Oil deposits of the Visean stage of the northeastern part of the Tatarstan Republic

. The sections of wells that have discovered oil-bearing deposits of the Bobrikian horizon, within the Aktanysh-Chishminsky trough in the northeast of the Tatarstan, are considered. In the terrigenous complex of the Bobrikian age, alternation of mudstones, siltstones, and sandstones is observed. According to the peculiarities of composition and porosity, mudstones and siltstones are classified as impermeable rocks, sandstone - as reservoir rocks. The study of sandy reservoirs showed that the rocks are characterized by a monomineral quartz composition, uniform oil saturation, fine-grained structure, massive texture. The porosity of sandstones is 15-20%. The pores are connected and form winding channels up to 0.1 mm in diameter. Sandstones form covers over the biogenic structures of the Tournaisian Stage, wedging out laterally and being replaced by dense siltstones. Good capacitive properties of the rock were acquired due to the leaching of calcite cement during the migration of water-oil fluids. Currently, sandstones contain oils with a predominance of light oil and medium paraffin fractions.


Introduction
The deposits of the Visean stage on the territory of the Volga-Ural anteclise are regionally oil-bearing [1][2][3].According to some estimates, about 35% of all explored oil reserves are concentrated within the Republic of Tatarstan [4].The most promising oil reservoirs are associated with sandstones of the Tulian-Bobrikian age, which form reservoir layers or perform erosion incisions within the South Tatar arch and the Melekess depression [5][6][7][8].The northeastern part of the territory of the Tatarstan is characterized by low oil content.For the entire period of geological exploration of the territory, only small oil deposits with a flow rate of 2 to 7 tons per day were discovered here [9][10].Oil deposits are controlled by low-amplitude dome-shaped uplifts, in the contour of which reservoirs are located at depths of 1260.0-1380 m.Most of the oil-bearing uplifts are concentrated within the Aktanysh swell-like structure, which complicates the northwestern side of the Aktanysh-Chishminsky trough.For a long time, small deposits were considered unpromising.Only starting in 1995 they gradually began to be introduced into development.However, until now, oil companies have a poor understanding of the geological structure of oil deposits.This is due to the high degree of variability in the structure of geological sections of oil-bearing intervals.Therefore, when drilling, geologists carefully study the cores of new wells.As part of this work, two wells were studied, which revealed potentially oil-productive deposits of the Visean stage.

Materials and methods
The main research methods were optical-microscopic, X-ray and thermal analyses.
With the help of optical microscopy, the study of the mineral composition of rocks, their porosity, and the features of the relationship of structural elements was carried out.The analysis was carried out on a polarizing microscope ZEISS AXIO Imager A2 (Germany).Interchangeable lenses made it possible to obtain information from thin sections at various magnification ranges.The most informative parts of thin sections were filmed with a camera.
X-ray analysis was used to refine the mineral composition and the ratio of mineral phases in the rocks.The most informative results were obtained in the study of mudstones and siltstones, the mineral phases of which are poorly diagnosed under a microscope.The analysis was carried out on a BRUKER D2 Phaser X-ray diffractometer (Germany).The range of scanning angles in the Bragg-Brentano geometry was from 3 to 40° in the shooting mode: X-ray tube voltage -30 kV, current -30 mA, scanning step -0.02°, speed -1 deg/min.
Thermal analysis was used to determine the phase composition of hydrocarbons.The method is based on the ability of light, medium, and heavy fractions of oil to boil off at different temperatures [11][12][13].The analysis was carried out on an NETZSCH STA 449 JupiterF3 instrument (Germany).The temperature increase step was 10 deg/min in the heating range from 30 to 1000°C.

