HYDROGEOCHEMICAL CONDITIONS OF THE OIL- AND GAS-BEARING REGIONS OF TOMSK OBLAST

A. D. NAZAROV, P. A. UDODOV, N. M. RASSKAZOV, V. G. BYKOV

Within the oil- and gas-bearing areas of Tomsk Oblast there are groundwater bodies diverse in chemical, gas, isotopic, and bacteriological composition; their study is of great importance in the search, exploration, and exploitation of oil and gas deposits, in the water supply of cities and settlements, and in the development of the hydromineral base for sanatorium-and-health-resort construction and district heating.

The aim of regional hydrogeochemical investigations for petroleum exploration is to assess the prospects of unexplored territories or to refine the prospects of territories with established oil-and-gas potential. Thus, for example, at present in Tomsk Oblast the question has arisen of assessing the oil-and-gas prospects of the Ob–Yenisei interfluve and the upper hydrodynamic zones of the Ob–Irtysh interfluve.

The formation of the waters of oil and gas fields, and of the fields themselves, is closely connected with hydrogeological and paleohydrogeological conditions, which may differ for each object, since the participation of sedimentation waters and of ancient and modern infiltration waters in the formation of the waters of different regions and even of individual fields is not the same.

The most favorable conditions for the long-term preservation of oil and gas accumulations are impeded water exchange or an elevated temperature that sharply suppresses the development of microflora.

Under such conditions, the waters of oil and gas fields usually have increased total mineralization and increased contents of calcium and sodium chlorides, bromine, and ammonium. However, cases are known (northern Western Siberia, the Caucasus, and others) where oil and gas accumulations are accompanied by low-mineralized waters with elevated contents of sulfates and bicarbonates. The possibility is not excluded of encountering, in weakly mineralized waters of oil and gas fields, also in the eastern regions of Western Siberia.

Even in the area of development of weakly mineralized waters, one can often encounter zones with a significant increase in mineralization, which is caused by the intensity of water exchange, which in turn is associated with the varying fluid conductivity of the rocks. Consequently, for a negative assessment of oil-and-gas prospects it is necessary to have data on the areal distribution of permeability-coefficient values.

Analysis of the factual material shows that within the Tom—

in Tomsk Oblast, three hydrogeochemical zones are distinguished fairly clearly, differing substantially in their conditions of formation [16].

In the upper zone, well flushed by infiltration waters, within the Ob–Irtysh interfluve the deposits are represented by sandy-clayey marine and continental Cenozoic rocks. In the Ob–Yenisei interfluve this zone already descends into the Upper Cretaceous and, beyond the limits of the oblast, into Lower Cretaceous and Jurassic deposits. The waters of this zone are fresh, bicarbonate-sodium or calcium, with a nitrogen or methane (in peat bogs) composition of dissolved gases. The ammonia content reaches 10 mg/l, while bromine is within the sensitivity limits of the method. When aquifer horizons are opened at depths of 280–380 m (Kolpashevo, Aleksandrovo), gas releases are observed [14, 6].

The second, less flushed zone is confined to sandy-clayey continental (the Pokur Formation and its analogues) and marine (the Kiya, Tarskaya, Kulomzinskaya formations and their analogues) deposits of the Upper and Lower Cretaceous. This zone is isolated from the upper one in the Ob–Irtysh interfluve by a thick aquiclude composed of Eocene–Upper Cretaceous marine clays. In a northwestern direction and with depth, there is a regular replacement of brackish bicarbonate and bicarbonate-chloride sodium methane-nitrogen waters by saline and strong-brine chloride-sodium and calcium-sodium waters with a nitrogen-methane and methane composition of dissolved gases. In the same direction, the contents of ammonia (up to 35 mg/l), bromine (up to 95 mg/l), and heavy hydrocarbons (up to 3.7%) increase. The formation of waters of this zone was influenced by free thalassogenic and meteogenic waters and by pore solutions of both the enclosing and overlying deposits [6, 14–19]. The processes of metamorphization and transformation of waters and buried organic matter, and in places the vertical migration of fluids, also had an effect. The area of development of mixed waters extends westward beyond the limits of the Kolpashevo district. The destruction of a small hydrocarbon accumulation in the Kolpashevo area (5–P) in deposits of the Tarskaya Formation may be considered a result of the action of these waters. In the Ob–Yenisei interfluve the main influence was exerted by processes of metamorphization of infiltration waters and buried organic matter, since here the deposits of this zone formed under continental conditions. The result is the distribution of brackish and saline waters with a methane composition of dissolved gases. In this zone, on the Ob–Irtysh interfluve, a number of productive beds of the BV, AV, and PK groups have been identified.

