The authors emphasize the need to take into account the uncertainty, contingency and chances objectively inherent in the oil and gas industry when estimating oil and gas reserves and resources. It is noted that the current system in Russia for calculating and state accounting for reserves at least does not fully take them into account, nor does it meet the needs of business and financial structures, since the principles laid down in it do not correspond to generally accepted international standards.
The authors come to the conclusion that the only reasonable choice for the institution of independent estimation (audit) of hydrocarbon reserves, which is beginning to form in Russia, is the use of the SPE-PRMS classification widely used in the world.

The Pashyian Regional stage (horizon) is the main productive unit of the middle Devonian clastic succession of the South Tatar arch. This article presents, for the first time, maps of the lower and upper parts of the Pashyian, including data on sand-shale ratio, number of sand layers (reservoirs) and thickness, based on the analysis of logging data from 25,000 wells. The maps were created by spatial interpolation of Natural Neighbor and ArcGIS Pro software.
The model of sedimentation of the Pashyian Regional stage reflects the interpretation of the plotted maps as well as the synthesis of the results of detailed core investigations (lithological, sedimentological, ichnotextural, petrophysical, etc.) and analysis of archive and published materials.
The main points of the proposed model are as follows. The Pashyian sediments were formed in a marine basin, in an environment comparable to that of the middle shelf of modern seas – in an offshore zone dominated by current activity. The basin floor was a relatively flat plateau, on which sandy, silty and clay sediments were simultaneously accumulated. Sediments of all types accumulated during sea transgression. Sea regression caused erosion and destruction of the already formed sediments.
Positive landforms of seabed relief, composed predominantly of sandy well-sorted material, comprised autochthonous underwater sand bars, formed by constant currents parallel to the bathymetric contour of the seabed. Underwater sand bars formed extensive systems nearly throughout the entire territory of the modern South Tatar arch. At the same time, allochthonous, poorly sorted, less mature sediments were formed in underwater troughs produced by transversal currents (directed from the shore towards the sea).
The proposed model explains the consistent thickness of the Pashyian Regional stage, the mosaic distribution of sand bodies over the area, and the lens-like shape of the sand and siltstone reservoirs. The model can be extrapolated to other stratigraphic intervals of the Devonian clastic succession with similar sedimentological features.

Domanic type deposits generally is abundant in Tatarstan Republic and other territories. They are feature with oil-source rocks (black shales), which partly generated hydrocarbons. However, due to their high hydrocarbon contain, domanic type deposits construing as unconventional reservoir rocks, that may be commercial significant. Based on Russian and foreign experience in the development of such deposits, the prospects for their exploitation are associated with the searching for formations with increased reservoir properties and light oil.
In this work we are analyzed the stratigraphic distribution of domanic type deposits in the Kama-Kinel system of depressions and beyond them. It is shown that such deposits in the Kama-Kinel system of depressions have a thickness about 300 m and cover the stratigraphic range from semiluk horizon of the Frasnian stage to Tournaisian stage. But out of depressions these rocks occurred only in semiluk horizon. The reason of it is high dissection of the bottom of the Domanic sedimentation basin in the Late Frasnian-Tournaisian ages due to the evolution of the Kama-Kinel system of depressions in the east of the Russian Plate.
The results of our own research show that carbonate and carbonate-siliceous rocks enriched in organic matter are the most common lithotypes in the Domanic type deposits. Also, we constantly found carbonate breccias and less secondary dolomites in the studied geological columns. In the last two types of rocks, we found higher values of porosity, openness, and a lighter composition of hydrocarbons. Based on the results of the author’s research and the literature observation, it follows that the development of carbonate breccias and secondary dolomites will be in the sides of the Kama-Kinel system of depressions. We consider that they are as the most promising objects for the search for industrial profit in the Domanic type deposits.

At the late stage of field development, residual oil reserves undergo a significant change from mobile to sedentary and stationary. These reserves are mainly located in technogenically and production altered, watered layers and areas of deposits.
Localization and development of such sources of hydrocarbons is an effective method of increasing the final oil recovery factor in mature fields, due to the presence of a readymade developed infrastructure for production, transportation and refining, as well as the availability of highly qualified personnel.
This article considers an approach that allows, based on neural network algorithms, the estimation the volumes and localization of residual oil reserves in multi-layer deposits in combination with the analysis of geochemical studies of reservoir fluids. The use of machine learning algorithms allows a targeted approach to the development of residual reserves by automated selection of wellwork. This approach significantly reduces the manual labor of specialists for data processing and decision-making time.

