Geological Background of the Further Exploration of Oil in the Nizhnekamsk Deflection

On the basis of new geological and geophysical data acquired in recent years, the Institute of Ecology and Natural Resources of the Academy of Sciences of the Republic of Tatarstan conducted studies to clarify the geological structure of Nizhnekamsk deflection of Kama-Kinel system in order to evaluate the oil potential. The article discusses the main features of geological and tectonic evolution and genesis of Nizhnekamsk deflection, which are crucial in clarifying the morphogenetic types of local uplifts and establishing their spatial distribution in the deflection. The question of the share of tectonic and sedimentary processes is considered in the formation of reservoirs and traps in the axial and marginal parts of Nizhnekamsk deflection; the prospects are justified of oil-bearing sediments involved in its structure.

Prospects for further search of hydrocarbons in Nizhnekamsk trough of Kama-Kinel System on a background of more effective training areas of oil reserves in the Republic of Tatarstan is far from being exhausted, and allow us to consider this territory as relevant to the preparation of industrial oil, indicating the need for the development of its undiscovered resources.
Within Nizhnekamsk deflection numerous commercial accumulations of oil are established.However, the main deposits are confined to the Devonian clastic rocks, sediments of which are not involved in the construction of Nizhnekamsk deflection.Established Upper Devonian and the Lower Carboniferous carbonate deposits are confined mainly to the board zones and near-board zones of the deflection.
Within the same axial deflection zone from 12 fields discovered only in seven oil deposits are established in carbonate Upper Devonian and carbonate-clastic Upper Carboniferous sediments.Of these, deposits of three fields (Elabuzhsky, Lugovoy, Omarsky) are located within the transition zone from the board to the center -near-board part of the deflection, causing that deposits are confined to the reef structure, common in the board areas of the deflection.Thus, the axis of the imposed intraformational Nizhnekamsk deposit against its sides is characterized by a small number of identified oil deposits.
Nizhnekamsk deflection, which is part of the Kama-Kinel system, is located in the northern part of the Republic of Tatarstan and divides South-Tatar and North-Tatar arches.Nizhnekamsk deflection is independent structure of the first order.Deflection length is about 140 km.The width of the deflection varies from 7-22 km in the west and up to 30-40 km in the east.Nizhnekamsk deflection connects the western part of it, with the same Mozhginsky, Ust-Cheremshansky and, in the eastern part, with Aktanysh-Chishminsky and Sarapulsky deflections (Fig. 1).
History of geological and tectonic evolution and origin of the Nizhnekamsk deflection, as well as the entire Kama-Kinel system is inextricably linked with the early period of formation of the Paleozoic sedimentary rocks of this area and covers a relatively short period of time.In accordance with the selected lithologic and stratigraphic rock complexes that period of time is divided into four stages: Middle Frasnian-Famennian, Tournaisian, Kosvinskian and Radaevskian-Bobrikovian.
Within Nizhnekamsk deflection in pre-Sargaevskian deflection North-Tatar arch held the highest structural position, limited to the south from the South-Tatar arch by largest Prikamsky sublatitudinal fault.Decrease in organic residues for all groups of fauna indicates about the high structural position of the North-Tatar arch.There are no algae, foraminifera, different types of ostracods are increasing (Khisamov et al., 2010).
In the Middle Frasnian time the entire territory in the area of Nizhnekamsk deflection experienced subsidence, which led to a deepening of the marine basin.This tectonic subsidence was performed unevenly, and the result was, as in Lower Frasnian time, the highest structural position of the North-Tatar arch.The relatively high structural position in the bottom of the Middle Frasnian basin of the North-Tatar arch is due to the development in Semilukskian time of organogenicdetrital limestones with remains of ostracods, amfipor and tentaculites, etc. Rechytskian deposits are composed of light-gray and gray massive reef limestone, filled with remnants of algae, stromatopore, amphipore colonies (Khisamov et al., 2010).The area corresponding to the South Tatar arch, occupies an intermediate structural position, being lowered with respect to the North-Tatar arch and raised with respect to restricting it to the west, north and east of depression, within which relatively deep bituminous silica-clay-carbonate rocks were commonly deposited.
