PROSPECTIVE TARGETS OF GEOLOGICAL EXPLORATION IN THE SIBERIAN PLATFORM AND CRITERIA OF THEIR FEASIBILITY

The relevance of the study is due to the reducing number of prospective blocks remaining unlicensed in Eastern Siberia and the need for feasibility study of the remaining potentially attractive blocks. The aim of the study is evaluation of the resource potential and allocation of new prospective license blocks in Eastern Siberia based on geological and economic criteria. The methods and instruments used in the study. To perform the economic analysis of resources and to assess the efficiency and risks of subsoil exploration and development, the authors used the results of in-house regional geological and geophysical modelling and economic research. A feasibility study of each potential pool was conducted on the author’s automated complex GeoProfi. The authors used probabilistic analysis and the cash flow discounting method to draw up an expert forecast. The results of the study. The study covers the present state and development prospects of oil and gas fields in the Siberian Platform within the Krasnoyarsk Territory, the Irkutsk Region, and the Sakha (Yakutia) Republic. The top priority petroleum zones were analysed. A feasibility study of selected areas was conducted. The income density and feasible recoverable reserves density were mapped. The critical parameters that indicate unprofitable targets were calculated. The most feasible subsoil blocks were determined based on the correlation of economic and probability parameters.

The East Siberian direction is the most important for the development of the country's oil and gas complex.High rates of exploration confirm the efficiency of fields development, necessitate the allocation of new objects of prospecting works.Since the most studied and promising subsoil plots have already been allocated, the task arises of in-depth geological and economic analysis of the remaining areas potentially attractive to subsoil users (Staroseltsev, 2015).
The problem of identifying new promising license blocks in Eastern Siberia became the main issue in the study covered in this article.For detailed economic analysis of resources and assessing the effectiveness and risks of exploration and development of subsoil resources, the authors of the article used the results of regional geological and geophysical modeling and economic research carried out by Siberian Research Institute of Geology, Geophysics and Mineral Resources (SNIIGGiMS JSC) in 2016.

State of development
Since 2005, the southern part of the Siberian platform from Russia's raw material reserve has become a new rapidly developing region of oil and gas production: the number of operating licenses has increased almost fourfold, more than 40 hydrocarbon fields have been discovered, about 20,000 km of seismic routes are being performed every year, 100 thousand linear meters of deep exploratory wells are being drilled.The oil pipeline system "Eastern Siberia -Pacific Ocean" was put into operation, which allowed increasing the annual oil production in the region up to 45.9 million tons in 2016 with the prospect of reaching 80 million tons per year.In January 2017, the construction of an oil pipeline from the Yurubcheno-Kuyumbinsky zone was completed.This will increase the loading of the main export oil pipeline by another 8.6 million tons per year.Work is underway to create the "Power of Siberia" gas pipeline, bringing the gas industry into Eastern Siberia with the potential of producing at least 60 billion m 3 per year (Gazprom JSC, 2014).
Regional geological exploration, licensing and monitoring of subsoil use is supervised by the Siberian Research Institute of Geology, Geophysics and Mineral Raw Materials, which allows the authors of the article to provide a comprehensive overview of the current state and prospects for the development of the East Siberia subsoil.
Currently, the total recoverable reserves and oil resources of the Siberian Platform are estimated at 15.8 billion tons (of which 3.2 billion tons are condensate).Including: in the Krasnoyarsk Territory -8.9 billion tons (of which condensate -2.1 billion tons), Irkutsk region -3.2 billion tons (of which condensate -0.5 billion tons) and Yakutia -3.7 billion tons (of which condensate -0.6 billion tons).The total volume of oil reserves of categories A + B + C 1 and C 2 is 2.8 billion tons.
Similarly, for gas -52.2 trillion m 3 , of which in the Krasnoyarsk Territory is 26.4 trillion m 3 , Irkutsk region -12.0 trillion m3, Yakutia -13.8 trillion m 3 .Natural gas reserves of categories A + B + C 1 and C 2 are 9.2 trillion m 3 (State Register of mineral resources of the Russian Federation, 2016, Efimov et al., 2017).
The State Register in the Krasnoyarsk Territory (within the Siberian Platform) includes 29 fields (including 10 with oil content).Large fields with more than 30 million tons (or 30 billion m 3 ) of initial recoverable reserves include 4 oil fields (93% of proven oil reserves) and 8 gas fields (69% of proven gas reserves).In Irkutsk region there are 38 fields (including 25 with oil content).About 87% of the explored oil reserves are concentrated in 9 large fields, 95% of gas reserves -in 13.Similarly, for the Republic of Sakha (Yakutia) -52 fields (including 17 with oil content), of which four large oil fields contain 76% of all explored reserves and seven large gas fields -40%, respectively.
In total during 2005-2016 years 159 subsoil plots were distributed.68 licenses have been expired, 68 licenses have been withdrawn or handed over by subsoil users.Some of the sites with licenses that were completed were redistributed (Efimov et al., 2017).
Since 2005, 320.6 billion rubles have been spent on geological exploration in the region.At the expense of this financing, an average of 18.5 thousand linear meters of seismic exploration work 2D, 6.9 thousand km 2 of 3D seismic survey, 3.7 thousand meters of parametric drilling, 140 thousand meters of deep drilling were conducted per year.As a result of the measures, 871.7 million tons of estimated oil resources and 4194.3 billion m 3 of gas were allocated.The increase in oil reserves amounted to 96.4 million tons, gas -123.9 billion cubic meters.

