On the Methodology of Prospecting and Exploration for Oil and Gas in Western Siberia

V. L. Kokunov, Yu. M. Shushakov

(Presented by Professor A. V. Aksarin)

Geological exploration work for oil and gas is being carried out with increasing success over the vast territory of Western Siberia, confirming its great national-economic importance for the further development of the oil-and-gas-producing industry of the USSR. Suffice it to say that in 1964, owing to Western Siberia alone, a major increase in oil and gas reserves in industrial categories was obtained, namely: 38% of the all-Union total [1].

Nevertheless, even in this largest oil-and-gas-bearing province, where prospecting and exploration work began relatively recently, the methodology of prospecting and exploration for oil and gas still has a number of shortcomings, although it should have incorporated all the latest achievements of science and technology.

At present, the methodology for prospecting for oil and gas fields should be divided into three main stages: the search for structures (preparation of the structural basis for deep prospecting and exploration drilling), comparative evaluation of the prepared structures prior to drilling (selection of the most promising structures), and deep prospecting and exploration drilling with searches for deposits in the section.

The question of preparing structures for deep exploratory drilling is being successfully resolved by areal seismic exploration work and, in the future, by application of the cheaper TT method. The only very substantial drawback is the noticeable lag of seismic exploration work behind the pace of drilling, which may lead to “structural famine,” already observed in the eastern part of the platform (the Ob–Yenisei interfluve), where the Novosibirsk Geological Administration is forced to drill on the first third-order structures prepared by seismic work. Naturally, such a situation is intolerable, since in the final analysis it leads to poorly justified drilling of deep wells on “empty” areas. As prospecting experience shows—

... of exploration work for oil and gas under the conditions of the West Siberian platform, for 64 structures that yielded oil or gas there are 123 barren structures on which drilling was carried out. Thus, out of every three structures, two prove to be barren. Under such conditions, checking by deep drilling each structure only just prepared by seismic exploration is impractical.

The second very important question in the modern rational complex of prospecting and exploration work for oil and gas under the conditions of the West Siberian platform—namely, the selection from the structural inventory of priority targets for deep drilling—is resolved on the basis of the following principal premises.

1. In the western part of the platform, approximately as far as the meridian of the Shaim group of fields, the evaluation of structures as drilling targets can be carried out according to the method of N. N. Rostovtsev [6], i.e., according to a map of isobars. But the indicated method is suitable only for gas and gas-oil fields. For oil fields, an analogous method has not yet been developed.

2. In the Central Ob region, the selection of priority drilling targets is carried out on the basis of the dimensions and amplitude of local uplifts of the 3rd order. The largest, most often linearly elongated uplifts with amplitudes of 100–200 m are subjected to drilling first.

The recently adopted policy of exploring first of all the largest local uplifts is, of course, correct, but the distinctive geological structure of the West Siberian platform makes it necessary to take into account what types of pools are associated with these large uplifts. The expectation that sheet-like anticlinal pools will necessarily be associated with large structures is not always confirmed by drilling results. A generalization of the results of prospecting and exploration work shows that, of the 67 fields currently known in Western Siberia, only slightly more than half (55%) are sheet-like anticlinal pools, while the remaining 45% are lithologically bounded pools, the exploration of which requires a larger number of wells than does the exploration of sheet-like anticlinal pools. In any case, as N. N. Rostovtsev and I. I. Nesterov [7] quite rightly note, in only two cases out of six is it possible to discover an oil or gas pool with the first well. In the remaining cases, drilling of another 1–2 wells is necessary, since the pools in these cases are lithological. Thus, a method for identifying oil-bearing structures from among those prepared for deep drilling before the drilling of the first wells has not yet been developed; moreover, on the “Map of Forecasts of Oil-and-Gas Potential of the Platform Part of the West Siberian Plate,” 1964, compiled by teams of workers of SNIIGGiMS, the Tyumen TTU, Novosibirsk TTU, Krasnoyarsk TGU, VNIGRI, and NIIGA under the editorship of N. N. Rostovtsev, the zone of possible discovery of lithological pools is shown only as a narrow belt somewhat east of the meridian of Khanty-Mansiysk. True, the authors make the reservation that this is only a zone of possible lithological pools in Lower Cretaceous deposits, while zones of lithological pools in Jurassic deposits are not shown on the map at all, which is undoubtedly a shortcoming of the map. Incidentally, the lithological pool at the Ust-Balyk field, although Cretaceous, did not fall within this contour. Evidently, the contours of distribution even of Lower Cretaceous lithological pools will prove to be considerably broader.

1. The methodology of prospecting and exploration work in the eastern part of the West Siberian platform on the lands of the Novosibirsk TGU and especially

Fig. 1. Schematic map of the oil-and-gas prospects of the Lower–Middle Jurassic deposits of the West Siberian Lowland.

Legend: 1 — marine deep-water facies; 2 — marine shallow-water facies; 3 — nearshore-marine facies; 4 — coal-bearing areas, mainly with coals of floodplain genesis; 5 — zone of distribution of lithological oil and gas pools.

2–3 — zones of distribution of stratified anticlinal pools.