Thus, by the end of Rechitskian time structurally this area already had South and North-Tatar paleo-swells and Prikamsky paleo-depression delimiting them.Slopes of paleo-swells, representing a system of flat terraces and steps, located above the fault zones in the crystalline basement rocks, were matched with the board of Prikamsky paleo-depression, within which from Upper Frasnian time Nizhnekamsk deflection of Kama-Kinel system began to form.
In Early Frasnian and Famennian time the amplitude of general tectonic subsidences in the Volga-Ural region has been much greater than in Middle Frasnian age.In terms of tectonic movements regime inherited from the Middle Frasnian time, subsidence was done with different intensities.
The differentiated nature of the tectonic subsidence was captured in the lithological composition and thickness of the Upper Frasnian and Famennian sediments accumulated in dramatically different and separated structural-facies zones.In contrast to the vertices, slopes of paleo-swells were sagged more intensely and in particular their edge zones, which have accumulated 300-500 m thick carbonate strata, mainly of reef rocks.Sustained mode of tectonic subsidence in conjunction with the shallow conditions of sedimentation and wide development in the basin of reef-building organisms (crinoids, foraminifera, algae, etc.) have created optimal conditions for the emergence in the edge zones of structural paleo-swells over structural stages of high-amplitude reef structures.
It should be noted that within the context of common subsidence, area of the northern slope of the South Tatar paleo-depression most intensively expanded, and subsidence rate of the south-eastern slope of the North-Tatar was more intense.This probably explains the displacement of the deflection bed to the north relatively to Prikamsky fault (Fig. 2).
In paleo-depressions in Upper Frasnian-Famennian time rate of tectonic subsidence rate was narrowly ahead of the subsidence rate of edge zones in paleoswell slopes, and there a deep-water environment of sedimentation remained.At this tectonic-sedimentary environment, bituminous, silica-clay-carbonate sediments of Domanic type continued to accumulate, drastically different in lithofacies, thickness of which did not exceed 140-230 m.
If it is assumed that the amplitude of general tectonic subsidence in paleo-depressions of the Upper Devonian Fig. 2. Thickness map of deposits of Semshukskian-Zavolzhskian complex. 1 -deep drilling wells, 2 -isopachs, 3 -licensing borders of oil fields, 4 -boundary of Tatarstan basin was the same as that in the edge zones of paleoswells (inward deflection of the board), the comparison of its size (180-290 m) with a thickness of 140-230 meters of accumulated sediments here clearly indicates on slippage of sediment accumulation rate from the rate of tectonic subsidence.This non-compensation of tectonic subsidence by sedimentation led to further deepen of paleo-depression originated in Middle-Frasnian time.
With the deepening of paleo-depression and as a result of a significant increase in the amplitude, deflection boards were even more sharply delineated.
In the structurally highest parts of boards, developed in depression silica-clay-carbonate rock of Domanic were replaced by coeval carbonate, mainly reef rocks of the edge zones of paleo-swell slopes.This substitution occurs in a narrow band with width of 1.5-3 km and is accompanied by a sharp increase in Upper Frasnian -Famennian deposits.As a consequence, ledges with a height of 150-250 m occurred in the indicated band.Within ledges layers occurring in the roof of Famennian stage are inclined to the central portions of the depressions and have steep angles of incidence (up to 10-15°).
A feature of Tournasian stage of geological history is that its onset began with filling sediments from previously arisen deflection (Fig. 3, 4).Thickness of Tournaisian deepwater sediments of the defleiction is 35-155 m.Given the same lowering amplitude in paleodepressions and areas of paleo-swells in Tournasian basin, which is 80-230 m, it is possible to conclude about a significant lag in the rate of sediments accumulation in paleo-depression from the speed of tectonic subsidence.
In the axial part of the deflection in the first half of the Tournasian stage deep-sea sediments were throughout deposited.In Malevskian-Upinskian time in the central axial part of the basin the most deep-sea sediments were accumulated.Their composition is dominated by bituminous siliceous-argillaceous limestone, shale, marl, often interlayered with silicified argillites.The thickness of this complex, similar to their underlying Devonian deposits of Domanic facies, is 15-55 m.
