of ﬂuid during core recovery and storage, up to the half (19%) of points in zone II could be also considered as potentially productive. We classify about 36% of the intervals as oil saturated rocks with very low permeability and high generation potential (zone III). These intervals could be considered as potentially productive in case of application of more complex recovery methods, including gas, chemical, and thermal EOR combined with hydraulic fracturing. The remaining 15% belong to nonproductive intervals (zone IV). The distribution of the reported interval types varies by area in a strong connection with organic matter maturity. At the immature stage and the beginning of the oil window, the part of intervals in zone I is only several percent and the proportion of intervals in zone III reaches 70%. At the middle of the oil window, the part of intervals in zone I increases up to 14.5%, while the percent of intervals in zone III decreases to 12.5%. At the end of the oil window, the proportion of intervals in zone I is 36%, while the part of rocks in zone III is almost 0%. However, at end of the oil window, the retention of hydrocarbons in the reservoir decreases considerably due to migration of ﬂuid when compared to the beginning of the oil window.

6. Conclusions We have studied a representative sample collection (more than 3000 samples) from the productive area of the Bazhenov Formation located in the central part of the West Siberian Basin. Uniﬁed pyrolysis analyses of rock samples were performed before and after Geosciences 2021, 11, 307 14 of 17

extraction with chloroform using the same methodology and equipment. The developed procedure has signiﬁcantly improved the determination of standard pyrolysis characteristics and additional parameters, including the calculation of heavy hydrocarbons fraction content in the rocks. The study allowed us to identify and analyze maturity trends for the Bazhenov Formation. In agreement with the previously obtained data for selected deposits, the analysis of the entire set of data provided quantitative characterization of the variability of rocks and organic matter characteristics, organic matter generation potential, the evolution of the source rock at different stages of catagenesis, and also allowed us to establish the proportion of the intervals of different productivity. To summarize, in the current study we have obtained the following data on the Bazhenov Formation: • More than 98.5% of organic matter in the deposits belongs to type II kerogen with different maturity. Up to 1.5% of organic matter belongs to type I kerogen, with a higher initial generation potential; • More than 75% of the intervals of the Bazhenov Formation are characterized by “excellent” generation potential, 15% of the intervals have “very good” and “good”, the remaining 10% of deposits are at the end of oil generation and their generation potential is exhausted; • About 46% of the studied samples of the Bazhenov Formation correspond to the initial stage of oil generation, 46% belong to the middle of the oil window and about 8% belong to the end of the oil window; • We have established the quantitative relations between organic matter content and maturity. The maximum and average values of TOC at the beginning of oil generation are approximately two times higher than the ones at the end of the oil window. According to the obtained data, the ratio of generative and nongenerative kerogen for immature kerogen was estimated as 65% and 35%, respectively; • The content of generated hydrocarbons in rocks (light oil, heavy oil fractions and hydrocarbon gas) depends on maturity. The amount of hydrocarbons in reservoir increases due to hydrocarbon generation and decreases as the result of migration from the source rocks. The beginning of the kerogen transformation to hydrocarbons is characterized by almost complete preservation of newly formed hydrocarbons in pore space of rock (more than 90%), while at the peak of generation (the middle of the oil window) the proportion of the remaining hydrocarbons in the reservoir decreases to 30%, and at the end of the oil window this proportion drops down to 10% and below; • The maximum content of light and liquid hydrocarbons in the reservoir is observed for the rocks corresponding to the middle of the oil window (438 ◦C < Tmax < 448 ◦C), at the beginning and at the end of the oil window the content of these HC fractions is two or more times lower; • Using the obtained geochemical trends, we have suggested the criteria (Table2) for the classiﬁcation of rocks in terms of hydrocarbon production prospects and estimated the percentage of each type of intervals (Table3 and further discussion) in dependence on the maturity level. The study extends existing knowledge on hydrocarbon generation, migration and accumulation in the Bazhenov source rock formation. The analysis of geochemical trends allowed us to improve approaches for the productivity evaluation of the formation and study the effect of organic matter maturity on distribution of productive intervals of different types. The developed approaches for the interpretation of pyrolysis data can be applied for the characterization of other unconventional organic-rich formations; however, the geological, petrophysical and geochemical features of deposits should be taken into account. Geosciences 2021, 11, 307 15 of 17

Author Contributions: Conceptualization, M.S., E.K., N.M.; methodology, M.S., E.K., P.M., E.L., T.B.; software, P.M.; validation, M.S.; formal analysis, E.K., P.M., M.S., E.L.; investigation, E.K., T.B., P.M.; resources, N.M.; data curation, P.M.; writing—original draft preparation, M.S., E.K., P.M.; writing— review and editing, M.S., E.L., P.M.; visualization, P.M.; supervision, M.S.; project administration, E.K., M.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Ministry of Science and Higher Education of the Russian Federation under agreement No. 075-10-2020-119 within the framework of the development program for a world-class Research Center. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is not available. Acknowledgments: The authors thank the Center for Hydrocarbon Recovery Laboratory members for technical support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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