The lower hydrogeochemical zone, with very slow water exchange, is confined to Jurassic deposits and the upper part of the basement and is isolated almost throughout the oblast from the overlying deposits by the thick Vasyugan–Berriasian–Upper Jurassic clay sequence. The zone is characterized by the widespread development of brines (36–80 g/l) of chloride-sodium methane waters, with ammonia contents up to 150 mg/l, bromine up to 153 mg/l, and heavy hydrocarbons up to 5.7%. Only near the external recharge areas are the brines replaced by less mineralized waters, and the methane gases by nitrogen-methane gases. The boundary of the zone with the presence of hydrocarbons extends in the east approximately along the line Vanzhilkynak—Maksimkin Yar—Pesochnaya-Dubrovka. Water formation occurred under the influence of syngenetic meteogenic (Lower–Middle Jurassic), marine (Upper–Middle Jurassic), and infiltrogenic (along the margins) waters, processes of metamorphization of organic matter, and migration of fluids from the basement. In this zone productive beds of the Yu and M groups have been identified [6, 14–19].

Thus, as a result of complex hydrogeological and paleohydrogeological conditions, the present-day vertical

... and horizontal hydrogeochemical zonalities, manifested in a regular increase, from top to bottom in the section and from southeast to northwest (areally), in mineralization, sodium and calcium chlorides, gas saturation, hydrocarbon gases, ammonium, and bromine. In the same direction, the conditions for the preservation of oil and gas pools improve. In general, it may be said that the second and third zones, with slow and very slow water exchange, are characterized by favorable conditions, deteriorating only in the zone of water mixing. This is also confirmed by discovered oil and gas fields [19].

However, besides favorable conditions, a source of hydrocarbons is also necessary. In the opinion of most Siberian and other scholars, the sources of oil and gas in beds Yu-I, Yu-II and in the beds of groups BB, AB and PK are the oil-source Middle–Upper Jurassic and Lower Cretaceous deposits [5, 10, 11]. The question of the formation of oil and gas in the freshwater Jurassic deposits and of their influx from the basement remains open. In this respect, the question of brine formation deserves attention. In the opinion of some researchers (N. N. Rostovtsev, B. P. Stavitsky, and others), the formation of brines is due to processes of metamorphization (concentration) of waters; in the opinion of others, the brines came from saliferous Cambrian deposits of the basement [6, 20]. The anomalous occurrence of brines, especially in the Kulomzino Formation, among the waters of the West Siberian artesian basin, and the increase in mineralization mainly at the expense of sodium chlorides, make it possible to suppose that brines may have entered from the basement along tectonic dislocations. Therefore, the formation of oil and gas pools in the Ob–Yenisei interfluve may be connected both with the formation of hydrocarbons in freshwater Jurassic deposits (although at present it is still difficult to judge the scale of such oil generation), and with their inflow along tectonic dislocations. The presence, in the composition of dissolved gases, of heavy hydrocarbons, and in the waters of organic substances of the petroleum series and hydrocarbon-oxidizing bacteria (North Kolpashevo area), indicates the possibility of the formation, in the Jurassic deposits of the Ob–Yenisei interfluve, of accumulations of oil and gas. At the same time, the discovery of hydrocarbon pools at the contact with Paleozoic rocks indicates the possibility of their migration from the basement [3, 6, 16].

Many researchers (V. A. Krotova and others) have long noted the important role of tectonic dislocations in the formation of oil and gas fields [12, 13]. Confirmation of this in Tomsk Oblast may be the presence of hydrocarbon pools in the Pokur and Ipatovo formations (Severnoye field).

Therefore, in prospecting for oil and gas in the Ob–Yenisei interfluve, primary attention should be given to structures located near, or associated with, major tectonic dislocations.

The same may be said about the prospects for oil-and-gas potential of the upper part of the second hydrogeochemical zone in the Ob–Irtysh interfluve.

Thus, from the foregoing it is evident that traditional hydrogeological indicators of oil-and-gas potential, such as mineralization, the chlorine–calcium type, iodine, bromine, ammonium, and gas composition, are regional indicators applicable only for evaluating the prospects of oil-and-gas potential.

At the prospecting stage, however, the field of application of these indicators is sharply limited, since the purpose of hydrogeochemical investigations at this stage is already to evaluate the productivity of individual structures both before, during, and after the completion of well drilling. Therefore, in the present...

In this case, such hydrogeochemical indicators are used whose changes are due chiefly to the migration of oil and gas components from the pool. These may include the elasticity of dissolved gases; the content of heavy hydrocarbons, benzenes, organic acids, isotopes of carbon, nitrogen, and hydrogen; certain microcomponents (vanadium, nickel, phosphorus, etc.); benzene-oxidizing bacteria; bromine; ammonia, etc. [12, 14, 15, 18].