In the northern near-board zone of deflection the thickness of siliceous-carbonate, carbonate-clay and clay-carbonate Malevskian-Upinskian deposits, also deposited in the deep-water (but less than in the central part of the basin) conditions, is 15-55 m; in the southern near-board area the thickness is 20-130 m.The difference between the amplitude (5-55 m) of tectonic subsidence and said thickness in the southern near-board deflection zone indicates that in Malevskian-Upinskian time sedimentation rate is slightly higher than the rate of subsidence.Naturally, such a direction of sedimentation contributed to the gradual equalization of Nizhnekamsk deflection profile.
Geomorphological alignment of deflection primarily covered band directly adjacent to the wings of Upper Devonian -Zavolzhskian reefs, i.e. near-board deflection zones, and did not affect its central parts.
In Cherepetskian-Kizelovskian time geomorphological alignment of Nizhnekamsk deflection began to develop even more.During this time, within the paleo-swells and central deflection part structure-depositional facies conditions have not changed substantially.In the central In the initial period of this sedimentary sub-step of Tournasian time sediments were deposited that carry features of relatively deep, but as near-board zones of trough were compensated and relief of basin bottom was smoothed, and, consequently, the area was reduced that was occupied by the most deep-water part of the sea; shallow marine, mainly organogenic limestones were Among organogenous, often clot-biomorphic limestone, the following are present -coral, brachiopod, crinoid-algal, algal-foraminiferal differences found in certain facial proportions with clay limestone.In general, in near-board areas of deflection due to a small amplitude of common tectonic subsidence, quite large reef constructions occurred in size, but low-amplitude (15-25 m) -biostromes and bioherms.The most optimal conditions for the development of Cherepetskian-Kizelovskian bioherm were in the band, immediately adjacent to the boundary of shallow biogenic limestones and relatively deep siliceous-clay-carbonate rocks and shales.
In addition, in near-board and axial areas on the already formed in Late Frasnian-Famennian time geomorphologically pronounced high-amplitude elevations, which gave the beginning of the formation of reef structures, there were small in area high-amplitude reefs.Ozerny, North-Ozevrny, South-Ozerny, Smolny and others can be attributed to such reefs.
As a result of deflection filling, further migration of reef facies is observed of deep troughs in their near-board zone where small-amplitude bioherms and sedimentogene uplifts -reefs occurred.
Thus, the end result of geomorphological analyzed Malevskian-Kizelovskian stage is expressed in a considerable compensation by mainly calcareous, claycarbonate and carbonate-argillic strata in near-board parts of deflection.In Kosvinskian stage of geologic history the geomorphological alignment process, which began in Tournasian time with near-board areas captured central, or axial zones of Nizhnekamsk deflection of the Kama-Kinel system (Fig. 5).From Kosvinskian time, when the general rise throughout the territory clearly outlined, regressive cycle came of Tournasian sedimentation phase, when approaching the sources ablation on the platform, clastic material was brought in a huge amount.On the background of common regressive sedimentation conditions in the swells in Kosvinskian time shallow marine deposits were accumulated, usually made of 3-5 m of mudstone.Here, as a rule, presented Kosvinskian layers are stratigraphically incomplete due to breaks and washouts, upper layers of sections are absent.
In near-board areas of deflection in relatively shallow (but deeper than in swells) conditions there are also stratigraphically incomplete strata of primarily clay sediments, with a thickness of 5-100 m.In the axial part of the basin, which is a relief in the bottom of the basin with the depth of 70-265 m, thick strata mainly of clay accumulated.
The presence of spikula limestone and cephalopod fauna, the amount of which decreases from the base to the roof of Kosvinskian deposits, the prevalence of finely Due to mechanical differentiation of sediments, the copious amounts of clay material entered in the depression in the first place.The rate of admission of this material is many times faster than common tectonic subsidence, which promoted the accumulation of 70-210 meters strata of argillaceous rocks.Such sedimentation regime led to the end of Kosvinskian stage the central part of deflection was loaded with clay, substantially aligned their profile.