Note: the map was compiled from data of SNIIGGiMS, NPU, KGU, TGU, and the Coal Geology Laboratory of the USSR Academy of Sciences.

in the Ob–Yenisei interfluve leaves much to be desired. First of all, there is a very narrow front of seismic-prospecting work for preparing areas for deep drilling, which has led to the practical absence of a structural inventory. Secondly, the history of the formation of this structural-facies zone is not being taken into account to a sufficient degree, which leads to protraction of the exploration periods for already discovered pools. As an example one may cite the results of work at the Ust-Silga gas-condensate field. To date, 15 wells have been drilled here, only three of which have yielded commercial inflows of condensate and gas. The basic document for siting prospecting and exploration wells here was a structural map, whereas in the exploration of lithologic pools it is necessary to combine the structural map with a lithologic map or a thickness map.

On the appended map of the oil-and-gas potential of the Lower–Middle Jurassic deposits (Fig. 1), on the basis of the results of prospecting and exploration work, and also of an analysis of the spatial distribution of facies, we have outlined zones where lithologic types of pools are most probable, which must be taken into account when siting the first prospecting and exploration wells.

Analyzing the existing situation, it should be noted that the weakest link in the process of oil and gas prospecting and exploration in Western Siberia is the second stage, namely: a rigorous scientific substantiation of the choice of priority structures for deep prospecting and exploration drilling. Meanwhile, the latest achievements of domestic geochemistry and the deep penetration into geology of physical methods of investigation, based on the use of precision instruments, make it possible to pose the problem of direct methods of prospecting for oil and gas pools.

Under the conditions of the West Siberian Lowland, the most suitable of the existing geochemical prospecting methods, which can quite well be used for evaluating structures prepared for deep exploratory drilling, is the study of the distribution and composition of gas dissolved in water [8]. The basis for introducing precisely this methodology is the generally known saturation with gases of formation waters in the section of the West Siberian Lowland, as well as the facts of the undoubted presence of aureoles of gas dispersion above gas pools of the Berezovo gas-bearing region [5] and the Shaim oil-and-gas-bearing zone [4]. In addition, there are indirect indications of vertical diffusion of petroleum hydrocarbons in the central Ob region. If one analyzes changes in the composition and certain physical properties of the oil of the Ust-Balyk field, it turns out that the greater the thickness of the seal above the pool, the lower the viscosity and specific gravity of the oil and the lower its sulfur content (Fig. 2).

Thus, one may conclude that there is migration of light hydrocarbons of oils upward through the seals, the consequence of which is the relationship between the properties of the oil and the degree of closure of the oil-bearing beds.

The above-mentioned facts of vertical migration of hydrocarbons through rocks make it possible to expect the presence of gas anomalies not only over gas pools, but also over oil pools, which can be recorded by sampling gases dissolved in waters even during seismic-prospecting work, i.e., before the drilling of deep prospecting and exploration wells. It is best to take samples of waters with gases dissolved in them from shot holes using the simplest gas sampler of V. A. Sokolov’s design, or the design of S. P. Borovsky. A single profile of gas-sampling points across a structure is quite sufficient for determining changes in the concentration and composition of gases. The composition and

![Graph showing dependence of viscosity, specific gravity, sulfur content, and asphaltenes on the thickness of the cover above the deposit. Axes: vertical—sulfur, viscosity, asphaltenes, specific gravity; horizontal—reliability coefficient. Legend: I, II, III, IV.]

Fig. 2. Dependence of the magnitude of viscosity, specific gravity, sulfur content, and asphaltenes on the thickness of the cover above the deposit. Compiled by Yu. M. Shushakov. Conventional designations: I—specific gravity of oil; II—oil viscosity; III—sulfur content; IV—asphaltene content; 1–7—oils of various fields; 1–3—Ust-Balyk; 4—Megion; 5—Vatinsk; 6—Trekhozernoye; 7—Ubinsk.

The concentration of gases in samples is best determined on a domestic gas chromatothermograph of the KhTM-4 type, which requires only 5 cm³ of gas for accurate and rapid analysis.

In view of the strong waterlogging of the territory of the West Siberian Lowland, it is necessary to verify the genesis of the gas being analyzed, which is entirely possible by using isotopic analysis for two or three sampling points, since the isotopic composition of marsh methane and methane from natural and associated gases is different [2].

Undoubtedly, the planned program of work for assessing the prospects of structures prepared for deep prospecting and exploration drilling represents a major problem topic and is not as simple as it may seem at first glance. But even the formulation of such a problem is extremely timely and highly relevant, since it is intended to eliminate one of the bottlenecks in the scientific substantiation of prospecting and exploration drilling for oil and gas.

Let us now proceed to an analysis of the methodology for placing wells for deep prospecting drilling on areas prepared by seismic exploration.

