Impact of the Pai-Khoi-Altai Regional Strike-Slip Zone on Localization of Hydrocarbon Fields in the Pre-Jurassic Basement of the Central Part of West Siberian Oil and Gas Province

1. Introduction

The West Siberian petroleum and gas basin provides approximately 70% of Russia's oil production and over 90% of its gas production [1]. The region is characterized by intensive exploitation of a productive thickness with a depth of 2-4 kilometers, consisting of Jurassic and Cretaceous deposits covered by younger sediments. This well-studied interval is gradually depleting, with experts noting a declining trend in oil and gas reserves growth, while there is no anticipated decrease in demand for petroleum products [2,3].

Relatively less explored but potentially petroleum and gas-bearing are the pre-Jurassic strata, including the so-called “intermediate structural layer” and the weathered crust of the consolidated basement. According to various estimates, 3 to 15% of the world's proven oil reserves are localized in the basement rocks. However, the volume of exploratory drilling to reach this level is orders of magnitude less than in the sedimentary cover, and the discovery of deposits of this type often happens by chance [4].

In the pre-Jurassic complex of Western Siberia, deposits have been identified at dozens of fields with oil reserves of about 0.2 billion tons (for comparison, current recoverable reserves from all horizons of Western Siberia are nearly 18 billion tons) [5,6]. The prospects for new discoveries in these deposits are significant, and the pre-Jurassic complex deposits are considered to be the principal source for locating large reserves in the Western Siberian region [7].

The West Siberian petroleum and gas basin is of interest to leading Russian geologists such as V.S. Surkov [8], A.E. Kontorovich [9], G.A. Lobova [10], A.I. Timurziev [4,11], S.F. Khafizov [12], and others. While the prospects for discovering oil deposits in the basement rocks of the West Siberian petroleum and gas province are highly rated, the factors controlling the formation of deposits in pre-Jurassic-age rocks are relatively poorly understood to date [13]. This circumstance is likely the reason for the lack of new discoveries at deep horizons in the sedimentary cover, in the “intermediate structural layer,” and in the upper part of the basement.

According to the results of scientific research, the localization of hydrocarbon deposits in pre-Jurassic complex rocks can be influenced by:

• the presence, thickness, and composition of “intermediate structural layer” rocks.
• the development of regional strike-slip dislocations, which create extensive zones of deformation and increased fracturing in the overlying sedimentary cover, controlled by “flower structures” [14].
• triassic riftogenesis with the development of volcanic-sedimentary deposits.
• the presence of uplifted basement surface features.
• the development of weathered crusts in the basement.

Our study examines the deep structural characteristics and the impact on the oil and gas potential of the Pai-Khoi-Altai strike-slip zone region.

The significance of strike-slip dislocations in predicting oil and gas deposits has been noted by researchers such as G.G. Gogonenkov [15], N.V. Nassonova [16], A.I. Timurziev [11], O.A. Smirnov [17], V.P. Igoshkin [18] and others. The manifestation of strike-slip tectonics in the West Siberian sedimentary basin has been identified at both regional levels (based on potential field data and 2D seismic surveys) and large-scale levels (based on 3D seismic surveys). It has been established that strike-slip dislocations appear in the form of curtain faults (in plan view) and “flower structures” (in cross-section). Publications mention the influence of the “piston” effect [16], which pressurizes fluids and operates during compression and extension, as well as the formation of “pull-apart” basins with developed sedimentary accumulation and the formation of curtain folds and faults, complicating the structure of deposits.

In the current work, using one specific area as an example, we examine the relationship between the formation of deep oil and gas deposits and the positions of principal strike-slip dislocations and feathering faults. We started with the assumption that strike-slip zones largely control the formation of fluid-permeable zones within the areas of principal and feathering dislocations. The study area chosen for research is located in the central part of the West Siberian petroleum and gas province and includes map sheets P-43 – P-44, which are intersected by the regional Pai-Khoi-Altai strike-slip zone from southeast to northwest (see Figure 1).