Radaevskian-Bobrikovian stage of geological history of the Kama-Kinel system did not occur in marine, but continental sedimentary environment, qualitatively different from those of the previous stages (Fig. 6).In Radaevskian-Bobrikovian time there was a system of large and small brackish waters with a wide network of islands.
In the territories occupied in Radaevskian time by board and near-board deflection zones, a break in sedimentation is recorded, accompanied by partial or total erosion of the underlying Kosvinskian sediments.In the axial part of the basin, which represented in relief broad, shallow depression, the largest body of water was located.In this reservoir sedimentation process was not interrupted, and bog-lake and alluvial formations were deposited.Sand-siltstone and carbonaceous-clay composition and textural-structural features (crossbedding of sand and silty rocks, traces of plant root systems, etc.) of these formations show very shallow conditions of their burial.
The accumulation of sediments in these parts of the deflection, limited by denudation areas, was done not in a sea basin, but in a coastal plain, within which only occasionally and briefly seawater penetrated.The character of areal development of Radaevskian horizon indicates that in the initial stages of sedimentation ponds were small, and it occupied the most loaded zones of preserved depressions -the central part of the Nizhnekamsk deflection.
With the accumulation of sediment, borders of this shoaling reservoir widened, and it has spread within the near-board zone along the central part of the basin.All this gives grounds to assume that the axial zone of Nizhnekamsk deflection loaded with Radayevskian deposits, with thickness of up to 150 m, wedging to nearboard zones, mainly in the southern near-board zone.
The overlying Bobrikovian deposits were accumulated in a relatively leveled terrain of coastal continental lowlands.Bobrikovian deposits occurring in near-board areas with erosion and stratigraphic unconformity, in the studied area are developed everywhere.Extensive development of Bobrikovian deposits is connected to the subsidence throughout the territory of the Volga-Ural region, marking the advent of more common and stable tectonic subsidence, which caused a new transgression of the sea basin.
If the total amplitude of frequent tectonic subsidence in the area is 10-30 m, in Radaevskian-Bobrikovian stage in the central remained unloaded parts sediments of 50-150 m were accumulated.During this phase, high sedimentation rate still remained compared with the tectonic subsidence, which provided deflection filling.By the beginning of Tulskian time Nizhnekamsk deflection, not completely, but geomorphologically was aligned.
In post-Bobrikovian time Nizhnekamsk deflection has ceased to develop under a single, general plan.In Tulskian time with the onset of sea transgression a new stage of geologic history begins.
Upon consideration of the main features of geological and tectonic evolution and genesis of Nizhnekamsk deflection we can draw the following conclusions.
1. Tectonic stratification of the territory in the South and the North Tatar paleo-arches and the Kama paleodepression caused by the end of the Middle Frasnian time separation of structural-facial zones and prepared the necessary conditions for the formation of Nizhnekamsk deflection.
2. Along with the region-wide irregular tectonic subsidence in the formation of the deflection significant role belonged to the sedimentation characteristics, expressed in uncompensated sedimentation of paleodepression; Nizhnekamsk deflection is formed in Upper Frasnian-Zavolzhskian time.
3. In the marginal zones of paleo-arches reef barriertype structures arose; the development of top-Frasnian-Volga reef structures are genetically related to the formation of these depressions.
4. Carbonate, clay-carbonate and carbonate-clay Tournaisian sediments compensated near-board zones of deflection.The process of filling depressions was accompanied by development in near-board areas of low amplitude bioherms and biostromes.Kosvinskian and Radaevskian-Bobrikovian clastic deposits compensate the most loaded deflection zones.
By the nature of relations with the underlying and overlying sediments Nizhnekamsk deflection is not reflected.The axial deflection zone is shifted northward relative to the axis of Kama fault on the underlying sediments.Geographically Nizhnekamsk deflection is located in the south-eastern slope of the North-Tatar arch.
Axis misalignment of the deflection is caused by younger, mostly late and post Permian tectonic uplifts of the northern near-board area, where a system of swells was formed (Pervomaysko-Bondyuzhsky, Yelabugzhsky, Suksinsko-Ozerny, etc.), and lowerings of the southern near-board area.This partial restructuring has led to the fact that in the present structural plan over the deepest central part of the Kama fault is located in the main southern near-board zone of Nizhnekamsk deflection.Nizhnekamsk deflection in the Devonian clastic strata corresponds to monoclinal bedding of layers.
