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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 py- rolysis [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
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