Geological and economic modeling
To determine promising geological exploration facilities, SNIIGGiMS JSC conducts systematic work on the compilation of the results of fieldwork, their integration within databases and geological and economic analysis.In particular, in 2016 the next stage of this fundamental work was completed: a seismic project was made with a total volume of about 34 thousand linear km of sections with reference to the 150 most representative wells, the main seismic horizons were traced at the regional level, a set of maps and charts was prepared that allows modeling the development of the Siberian oil and gas basin.
The resources of oil, gas and condensate were differentiated by oil and gas bearing complexes, types of fluids, types of traps and size classes.A single (local) object of geological and economic evaluation was a typical predicted deposit.It was supposed that the reservoir could potentially be discovered as a result of the geological exploration complex and later become an independent object of development.
Thus, 91 oil and 104 gas typical reservoirs were modeled, and the total number of the forecasted objects was 3360 and 4500, with the predominance of the oil or gas part, respectively.Parameters of typical deposits (volume and density of reserves, trap type, reservoir type, fluid type, reservoir properties, depth of occurrence, etc.) were taken in accordance with approved oil and gas geological zoning and quantitative assessment of resources.When forecasting the initial geological characteristics, the analogy method, the mean value method and the expert evaluation, as well as combination of these approaches were used.
The forecast of costs for the preparation and development of reserves was carried out using the standards obtained from the reporting data on the estimated cost of work on exploration organizations for 2015-2016, which conducted work in adjacent sectors.Investments in the development of objects were calculated on the basis of calculated indicators of the extraction process (drilling volumes, well stock, field equipment, etc.) and specific standards for each type of work.Corresponding initial data are obtained by means of analogies with the developed fields, information on which is given in the technological schemes, feasibility study of oil recovery factor, development projects, and production reports for 2015-2016.
The forecast of the dynamics of net cash flows (NPV) and indicators of economic efficiency was carried out at an oil price of $ 45 per barrel and the exchange rate of 65 rubles per dollar.The discount rate was assumed to be equal to 10%, taking into account the specifics of the implementation of oil and gas field exploitation projects (risk bonuses) and the refinancing rate set for the calculation period, taking into account that the calculation is carried out without taking inflation into account.
Geological and economic assessment of each potential deposit was carried out on the author's automated complex GeoProfi according to the algorithm presented in Figure 1.Geological and geographic characteristics simulated the dynamics of prospecting and exploration, oil and/or gas production, and also determined economic indicators (profitability, payback period, dynamics of the necessary investments, the dynamics of tax deductions, the dynamics of NPV, etc.), characterizing the investment attractiveness of the developed and the object being studied (Varlamov, Gert, Milyaev et al., 2015).
The shares of profitable deposits and the potential income from their development are the most interesting, calculated for each oil and gas bearing complex (OGC) and class of size (Table 1, 2).This information characterizes the most promising directions for setting up prospecting works: -Vendian (V) oil and gas bearing complex within the Baikit, Katanga, Nepa-Botuobin and South Tunguska oil and gas regions; -Cambrian (E) complex in Nepa-Botuobin, Baikit, Katanga, Angara-Lena, South Tunguska and North Tunguska oil and gas bearing areas; -Riphean (R) in Baikit and Katanga oil and gas bearing areas; -Ordovician and Devonian (O-D) in the North Tunguska and, to a lesser extent, Katanga oil and gas bearing areas (SNIIGGiMS JSC, 2016-2017).