Researches has revealed that the Hercynian folded basement has a heterogeneous composition, consisting of metamorphic and variably metamorphosed sedimentary rocks (clay shales, ortho- and metashales, carbonate rocks) that have been disrupted by intrusions of both acidic and mafic composition. During the Early Mesozoic, uplift movements predominated over a large part of the area, resulting in significant erosion of the pre-Mesozoic strata and the formation of extensive weathering crusts. In the Early Triassic, rift zones began to form and develop, which were filled with sedimentary-volcanogenic deposits. The accumulation of rocks was accompanied by folding and the development of faulted (including strike-slip) dislocations, driven by stresses along the boundaries of adjacent folded areas. Simultaneously with the deepening of graben-rift basins, there were multiple shifts between basin and continental facies [24]. A marine regime was established in most of the region during the Late Jurassic, coinciding with the deposition of sandy-silt-clay sediments. At the end of the Jurassic period, carbonate-rich and bituminous clay silts were deposited in the marine basin [24,25]. During the same period, active movements occurred along the regional Pai-Khoi-Altai strike-slip zone.

Thus, the information about the geological history supports the presence of a substantial volume of sediment and organic materials buried in pre-Jurassic rocks, indicating the possible role of the Pai-Khoi-Altai strike-slip zone in the formation of hydrocarbon-bearing structures during the Jurassic and Late Paleozoic stages of the West Siberian region's crustal evolution.

2. Hydrocarbon potential of the Pre-Jurassic basement in the study area

Within the considered area, several hydrocarbon-bearing complexes (HBC) are distinguished: the pre-Jurassic (Paleozoic), Jurassic (Tyumen, Vasyugan and Bazhenov), boundary time interval (upper Jurassic - lower Cretaceous), Lower Cretaceous (Figure 2). Most of them are terrigenous (sandstone), but there are accumulations of hydrocarbons in the tops of the Triassic volcanic rocks, their porosity reaches 15 %. Major source rocks are: unnamed Triassic organic-rich sediments; Togur-Tyumen formation (J1-2) (coals in shales and sandstones), Bazhenov (J3-K1) organic-rich shale, Pokur formation (K1-2). In the Caledonian part of the study area, there is a hydrocarbon-bearing “intermediate structural layer” (ISL) located between the consolidated basement and the platform cover. This layer is primarily composed of Late Paleozoic sediments and weathering crusts, partially re-deposited, with zones of organic matter (OM) generation [17,27]. This complex is poorly studied over most of the area due to its deep burial, and it has only been encountered in a limited number of wells.

In Paleozoic rocks, the following oil deposits have been explored and studied (Figure 3): Severo-Varieganskoye field, where oil accumulations are found in fractured volcanites and shales, as well as in their weathering crusts. Their structure is poorly studied [24].

Sovetskoye field - reservoir M1 (Paleozoic) consists of loose weathered volcanites of the basic composition and fractured organogenic limestones. The pre-Jurassic HBC here is encountered in five wells at depths ranging from 2702 to 2776 meters. Commercial oil flow is obtained from weathered fractured limestones [24].

Kotygyeganskoye field (Vasyuganskaya oil and gas region), where the oil reservoir is located in fractured-cavernous Paleozoic collectors represented by dolomite breccia [25].

Several fields have oil reservoirs in pre-Jurassic rocks, including the unique Kharampur field, large fields like Bakhilovskoye, Varyngskoye, Verkhnekolik-Eganskoye, as well as the medium-sized and small Festivalnoye and Severnoye fields [25].

The thickness of the intermediate layer, based on the limited number of wells studied, is estimated to be highly variable, averaging several hundred meters [28,29]. Due to its strong metamorphism, ISL was not considered a promising target for oil exploration until the 1970s. However, subsequent assessments have shown that the ISL, in terms of sedimentary fill volumes, is “comparable to the volume of the Mesozoic-Cenozoic cover of the plate” and could be a significant source of organic matter [30]. High prospects for oil and gas in the actual intermediate structural layer based on the study of oils, bitumens, and organic matter dispersed in rocks and underground waters [31]. Moreover, organic matter could have migrated into overlying horizons through subvertical fluid-permeable zones formed by strike-slips [32].