Unfortunately, it must be stated that the regularities in the variation of these indicators near pools in general, and under the conditions of Western Siberia in particular, have as yet been poorly studied. Usually, distinct traces of other processes are superimposed on the process of exchange of constituent components between water and oil, and the influence of pools is often detectable only by means of highly sensitive methods; therefore, the quality of horizon testing and of water sampling becomes of great importance.

Within Tomsk Oblast, an increase near hydrocarbon pools is observed in the gas factor to 5 and more, and in heavy hydrocarbons—to 2–20%.

The question of the distribution of bromine and ammonia near oil and gas is still unclear. Whereas it was previously believed that they accumulate under the influence of hydrocarbons, it has recently been proved, including for Western Siberia, that the processes of metamorphization have the determining influence. This is also confirmed by factual material. Thus, if in the upper hydrogeochemical zone the bromine content is within the sensitivity limits of analytical methods, and ammonia ranges from 0.1 to 10 mg/l, then in the second zone their contents increase, respectively, to 95 and 35 mg/l, and in the third—to 153 and 150 mg/l. The question of the influence of hydrocarbon pools, which, moreover, are characterized by conditions more stagnant than in other parts of the bed, remains unresolved to this day. Here the study of nitrogen isotopes may play a major role [15, 19].

In the distribution of microcomponents in groundwaters, the increasing influence of concentration processes with depth is also observed. There are known works (L. K. Gutsalo, V. A. Krivosheya) on the Dnieper–Donets depression in which the idea is expressed that strontium has prospecting significance [9]. However, the latter occurs in a paragenetic association with calcium and is more likely an indicator of the more stagnant conditions of traps. The prospecting significance of vanadium and nickel is likewise still unclear. In the opinion of M. S. Gurevich, for Western Siberia no interrelation of microcomponents with hydrocarbons is observed [8].

The study of organic matter in waters has great promise, although the analytical methodology is very complex. The available materials on the organic composition of groundwaters of oil and gas fields make it possible to outline as more promising indicators the content of benzene, fatty acids, and organic phosphorus.

At present, isotopic (carbon, hydrogen, oxygen, nitrogen, etc.) and microbiological studies are widely conducted for petroleum exploration purposes. In the distribution of deuterium, a regular increase with depth is noted. The question of its distribution near pools remains open. According to I. V. Grinberg and M. E. Petrikovskaya (flotation method of analysis), waters are enriched in deuterium; according to others, they are depleted; according to still others, no isotopic exchange occurs [1, 2, 7]. Proceeding from theoretical premises, isotopic exchange between water and hydrocarbons should proceed in one direction or the other depending on the displacement of isotopic equilibrium. According to our data, obtained by the activation method, an increase in the deuterium content in waters with depth is noted, from 0.0140 to 0.0148 at. %, which is due to an increase in the proportion of thalasso-

groundwaters. No unequivocal influence of oil pools is observed. Therefore, deuterium should be regarded as a regional indicator [17].

We have also, jointly with the Department of Combustible Minerals of TPI and SNIIGGiMS, begun studies of the distribution of carbon isotopes in waters and gases on a regional scale and in the vicinity of oil and gas pools. At present, deep and surface isotope surveys are being carried out, the purpose of which is to identify the front of hydrocarbon migration from a pool. The theoretical premise of the investigations conducted was the difference between the isotopic composition of petroleum gases and other biochemical and metamorphic gases; the increase in the content of heavy isotopes toward the homologues of methane (works by I. V. Grinberg, F. A. Alekseev, E. M. Galimov, and other Soviet and foreign authors); and the possibility of hydrocarbon migration all the way to the surface (works by V. A. Sokolov, P. A. Antonov, F. A. Alekseev, G. A. Mogilevskii, and others) [4, 6, 7].

According to our data, vertical migration of hydrocarbon gases, including heavy gases, has been established from deep-seated oil-and-gas-bearing deposits. The regional gas zonality of deep horizons is reflected in general terms in the gas composition of the first laterally persistent aquifer from the surface. Against this regional background, hydrocarbon pools and zones of faulting are recorded as anomalies in hydrocarbon gases and microflora. The results of these investigations made it possible to develop a method of hydro-gas-biochemical prospecting for oil and gas fields in the region under consideration, recommended to production organizations.

Considering that at present a more difficult stage is beginning—the stage of prospecting for lithologically, stratigraphically, and tectonically screened pools—the development of a hydrogeochemical method is of great importance.

LITERATURE

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