The method of acid hydraulic fracturing has been actively used in recent years, for the deposits of the Vereysky horizon of the Moscowian strata in the south-east of Tatarstan. Design development and the use of hydraulic fracturing requires full-fledged studies of the geomechanical characteristics of rocks, including the prediction of the propagation of emerging fractures. It is considered the main geomechanical characteristics for reservoir rocks and dense rocks involved in the structure of the sections, on the example of the study of polyfacial reservoirs of the Vereysky horizon of the Ivinskoye oil field. It was revealed that the carbonate reservoir rocks of the middle and upper parts of the Vereysky horizon (C₂ vr3 and C₂ vr5) have the least strength properties and a greater susceptibility to fracturing. More durable were sandstones of the upper part of the horizon, marls and mudstones of the middle and upper parts of the horizon, as well as mudstone and dense packstone, typical for the lower part of the Vereysky horizon. The presented data can be valid for most of the sections of the Vereysky horizon of the southeast of Tatarstan and are used for the choice of optimal solutions when applying the hydraulic fracturing technology.

An important tool in determining residual oil saturation today is the single-well tracer test, as the preferred method for assessing the potential for using enhanced oil recovery methods (EOR) and developing pilot projects. The success of the test performed directly depends on the optimal choice of the tracer composition, which will contribute to the qualitative determination of the parameters required in the calculation of the residual oil saturation of the formation. To assess the boundary conditions for the applicability of the tracer in the field, the kinetic and thermodynamic properties of tracers are considered under various reservoir conditions of the field. Based on the results of this work, an algorithm for assessing the applicability of the tracer for reservoirs in a wide range of salinity and temperatures is presented.

This article is devoted to the review of one of the currently relevant methods of enhanced oil recovery – low-salinity waterflooding (LSW) – on the example of terrigenous Tulian, Bobrikovian and Devonian reservoirs of the Tatarstan Republic fields.
The first part of review contains information about the key processes underlying this method, such as swelling and migration of clay particles and wettability alteration, as well as the mechanisms that explain these processes, which include cation exchange, multicomponent ion exchange, electric double layer expansion, etc. Their understanding, in turn, contributes to the identification of the main factors, the presence or absence of which at the field allows us to give a preliminary assessment of LSW application. There are main factors: the presence of clay particles, a significant content of Ca2+ and Mg2+ ions, as well as multivalent ions in formation water, low formation permeability, reduced oil viscosity, an increased content of acidic and especially basic components in oil and, as a result, a hydrophobic type of initial rock wettability.
Then, examples of using the method in other fields are given and the results of laboratory studies, including the measurement of the contact angle, core flooding experiments, are briefly highlighted.
A preliminary screening is carried out on the basis of geological and field data from several fields of Tatarstan Republic, candidate fields are identified and a preliminary conclusion about LSW application in this region is made. The positive factors for the reservoirs under consideration, identified at the preliminary screening stage, include low formation temperature, high salinity of formation water with a significant content of divalent cations and the potential to shift the pH level from the current slightly acidic level towards increased alkalinity, and an increased content of polar oil components. In addition, the reservoirs of the Tulian and Bobrikovian horizons are characterized by the presence of clay particles, and the Devonian reservoir are characterized by a reduced oil viscosity coupled with an increased oil base number.

Nowadays, oil fields on the territory of the Republic of Tatarstan are developed with application of systems for reservoir pressure maintenance, involving injection of large volumes of water into the formation. Efficiency of waterflooding depends on the quality of the injected agent, and, despite the use of various technologies of preparation and treatment of injected water, with the lapse of time critical matrix porosity and permeability inevitably decreases. In this work the main sources of injection wells critical matrix clogging are considered: mechanical impurities, oil products and scales formation, and the results of the component composition analysis of deposits from cluster pumping stations are given. Using the Oddo-Thomson methodology, it is revealed that injected water is prone to calcite and barite deposition at bottomhole conditions. Physicochemical and core-flooding study of several reagents for removing colmatants: acid compositions, hydrocarbon solvents and complexing agents were carried out and their optimal formulations were determined. The matrix treatment technology, including the sequential injection of compositions based on hydrochloric and mud acids, was substantiated. An algorithm for calculating the optimal reagent volume has been developed on the basis of research data on the mineralogical composition of mechanical impurities and ionic composition of injected water. Four types of matrix treatment technologies were determined for the conditions of injection wells at the Romashkinskoye oil field Devonian formation, taking into account the composition of colmatant, and acid treatment designs were developed.