5. Predictive assessment of the study area based on a set of geological, structural, and geophysical factors

The study area is characterized by a high density location of oil and gas deposits and, consequently, the area is highly studied by geological and geophysical methods [46]. Third-generation of state geological maps (sheets P-43, P-44) are available online [47]. Three profiles of deep seismic sounding (Bitum, Kimberlite, Quartz) are laid here, as well as a series of regional 2D seismic (CDP) profiles. Within individual deposits and prospective zones, numerous 3D seismic survey areas are localized. Digital gravity and magnetic survey data are available on a scale of 1:1,000,000 [48,49].

Rift structures served as zones of organic matter accumulation and provided heating of these layers to achieve a high degree of catagenesis for oil maturity [50], ensuring their migration capability. Several deposits are localized on the periphery of rifts [51], which makes it reasonable to use gravity and magnetic data as indirect predictive factors. Intrusions of various compositions are distinctly manifested in potential fields as isometric anomalies of different signs and amplitudes.

To study the heterogeneities of the sedimentary cover and conduct predictive assessments for oil and gas, several techniques are applied, such as the method of gravimetric detection of oil and gas (GDONG) [52], the method for identifying “deposit-type” anomalies (DTA) [53] and others. However, direct hydrocarbon reservoir prediction methods based on potential fields generally do not demonstrate high effectiveness.

The comprehensive interpretation method applied in this work, developed by A.S. Dolgal [54], consists of the following procedures: filtering (transformation), zoning (classification), combining the results of methodical interpretation, identification of prospective zones (forecast), and modeling.

Formalized forecasting within the study area is performed using a pattern recognition algorithm that utilizes geological-structural and geophysical parameters available in digital format as factors for training.

The optimal set of geophysical transformations for predictive assessment in the study area includes calculating the horizontal gradient magnitude - to enhance the contrast of block boundaries and fault dislocations; calculating the vertical derivative of the field - for contrasting the manifestations of local anomaly-forming objects, which can assist in mapping blocks and sutural zones, fault dislocations, basement protrusions, and granitic massifs; mapping “blocks” identified on gravity and magnetic fields [55], considering uniform amplitude-frequency characteristics of the fields; and analyzing the relief morphology of the basement.

In addition to tectonic parameters, we introduced the “horizontal gradient module of the isochron reflecting the “B” horizon” as a predictive factor. This corresponds to the angle of inclination of the Bazhenov suite's surface. There is a visual correlation between the hydrocarbon reservoirs of the Jurassic basement and the areas with significant tilting of this surface [56,57,58]. This can be explained by the impact of the strike-slip fault on the geometry of the Bazhenov suite's roof, leading to the formation of specific traps.

As “training objects,” we selected deposits with reservoirs located at various depths, including those in rocks of Jurassic age (deposits 3-9 in Figure 3). The largest of the selected “training objects” is the Kharampur deposit (see Figure 3, deposit 4). The basement here is exposed to a depth of about 4 km, where it is represented by basalts; sediments of the Lower Jurassic age lie unconformably above. Within the deposit, 28 reservoirs of the sheet, massive, and lithological barrier types have been identified, primarily in sandstones. Due to tectonic disruptions, productive layers are fragmented into separate reservoirs with sharp heterogeneity, variability, and fracturing of the reservoirs [58]. On the Upper Kolik-Egan oil and gas condensate deposit (Figure 3, deposit 8), the Paleozoic basement is penetrated by wells to a depth of 3.2 km. It consists of shales, limestones (Lower Carboniferous age), and sedimentary terrigenous formations. In the productive part of the sedimentary sequence, the deposit has 62 layers [35]. The oil reservoir, located on an erosional-tectonic protrusion of the Paleozoic basement, is exposed on the Festival Square (Figure 3, deposit 5). The host rocks are represented by sedimentary and volcanic formations [59]. The heterogeneity of the reference sample, due to the different geological-structural positions of the reference hydrocarbon deposits, necessitates the use of a combination of forecasting methods and a large number of criteria.

