The offshore deposits indicators substances and their spatial distribution modelling

. The study of marine areas in order to replenish the raw material base with additional sources is an increasingly relevant task. The modern geological exploration equipment development for the offshore deposits search requires a further study of both the deposits features and the seawater characteristics. According to the results of the existing offshore deposits analyzed data, it was revealed that the deposits are characterized by an increased content of hydrocarbon gas bubbles on the surface of the water, as well as an increased content of these substances in the waters themselves. The main indicator substances were determined to be water-soluble methane, propane, ethane and butane. In addition, the paper presents the results of the vertical distribution of these substances modeling over marine deposits.


Introduction
Today the hydrocarbon potential of the World Ocean is estimated at about 264 billion tons of oil equivalent, where 152 billion tons are gas resources, and the rest are oil. Meanwhile 31% of global oil production and 28% of natural gas production are marine deposits [1]. The distribution of the world ocean resources is shown in figure 1 [1]. Despite the fact that the offshore fields development is a very difficult process, the listed statistics give reason to conclude that the replenishment of the hydrocarbon base through the study of the marine potential is becoming more and more relevant.
To determine the possible oil and gas potential, various methods that record gas occurrences on the water surface are used. Modern remote sensing methods involve determining the possibility of territory oil and gas potential by detecting certain indicator substances in the water composition, the presence of which gives an understanding of the presence of hydrocarbon deposits. The hydrocarbon gases sources emitted and consumed by the oceans are the distribution of marine sources, free gas zones, which is located below the gas stability zone of the hydrate [2]. To optimize the apparatus, that will allow to identify such substances quickly and precise, it is necessary to have a clear view of the quality and quantity of indicator substances and what accompanying signs may be indicators of oil and gas potential. The purpose of this study is to conduct an analysis of existing offshore deposits to determine and substantiate indicator substances. To do this, various offshore oil and gas fields located in different regions were analysed, thereby expanding the sample of the studied data. The data obtained were adapted to each other, since the observed deposits are scattered across different regions, depending on the composition of oil and gas, as well as on climatic and water conditions.

Materials and methods
The release of gas from hydrocarbon deposits passing through the liquid phase can be fixed in two ways: 1. in the atmosphere -gases dissolve in water and evaporate in the atmosphere by diffusion or 2. through bubbles -difficult soluble gases collect in clusters of bubbles and exit in the form of gas sips.
The gas saturation of an aqueous solution does not depend on the concentration of the dissolved substance, but on the nature of the formation of the gas bubble itself. At the same time, the amount of gas released into the atmosphere and dissolved in water partially depends on meteorological parameters [3].
Studying the process of gas transition into seawater by diffusion, Henry's law is used. It is a dynamic function, which characterizes gas exchange in an equilibrium state: Where -solubility, -surface tension, -gas constant equal 8, 31 J mol -1 K -1 To describe processes where gas is on the surface of water in the form of bubbles, the gas equation is used: Where -the number of moles of gas in the bubble, -volume gas, mol % It should be noted that in addition to pressure and temperature, additional factors, such as the water salinity affect the gases solubility since hydrocarbon gases are less soluble in saltier solutions.
The calculation of the methane flow into the atmosphere in [4] is calculated using the formula below: Where -Schmidt number; = 2039.2 -120.31t + 3.4209t 2 -0.040437t 3 ; -wind velocity, t -water temperature, ∆C -the difference between the measured and atmospheric equilibrium methane concentration.
Thus, calculated the range of possible hydrocarbon gases in bubbles concentrations or in the atmosphere above the water surface, it is possible to make a conclusion about the area oil and gas potential.

Results
In the offshore deposits, hydrocarbons are in the gas state, in the form of gas-liquid mixtures, as well as in the form of gas hydrates. Their properties and condition depend on temperature, pressure and the composition itself.
To date, more than 30 classifications of natural gas by chemical composition have been proposed in the scientific researchers [1]. Hydrocarbon composition are mainly methane (C n -H 2p+2 ), petroleum (C n H 2n ) and, to a lesser extent, aromatic (C n H 2n-6 ) series. According to the physical state in surface conditions, hydrocarbons from CH 4 to C 4 H 10 are gases; from C 5 H 12 to C 16 H 34 are liquids and from C 17 H 34 to C 35 H 72 and above are solids [5].
Statistical processing of available data of the hydrocarbon and non-hydrocarbon components in free and dissolved gases allowed identifying the characteristic features of their ratio for gas, gas condensate, oil and gas condensate and oil deposits.
To determine the indicators that characterize oil and gas fields, the deposits of the Sea of Okhotsk [4;6-7], the Arctic Shelf [8], the Barents Sea [5], the basins of Northern California [9] and India [10] were analyzed.
Analyzing the geochemical composition of gas condensate offshore deposits, table 1 was compiled. Suppose that the pressure in the ocean depends on the depth -the greater the coefficient characterizing the depth of hydrocarbons, the higher the pressure. Then it becomes possible to construct a simplified conceptual model of bubble saturation with gas taking into account equations (3) and (1). The model for the hydrocarbon gases methane, ethane and propane are shown in Fig 2. As in [11], it was assumed that the size of the bubble does not change its composition passing through the water column, however, not all bubbles that form on the seabed reach the water surface, 10% dissolve along the way [11].

Discussion
Based on the presented model, it can be concluded that the vertical distribution of gas has a linear dependence. However, comparing the data obtained during the expeditions presented in [2] (Fig.3), the opposite can be noticed. Different colors indicate methane concentrations at different research stations. This is due to the fact that the graph presented in figure 2 does not take into account such parameters as the presence of underwater currents, waves, the water masses movement velocity and meteorological parameters changes. The model took into account the average values of water temperature under the condition of a linear vertical distribution.
When assessing gas concentrations for a specific area, it is necessary to take into account the dependence of temperature on the depth of occurrence. Then, knowing these parameters, the described model will have the form (figure 4). Figure 4 shows the distribution of methane on the example of the Sea of Okhotsk according to data in [4; 6; 7; 12].

Conclusion
Thus, based on all of the results above, the following conclusions can be drawn: • Marine waters have a large hydrocarbon potential.
• The main theoretical basis for the use of indicators in assessing the potential oil and gas potential of the territory are methane, ethane, propane, butane.
• The vertical gases distribution is heterogeneous due to factors such as the movement of underwater currents, meteorological conditions and the nonlinear distribution of water temperature.