This study provides a new rock-typing approach for low-resistive and low-permeable clastic rocks. The approach includes integrated interpretation of routine core analysis data with microstructural characteristics, acquired from computed tomography (CT) and nuclear-magnetic resonance (NMR) data.
The studied formation comprises siltstones in its bottom, which are replaced by sandstones in its top. Sandstones form the main part of the oil reservoir, whereas siltstones were originally considered as water-saturated. The reserves calculation was performed based on a single Archie equation for the whole formation.
Despite on apparent water saturation and low permeability of the siltstones, incidental perforation showed considerable oil inflow from them as well. In order to delineate missed productive intervals within the low-resistive siltstones, we had to develop a new rock-typing approach, acknowledging rock multimineral composition, diversity of microstructures, a wide range of porosity, permeability, and residual water saturation values.
Designed laboratory program included porosity, permeability, electrical resistivity measurements, capillary, NMR and CT tests. The experiments were performed on the same core samples that enabled reliable correlation between measured parameters.
The joint interpretation of flow zone indicator, calculated as a function of porosity and residual water saturation, together with the results of petrophysical and microstructural measurements allowed reliable rock-typing of the clastic formation. It will serve as a petrophysical basis for identification of the missed productive intervals.
The developed laboratory program and rock-typing algorithm can be implemented in other oilfields.

The proposed method of predicting spatial distribution of leaching zones in carbonate reservoirs includes integrated interpretation of well logging data, including electrical microimages (Formation MicroImager – FMI), together with analysis of lithofacies within depositional cycles. Based on the comparison of FMI data with results of lithological and petrophysical studies of the core, an atlas of diagenetic porosity images for the studied formation was developed. Choquette and Pray international classification was used for pore typing. Applying the developed atlas to FMI logs, karstification zones were identified and classified in reference wells. In wells, where only standard well loggings was performed, karstification zones were identified using a developed decision tree. Sedimentological and sequence stratigraphic analysis made it possible to identify sedimentation cycles and to correlate the main lithofacies within the target reservoir. It was established that microbial and shallow facies form the most productive part of the reservoir. These facies with initially high primary porosity were subjected to the most intensive karstification along sequence stratigraphic boundaries due to subaerial exposure. The established relationships between leaching zones, lithofacies, and sequence-stratigraphic boundaries made it possible to carry out inter-well correlation of karstified intervals. The correlation will allow construction of leaching zones maps, that will be further applied for geological modeling.

This paper presents the results of geological and production analysis of development and reserves of the pilot area using the results of dynamic marker monitoring of horizontal wellbores of production wells using high-precision indicators. As part of the research, the geological structure of the reservoir section was updated, which includes a lithological and facial analysis and geological and hydrodynamic simulation of the reservoir. The energy state of the reservoir was analyzed and the efficiency of the waterflood system was evaluated using dynamic data analysis and correlation analysis. The reasons for changes in the dynamics of inflow profiles over time have been analyzed. By the example of the pilot area the decision-making algorithm for adjusting the process of hydrocarbon reserves extraction, based on the analysis of the relevant geological and technical information, obtained during the long-term dynamic marker monitoring of the horizontal wells operation, has been implemented. A set of recommendations has been developed, which makes it possible to achieve stable dynamics of development indicators, to increase the coefficients of coverage by displacement in the area and the reservoir section, to involve previously undrained areas of oil reserves in the development. The results of the analysis of the efficiency of the implemented geological and technological measures are presented.

This work is devoted to development of an automatic algorithm for determining the velocity model from the data of the field seismic profile. First of all total energy of received signal and the positions of the first arrivals of reflected waves has to be determined. The traces sort by common depth point and the spectra of possible velocities determine at each point of the trace and each assembly of the common deep point. The procedure is fully automated and as a result, instead of about 150 points of manual processing, 4 million points are obtained on the test profile. The values of chosen effective speeds correspond not only to existing criteria for their determination, but also to requirement of smoothness in first derivative. This makes it possible to determine reliable interval velocity profile at each track point and each collection of the common depth point. Speed determination accuracy in the range of 10-50 m/s. After recalculating the time profile into a depth profile, it becomes possible to obtain high-resolution and detailed model of the velocity structure of the geological profile during migration transformations. An iteration process refines the calculations obtained by geologists and geophysicists in order to obtain consistent and consistent results.