Risk analysis
Uncertainty, in the conditions of which it is necessary to work for oil and gas companies, is extremely high for new underdeveloped territories.Sources of risks may include various factors: geological, economic, infrastructure or political.Therefore, in the preinvestment decision-making phase, it becomes necessary to conduct an analysis of risks that could lead to loss of potential income (Milyaev, 2007).
Within the framework of the research, the critical (threshold) values of the parameters of search facilities were determined for which the development becomes economically ineffective (SNIIGGiMS JSC, 2016-2017).We can draw a general conclusion that with a probability of 80%, the cost-effective oil deposit Table 1.Distribution of the forecasting specific income from the development of predominantly oil deposits Georesursy = Georesources.2017.  in Eastern Siberia has a reserves density of at least 3000 tons/km 2 , lies at a depth of less than 3650 m and has an initial production rate of wells of at least 11 tons/ day; similarly profitable gas deposit -the density of reserves is not less than 10 million m 3 /km 2 , a depth of at least 3,650 m and an initial production rate of more than 21 m 3 /day.
Tables 3 and 4 list unprofitable objects and outline the characteristics that caused the negative NPV. Unprofitable were: • oil deposits of up to 10 million tons, usually due to low initial production rates and large depths, and also to a certain extent due to low resource density in the search phase of operations; • gas reservoirs confined to the Riphean complex and therefore located at great depths (up to 6000 m), and also, in some cases, gas deposits of the Vendian complex, again because of the depths and, in addition, the low density of resources.
The unprofitable nature of the gas condensate facilities of the Angara-Lena and Katanga oil and gas bearing areas is associated with the need to build a long-distance gas pipeline, which is exacerbated by the inflexibility of the tax system, which does not take into  account the fact that the company incurs costs of this magnitude (Table 5).At this stage, there are no projects for the construction of oil trunk pipelines and gas pipelines (OP and GP) passing through the Katanga, North Tunguska and South Tunguska oil and gas bearing areas.In fact, mastering most of the promising areas in the current realities is impossible.The express assessment showed that the cost of building the necessary OP and GPs is from 5% to 31% of the total NPV that can be obtained from the development of the resources in the relevant oil and gas area.Consequently, in the future, there is likely the emergence of systems for transporting raw materials from these territories.