The main provisions stemming from the structural features of the Upper and Lower Carboniferous sediments and history of geotectonic development of Nizhnekamsk deflection are initial and in many ways decisive in clarifying the morphogenetic types of local uplifts and establishing their spatial distribution in the deflection and associated with them edge zones of arches.
Within Nizhnekamsk deflection swells are developed, the long axes of which are oriented to the northeast and cut structural elements of Nizhnekamsk deflection (intersecting swells).Intersecting swells of the northeast strike repeat the main features of the structural-tectonic plan of Devonian formations.Swell-like zones of Devonian deposits, in turn, reflect the picture of the crystalline basement surface, forming a system of ridges (Fig. 1) of the same stretch: Shurnyaksky, Vyatsky, Yelabuzhsky, Novo-Elkhovsky, Bondyuzhsky, Suksinsko-Ozerny, Bukharsko-Saraylinsky etc. (Larochkina, 2013).General feature for intersecting swells is that participation of tectonic factor is required in the formation of the constituent local uplifts.
Located on intersecting swells local uplifts are confined to various structural-facies zones of Nizhnekamsk deflection, characterizing depending on which zone they are located, well defined by structural relations for various age horizons.Thus, these swells are generally at a sufficient extent combine local uplifts of various morphogenetic types.
Local uplifts, owing their origin to tectonic factor (without bioherms of Upper Frasnian-Zavolzhskian and Malevskian-Kizelovskian age), are confined to swells, which are geographically located in the axial and northern near-board areas of Nizhnekamsk deflection (Bondyuzhsky, Pervomaisky, Lugovoy, Saraylinsky et al.) Unfortunately, swell-like structures are not expected, developed on the boards of the Nizhnekamsk deflection and owing their formation and genesis to sedimentation factor (analog to Arlansky).Sedimentation is the predominant factor for the southern board zone (Svinogorsky, Novo-Suksinsky, Zychebashevsky, Elginsky, Vinokurovsky, Urgundinsky, Abdulovsky and other uplifts), but tectonic factor plays an important role.Genesis of swell-like structures of the southern board is tectonic-sedimentation.Such uplifts are formed mainly due to structure forming ability of reef facies of Famennian-Zavolzhskian and-Malevskian-Kizelovskian deposits with the subordinate role of tectonic factors.
Special structures are attributed to sedimentationtectonic uplifts, in the formation of which tectonic factor was manifested many times, and prevailed, and sedimentation factor led to the emergence of relatively low (a few tens of meters) bioherms (Mamadyshsky and others).These uplifts are clearly expressed in the sediments underlying the bioherms, in particular layers of Devonian clastic strata, in which the amplitude is greater than for the Upper Carboniferous deposits.
An important feature of most of the uplifts of this type is the displacement of the dome portion on the roof of the Famennian-Zavolzhskian and younger sediments toward the steep wing and regional tilt of layers and can often serve as an indicator of simultaneous action of tectonic and sedimentary factors, in which bioherm in the process of its growth constantly attempted to be above the band of accelerated and sustainable dipping of seabed.
Bioherms of Rechtskian-Danskian-Levedyanskian and Malevskian-Kizelovskian ages are attributed to the structure forming carbonate reef (biohermal) arrays, genetically related to Nizhnekamsk deflection.Urazbahtinsky, Omarsky, Smolny, Shirmansky, Demyanovsky, Ozerny, North-Ozerny, Prirazlomny, West-Yurtovsky, Timerovsky and Otarny uplifts are attributed to the above-mentioned uplifts with biohers in the core in the form of single structures.All single highamplitude structures -reefs are confined to axial and internal near-board deflection zones.From mentioned uplifts only Omarsky has proven sedimentation genesis, the other uplifts are tectonic-sedimentation.
It is important to emphasize that local uplifts, the structural relations of which are due to the participation of Upper Frasnian-Famennian and Malevskian-Kizelovskian bioherms, genetically closely related to the Nizhnekamsk deflection.