Results
The study of the wells core that discovered the oil-producing intervals of the Visean stage showed that the main prospects for oil production are associated with the deposits of the Bobrikian horizon.The Bobrikian horizon in the wells is represented by a terrigenous complex, within which mudstones, siltstones, and sandstones are interbedded (fig.1).
Argillites according to X-ray analysis are characterized by a polymineral composition.In them, the clay component, which makes up 50-55% of the rock, is represented by kaolinite (45-50%) and illite (5-10%).Fragments of quartz (40%) and feldspars (5-10%) are present as clastic minerals (fig. 2 black curve).The flakes of clay minerals adhere tightly to each other with their basal planes.As a result of this orientation, kaolinite and illite flakes form a dense aggregate with an axial structure.Humic organic matter is located between clay minerals in the form of thin black films, clots, less often thin wavy lenses.In the dispersed organic-clayey mass of rocks, there are inclusions of fine silt grains of quartz and feldspars.Fragments of minerals are scattered, located at a distance from each other, less often concentrated in small lenses.The porosity of mudstones is 2-3%.Intergranular pores, subcapillary dimension, contain pinched aqueous solutions.
According to X-ray analysis, siltstones are mostly composed of fragments of mineral grains (65-75%), represented mainly by quartz grains (85-90%) with an admixture of feldspar fragments (10-15%) size 0.01-0.1 mm.The clay component of the rocks, which is 15-35%, is represented by kaolinite and illite (fig. 2 blue curve).Fragments of minerals are tightly adjacent to each other, in some areas forming contacts of mutual adaptation.The intergranular space of siltstones is filled with clayey illite-kaolinite cement enriched in humic organic matter.Clay material is unevenly distributed, often forming separate layers.Sandstones according to X-ray analysis are 98-100% composed of quartz, feldspar and muscovite account for no more than 2% of the rock.Optical-microscopic studies have shown that the rocks are characterized by uniform oil saturation, fine-grained structure, massive texture (fig.3).Quartz grains are semi-rounded, less often angular, mostly isometric in shape, 0.1-0.25 mm in size, form a dense structural pack, touching edges.Point and concave-convex contacts of mutual adaptation are formed at the points of contact.Some of the quartz grains have traces of regeneration.Mineral fragments are cemented with polymineral clay-silica cement.Silica cement of contact-regeneration type confined to areas of contact interactions between quartz grains.Clay cement of the porous-clotty type, pelitic in structure, enriched with organic matter, fills the intergranular space in the rock.Fragmentally around quartz grains, relics of syngenetic calcite cement are fixed, which is currently mostly leached.The sandstone contains rare inclusions of authigenic pyrite aggregates up to 0.1 mm; porosity 15-20%.Pores are intergranular, communicating, form an extensive network of intersecting channels with a diameter of up to 0.1 mm, filled with oil.
A comprehensive study of sedimentary rocks of the terrigenous Bobrikian complex↑ showed that the main reservoirs are sandstones.Argillites and siltstones, due to the dense composition of structural elements and very low porosity, are cap rocks.The presence of high porosity and intense oil saturation is not always a sign of the productivity of an oil reservoir.In the case of natural waterflooding, due to the penetration of edge formation waters, oil oxidation can occur [14][15][16][17].Due to the loss of oil viscosity and the of heavy hydrocarbons in it, terrigenous reservoirs lose their significance for development [18][19][20].For qualitative analysis of hydrocarbons, the composition of oils localized in Bobrikian quartz sandstones was studied using thermal analysis.When oil-saturated sandstones are heated, the DCK curve (curve of differential scanning calorimetry) clearly shows three exothermic effects caused by successive boiling off from different groups of hydrocarbons (fig.4).In the range of 200-360°C, light oil fractions boil away from the oil composition, in the range of 360-475°C -medium fractions, in the range of 475-540°C -heavy fractions.At the same time, the weight loss on the TG curve (thermogravimetric curve), which accompanies each exoeffect, allows us to evaluate the content of each fraction group: light -3.19%;medium -4.59%; heavy -0.19%.

Discussion
The obtained results show that in the sections of the Bobrikian horizon of the onboard part of the Aktanysh-Chishminsky trough, the main oil-bearing reservoirs are fine-grained quartz sandstones.The rocks were formed during the regressive stage of the development of the Visean Paleosea.The uplift of the territory led to the formation of a desalinated coastal-marine basin on the southwestern slope of the trough.The proximity of the coastline contributed to the influx of a large amount of terrigenous material.The controlling factor in the distribution of demolition products along the coastline was the relief of the seabed, its dynamics, openness, and the presence of underwater elevations.In relatively deep areas and in back-reef areas, fine silty and clayey material accumulated, which subsequently transformed into siltstones and argillites.Coarse-grained material accumulated on underwater flat-topped elevations of organogenic structures located along the coastline and forming the Aktanysh swell-like structure.Due to such differentiation of the clastic component, the tops of the organogenic structures were covered with layers of sandstones, which laterally gave way everywhere to siltstones and argillites.In the process of lithogenesis, quartz grains of sandstones were cemented by carbonate cement with an admixture of clay material.Due to the crystallization pressure, the calcite grains pushed the quartz grains apart, breaking the primary dense structural packing of the rhombohedral type and creating prerequisites for the formation of increased porosity.During the migration of aggressive water-oil fluids, calcite cement was leached and intergranular pores formed in its place.Fluctuations in acidity-alkalinity in the pore medium contributed to the redistribution of silica with the formation of a new cement of the contact-regeneration type.The reservoir prepared in this way eventually filled with oil.When analyzing the DSC curve, it is seen that the exothermic effect caused by the boiling off of heavy groups of hydrocarbons is somewhat remote from other exo-effects that merge with each other.This may indicate the repeated arrival of oil fluids in terrigenous reservoir rocks.Probably, the first oil deposits were destroyed, leaving behind bituminous rims around quartz grains.After that, new oil fluids came again, filling the vacated pore space with lighter oils.The first two paired exoeffects are new oil enriched in light and medium fractions of hydrocarbons, the third exoeffect is residual heavy hydrocarbons (resins, asphaltenes) of earlier oxidized oil.Considering the above, the following conclusions can be drawn: • In the sections of the Bobrikian horizon of the onboard part of the Aktanysh-Chishminsky trough, the main oil-bearing reservoirs are fine-grained quartz sandstones, the cap rocks are dense argillites and siltstones.
• Sandy oil-bearing layers are lithologically isolated, spatially confined to flattened peaks of organogenic structures of the Tournaisian stage, forming the Aktanysh swell-like structure.
• Oils enriched in light oil fractions and medium groups of hydrocarbons are localized in sandy reservoirs, which may indicate their relatively young age of migration.