Final evaluation
Experts of SNIIGGiMS JSC compiled a forecast of a fifteen-year study of oil and gas bearing areas, including potentially possible discoveries during this period and the necessary geological exploration work.
Table 6 presents the main parameters of the geological and economic assessment of the facility: the quantity and reserves of profitable deposits, potential production, costs, the calculation of which for the entire calculation period is carried out at constant prices (without the use of deflators) and the resulting cash flow, which is discounted to the year the calculations begin.
The total potential increase in cost-effective reserves can reach 1.06 billion tons of oil and 3.03 trillion cubic meters of natural gas.Search for deposits, development and transportation of raw materials will cost for subsoil users an average of 74.8 dollars per ton of conventional hydrocarbons, depending on the oil and gas bearing area, vary in the range of 62.3-88 USD/t of conventional hydrocarbons.The total NPV is estimated at 22.9 billion dollars, more than 70% of this amount falls on three promising areas: Nepa-Botuobin, Baikit and Katanga.
In the territories under consideration it is possible to extract more than 886 million tons of oil and 2.5 billion cubic meters of natural gas.The highest volume of hydrocarbons is accounted for by the same three oil and gas bearing areas: Nepa-Botuobin -381 million tons of oil and 753 billion cubic meters of gas, Baikit -203 million tons of oil and 257 billion cubic meters of gas, Katanga -167 million tons of oil and 331 billion cubic meters of gas.
To visualize the result obtained, geological and economic maps have been constructed, namely, a map of the income density from the development of subsoil (Figure 2) and a map of the densities of profitable recoverable resources (Figure 3).It can be noted that in 80% of cases the income density is in the ranges: • 0.259-4.229 million dollars, per km 2 for prepared structures; • 0,014-0,096 million dollars, per km 2 for the identified structures; • 0.009-0.014 million dollars, per km 2 for nonlocalized resources.
The income density for non-localized resources in comparison with the resources of the prepared and identified structures is much smaller.This is due to both a higher level of uncertainty and a significant (up to 10 years) time lag preceding the beginning of their development.Obtained geological and economic maps clearly reflect the most highly profitable territories and serve as a basis for allocating investment-attractive sites (SNIIGGiMS JSC, 2016-2017).

Selection of prospective sites
The definition of the most promising areas for development is a step-by-step process, the initial base for which is the extensive research complex, which is carried out on an ongoing basis in SNIIGGiMS JSC.The works are oriented both to the tasks of the government customer, and exploration and mining companies: collection, processing and systematization of data on geology and subsoil use, maintenance of electronic databases; planning and analysis of the results of regional geological exploration; development and application of geophysical technologies; technical and technological support of geological exploration; laboratory research and so on.
To allocate plots, maps were built: • Probabilistic map characterizing the success of confirming the reserves of an oil and gas-bearing complex; • A map of license blocks, the right to use which can be purchased through a state procedure or purchased from existing subsoil users; • Geological and economic maps for each oil and gas bearing complex.
Their joint consideration allows us to determine the licensed areas (Figure 4), the most promising in terms of a set of criteria: • Resource potential; • Distribution of productive oil and gas bearing complexes; • Results of geological exploration work; • Transport infrastructure; For all prospective sites, electronic passports are compiled, including geophysical and borehole materials, followed by in-depth geological and economic research and preparation of technical and economic proposals for the acquisition of plots.In a condensed form, these data are presented in the album "Perspective licensed areas of Eastern Siberia and the Republic of Sakha (Yakutia)" (Milyaev, 2017).

Conclusion
The study revealed that 80% of recoverable hydrocarbon reserves in Eastern Siberia are profitable even at an oil price of $ 45/bbl.It has been established that a negative NPV may result from a low reserves density, expected production rates, as well as large depths of productive horizons, and specific threshold values of these parameters are determined.The reason for the unprofitability of certain areas was not geological, but infrastructural and political factors: remoteness from the existing main pipelines and insufficient flexibility of the tax system.
At the same time, a stimulating government policy is being implemented to develop remote perspective territories with high resource potential by subsoil users: regional geological exploration, construction of main pipelines ("Eastern Siberia -Pacific Ocean", "Power of Siberia", "Kuyumba-Taishet"), tax vacations in the initial stages of development.This increases the

Fig. 1 .
Fig. 1.Algorithm for geological and economic assessment

Fig. 4 .
Fig. 4. Map of prospective license areasattractiveness of parts of Eastern Siberia, along with their highly undeveloped resource potential for production of conventional light oil.In the short term, the Baikit, Katanga and Nepa-Botuobin regions are the priority for development, in the future -projects for the development of the North Tunguska and South Tunguska oil and gas bearing areas.

Table 2 .
Distribution of the forecasting specific income from the development of predominantly gas deposits Table 3. Unprofitable oil facilities and values of critical parameters

Table 4 .
Unprofitable gas-condensate facilities and values of critical parameters

Table 5 .
Influence of the facility remoteness from the system of transportation of raw materials on the expected income

Table 6 .
The result of determining the investment attractiveness of promising oil and gas bearing zones (within a fifteen-year period of geological study)