Thus, in the axial zones of troughs only tectonic uplifts and sedimentation and tectonic single reefs are developed.In near-board areas along with sedimentation and tectonic uplifts also tectonic-sedimentary and sedimentary uplifts are developed with Malevskian-Kizelovskian and rarely Famennian-Zavolzhskian bioherms.
Analysis of the geological development, structure, and established laws of placing deposits allows linking the prospects for further search of oil in Nizhnekamsk deflection with the following three types of traps: tectonic uplifts, Middle Frasnian-Kizelovskian reef constructions, areas of structural and lithologic traps.The potential for the spread of the combined traps is obvious: sedimentary-tectonic -and lithologic -stratigraphic -in near-board areas.
Let us note that in the fields of Nizhnekamsk deflection, the main productive horizons are mainly deposits covering the reef arrays (Ozerny, Lugovoy, Bakhchisaraisky, Biklyansky and others).Deposits directly in the reef rocks are rare and controlled mainly by single high amplitude biohermal sturctures (Menzelinsky, Timerovsky, etc.).In such fields sandsiltstone rocks of the Lower Carboniferous clastic strata are favorable for the accumulation of oil and gas reservoirs; in the case of the Upper Devonian-Zavolzhskian or Upper Tournaisian reef massescarbonate rocks that form these arrays.
Traps of tectonic type account for the vast majority of projected by seismic survey for further searches and established oil and gas structures.Given the stable block structure of the crystalline basement (ridge), in the western part of the deflection, in contrast to the central, where there are mainly large deposits of oil, due to the weak differentiation large extended and sharply expressed contrasting uplifts are not expected.
At the same time, the probability of detection of new medium-and small-amplitude elevations, especially taking into account the favorable tectonic conditions of their formation at the joints of structural terraces, along the basement faults, is very high.Identification of tectonic traps is expected in the eastern parts of the deflection.
Axial deflection zone is characterized by the presence of structural traps in the form of tectonic uplifts, which at sufficiently high amplitude may appear very promising.Oil deposits may have sand and silt reservoirs of Carboniferous clastic strata, and in some cases, fractured, sometimes porous-cavernous carbonate rocks of the Upper Devonian, Tournasian, Bashkirian and Moscovian ages.
In the axial zone of the deflection within the tectonic and sedimentary-tectonic structures Upper Frasnian-Kizelovskian rocks of Domanic facies may serve as reservoirs in the event of strong fracturing and sufficient permeability.Domanic facies of carbonate stratum of the Upper Devonian, forming the bed deflection, are regionally bituminous.Signs of oil content in the form of spotted oil saturation and inclusions of tarred viscous oil were observed in the core during the drilling of many wells.
Particular attention, in connection with the established unique high-capacity oil traps in Nizhnekamsk deflection should attract high-amplitude single reef sturctures of tectonic-sedimentary origin.The development of long-term trends associated with reef structures is an important reserve for future searches.
The west end of the deflection is the perspective in this direction, where by seismic data a number of uplifts are mapped, and its eastern part in the strike of the Prikamsky fault, expressed in sharply differentiated surface of the basement, in the conjunction of axial and near-board deflection parts (Fig. 1).In addition, the main interest in conducting seismic survey should represent the intersection portions of near-board and board areas with intersecting tectonic dislocations favorable for the formation of contrasting sedimentary-tectonic structures and their systems.

Fig. 5 .
Fig. 5. Thickness map of the Kosvinskian sediments.1 -deep drilling wells, 2 -isopachs, 3-licensing borders of oil fields, 4 -boundary of Tatarstan, zone boundaries of Nizhnekamsk deflection of Kama-Kinel system; 5 -axial, 6 -board Filling of deflection accompanied by simultaneous growth of the territory occupied by shallow-marine facies, seized the second half of Malevskian-Kizelovskian stage and their near-board zones.By the end of Tournasian stage the band occupied by deepwater facies significantly (2-3 times) narrowed, occupying the central part of the deflection and having uncompensated Malevskian-Kizelovskian accumulations.