The space of linear, contour, and digital predictive features, formed on the basis of a complex of geological and geophysical data, is presented in Table 1. Within the pattern recognition technology, we performed: the formation and mutual alignment of data matrix nodes; the selection of the optimal interpolation procedure with averaging of the final areal distributions; a detailed analysis of the morphology of primary features; the formalization of initially non-digital features; and, specifically, object recognition with training. To implement the latter, a discriminant analysis in an author's modification was applied, aimed at considering a small neighborhood of an individual point in the reference sample as an independent reference object in a multidimensional feature space. The result of the analysis is predictive aureoles distributed over the area of the study area and verified using the K-means method [60].

Joint processing, implemented in the multidimensional space of the mentioned geological-structural and geophysical features, aims to create predictive aureoles of potential object detection, similar to hydrocarbon reference objects. The result of using the modified discriminant analysis in conjunction with the K-means method is generalized by us in the form of a scalar field of the probability distribution of object detection, similar to hydrocarbon reference objects (Figure 7). It can be seen that in the probability value range exceeding 0.5 (highlighted with dark blue shading), the areas extend beyond the boundaries of reference objects, forming the most representative anomalies in the areas of junction of horst-graben boundaries and principal fault zones.

As evident, the location of reference deposits aligns with the nodes of intersection of the major fault with adjacent extensional structures. When analyzing the interpretation results, we pay attention to the specificity of the distribution of contours of hydrocarbon reference objects, forming groups with a regular spatial step, manifested both along the mentioned fault zone and in its vicinity. Quasi-periodic systems are a manifestation of wave structuring of the geological environment and find application in forecasting deposits of both ore [61,62] and oil and gas. Furthermore, the belt of increased probability of detecting deposits similar to reference ones marks known deposits with local areas of increased morphological variability. Predictive aureoles are elongated in several different directions and have a different distribution pattern in the blocks separated by principal dislocations, confirming the significance of the strike-slip fault as a controlling factor for the hydrocarbon potential of the region's structures. Among the extensional structures, horsts tend to gravitate toward reference objects to the greatest extent.

6. Conclusions

The Pai-Khoi-Altai strike-slip zone covers a vast area and manifests itself as a system of two principal strike-slip dislocations and numerous adjacent horsts and grabens. Fundamental importance has the nature of feathering dislocations; our detailed constructions indicate right-lateral kinematics of the principal strike-slip faults (Figure 6). In addition, this zone extends to the Altai-Sayan folded area, the dislocations of which, including the Kuznetsk Alatau, have right-lateral kinematics. According to seismic survey data, strike-slip dislocations are manifested at depths below the Bazhenov horizon; above it, predominantly fold structures are observed, which can also form traps for hydrocarbons, increasing the significance of the studied structure.

The obtained predictive estimates indicate the high prospects of areas adjacent to the eastern major strike-slip fault zone and adjacent horst structures. This is explained by the probable presence of tectonically weakened fluid-permeable zones; hydrocarbon traps in plicatic dislocations; sources of hydrocarbons in the lower horizons of the sedimentary cover; and favorable conditions for the maturation of organic matter and hydrocarbon generation.

In this work, a methodological approach has been tested, allowing the application of formalized tectonic and geodynamic criteria in automated forecasting of prospective deep-seated hydrocarbon deposits in the study area, including the application of neural network technologies [63]. Based on the results of geological interpretation of geophysical materials, a geological-structural basis has been developed, the formalized elements of which have been applied as factors in object recognition with training on reference deposits. Seismic section data has been applied to verify the identified tectonic elements and to confirm the configuration of the Pai-Khoi-Altai regional shift zone.

Predictive modeling based on the analysis of deep structure and tectonic dislocations of various kinematics is a justified tool in regions with promising lower structural strata for oil and gas. From the analysis conducted, it is evident that reference deposits are located in those areas of the regional strike-slip zone adjacent to which extensional horst structures are present. Studying the geological processes of accumulation, both in the traditional organic paradigm and from the perspective of an inorganic “mantle” source of oil and gas, along with an analysis of the geophysical manifestations of hydrocarbon criteria, may be the key to discovering new deep-seated hydrocarbon deposits in Western Siberia.