According to the recommendations of N. N. Rostovtsev and I. I. Nesterov [7], in the West Siberian Lowland two areas should be distinguished according to the geological conditions of occurrence of oil and gas accumulations, in which different methods should be applied for locating exploratory wells. The authors assign the Berezovo–Shaim and Krasnoleninsk oil-and-gas-bearing regions to one of these areas, and the Surgut region, that is, the Middle Ob region, to the other. Strictly speaking, the difference in methodology for the two areas lies not in where to drill the first well, but in where to locate the subsequent 2–4 wells. The first well is usually drilled in the most uplifted part of the uplift. The further method of well placement depends on the drilling results of this well. In all, I. I. Nesterov [7] outlined six variants of subsequent work, with which one must agree:

1. The crest well yielded water from sandy near-contact rocks. The structure is water-bearing; further drilling is discontinued.

2. The crest well yielded water from permeable rocks of the weathering crust or basement. The structure is water-bearing; further drilling is discontinued.

3. The crest well yielded oil or gas from sandy horizons at the base of the Mesozoic. Subsequent wells are drilled on the flanks near the contouring uplifted isohypse, according to the usual method for stratiform crest deposits. It is recommended that the second well be drilled on the gentler flank in order to detect more rapidly the water–oil contact, whose position is assumed.

4. The crest well penetrated clayey deposits in the basement rocks. During testing, insignificant inflows of gas were obtained. Further drilling on such an area is continued according to the method set forth in item 3.

5. The crest well is “dry.” In this case the second well is placed at the presumed gas–water, or oil–water, contact. If water is obtained in the second well, then the third well is placed structurally higher.

6. The crest well penetrated clayey deposits at the contact with the Paleozoic, and during testing insignificant inflows of water were obtained. If the quality of the test is not in doubt, then drilling is carried out further according to the method set forth in item 5.

In our view, a very substantial shortcoming of the method described (despite all its positive qualities) is the very imprecise determination of the location for drilling the second well—as the “presumed gas–water or oil–water contact.”

On the basis of an analysis of the specific position of the water–oil contact in structures already studied, we propose to specify more precisely the location of the second well, which will introduce a certain clarity into the methodological procedures in exploratory drilling that are decided by the geological service during drilling after the work program has already been approved.

In studying the dependence of the height of oil or gas pools, we established a dependence of the degree of filling of structures on their amplitude and on the type of pools. The calculation was carried out for 15 areas well illuminated by drilling. Characteristic is the filling of structures with crest stratiform pools to 60–90% of their amplitude, and the filling of structures with lithological types of pools to 40–60% of the amplitude of the uplifts (Fig. 3).

By the filling of structures we mean the ratio of the value of the pool height in meters to the value of the amplitude of the uplift in meters. The ratio is expressed as a percentage and may be called the coeffi-

Fig. 3. Relationship between the filling of local structures of the West Siberian Lowland and the amplitude of the structures and the type of pools. Compiled by V. L. Kokunov. Symbols: I — anticlinal sheet pools; II — lithological pools; 1, 2, 3, 8, 9 — pools of the Ust-Balyk field; 4, 10, 11 — pools of the Megion field; 12, 13, 14, 15, 16 — pools of the Bystry field; 5 — pool of the Vatinsk field; 6 — pool of the Trekh-Ozernoe field; 7 — pool of the Ubinsk field.

by the percentage of filling of the structures. Third-order structures were taken into calculation for the study.

We propose that in areas where lithological pools are distributed—by which we include the Berezovo-Shaim, Krasnoleninsk, and Tomsk oil-and-gas-bearing regions of the lowland (with Jurassic deposits in mind)—the second and subsequent wells should be located taking into account an expected coefficient of structure filling of 40–60%. When carrying out deep exploratory drilling in Cretaceous deposits, where anticlinal sheet pools are mainly distributed, the second and subsequent wells should be sited taking into account an expected coefficient of structure filling of 60–90%.

REFERENCES

1. N. P. Budnikov. Main results of geological exploration work for oil and gas in the USSR in 1964 and tasks for 1965. Geologiya nefti i gaza, 1965, No. 4.

2. G. P. Vdovykin et al. On the isotopic composition of carbon in natural combustible gases. Geokhimiya, 1964, No. 1.

3. L. E. Kazakov, N. I. Nesterov. Effectiveness of geological-prospecting and exploration work for oil and gas in the West Siberian Lowland. Geokhimiya nefti i gaza, 1965, No. 2.

1. A. M. Karametov et al. On the question of a geochemical anomaly in the territory of the Shaim oil-and-gas-bearing zone of Western Siberia. Scientific and Technical Collection on Oil Production. Proceedings of the All-Union Petroleum and Gas Research Institute, issue 25, 1964.

2. P. A. Levshunov. The content and differentiation of heavy hydrocarbons of the Berezovo gas-bearing region. Geology and Geophysics of the Siberian Branch of the USSR Academy of Sciences, No. 4, vol. 104, 1960.

3. N. N. Rostovtsev. Experience in compiling prognostic maps for oil and gas for the territory of the West Siberian Lowland. Bulletin of Scientific and Technical Information of SNIIGGiMS, Novosibirsk, 1961.

4. N. N. Rostovtsev, I. I. Nesterov. Methodology of prospecting and exploration work for oil and gas. Nedra Publishing House, 1964.

5. V. A. Sokolov, G. G. Grigoriev. Methodology and results of gas geochemical oil-and-gas prospecting work. Gostoptekhizdat, 1962.