NEW DATA IN THE GEOLOGICAL INTERPRETATION OF MATERIALS FOR ESTIMATING COMMERCIAL OIL AND GAS RESERVES IN FIELDS OF THE TOMSK REGION

Yu. V. Naidanov

(Presented by the Department of Combustible Minerals)

The completed set of geological-exploration works should provide an assessment of the general prospects for the oil-and-gas potential of a field over its area and at depth, and should identify and prepare for commercial development the most productive deposits and those with the largest reserves [5].

The question of the methodology of commercial exploration, and of systems for arranging exploratory wells, is closely connected with the rational development of oil and gas pools. Some researchers give preference to ring and triangular systems of well placement [1]; others are inclined to the view that the most rational is a profile system of well placement [4, 6, 8].

In works devoted to problems of the economic efficiency of exploratory drilling for oil and gas, questions are raised concerning capital expenditures for exploration of 1 ton of oil reserves and 1000 m³ of gas [2]. The most important indicators of a rational exploration methodology should be considered to be the use of the minimum permissible, under the given specific conditions, number of wells for obtaining a sufficient amount of reliable data necessary for the correct assessment of reserves and for the selection of a justified development system [7].

When choosing a system for placing exploratory wells on discovered oil and gas pools in the regions under consideration, the influence of at least five factors was taken into account:

1) the size of the pool;
2) its shape (isometric, elongated, etc.);
3) the established petroleum-bearing stage;
4) the complexity of the geological structure;
5) the heterogeneity of the reservoir rocks.

The size of the pool, with other favorable factors, makes it possible, as a first approximation, to outline the most probable number of wells for preparing the field for reserve estimation and for development. In doing so, the recommendations of the State Reserves Commission (GKZ) concerning the reliability of estimated parameters and the obtaining of a productive characteristic of the bed are taken into account (testing of different lithological intervals, testing of the entire bed, etc.). The table below gives certain data on the degree of exploration of fields in the Sredne-Vasyugan and Pudino districts, the reserves of which have been approved by the GKZ.

Apparently, in the region being described, at the stage of preparation for development, fields should be recognized as explored when the specific area is 10–13 km² per well. It is understood here that the field is not disrupted by disjunctive tectonics.

Table

The shape of a reservoir chiefly affects the choice of the system for arranging exploratory wells.

Thus, for example, with a linearly elongated reservoir shape (Sredne-Nyurolskaya, Myldzhinskaya, and others), a profile system of well placement was chosen as the principal one. However, at the Myldzhinskoye field, when drilling out the central part, it became necessary to abandon a purely profile system and apply a system of uniform distribution of wells over the area (an areal system), which in this case provides the greatest amount of information and most fully meets the requirements for preparing commercial reserves. Areal placement of wells is especially rational in exploration of gas-condensate fields that have an oil rim, since in this case each productive well bears an additional load: the study of the oil rim and of gas-condensate content (Luginetskoye, Myldzhinskoye fields). However, under our conditions it is not always possible to maintain the required distances between wells because of the presence of a large number of lakes and swamps.

The oil-and-gas-bearing stage substantially affects the number of exploratory wells when reservoirs of different types occur in the section. For example, the main reservoir in terms of area and reserves at the Myldzhinskoye gas-condensate field is a bedded reservoir of massive type in the Jurassic deposits. Higher in the section (bed B₈), another gas reservoir has been identified, tectonically screened, of lithological type, developed mainly on the eastern flank of the field. To delineate this reservoir, several additional wells had to be drilled. A similar pattern is also observed at the Ostaninskoye field, where drilling of additional wells will be required to study the gas-condensate reservoir in bed M.

In addition, extra footage has to be expended if the overlying reservoir is larger in size than the lower one (Sovetskoye field, beds A₁ and B₈ on the Nizhnevartovsk arch).

The complexity of the geological structure of reservoirs is associated, above all, with the presence of disjunctive faults. The materials currently available indicate that the basement of many local structures is broken into separate, often small blocks, and that disjunctive faults in the cover are a form of manifestation of differential movements of these blocks. Such structures include the Sredne-Vasyuganskaya, Myldzhinskaya, Severnaya, Mezhovskaya, Veselovskaya, and many others [3].

The presence of disjunctive tectonics affects the methodology for placing exploratory wells and the comparability of test results.

Heterogeneity of reservoir rocks—one of their practically most important and most general properties—has long attracted the attention of geologists. Recently, in the study of the geological heterogeneity of beds, a number of directions have emerged, associated with investigations aimed at identifying the nature and degree of heterogeneity with subsequent

its quantitative evaluation and with the improvement of methods for accounting for bed heterogeneity in designing and analyzing development [9].

Despite the considerable number of researchers (M. A. Zhdanov, A. P. Krylov, V. S. Melik-Pashaev, Yu. P. Borisov, and others), there is still no unified point of view on questions of terminology, classification, and evaluation of bed heterogeneity.

By geological heterogeneity of the object under study one should understand any variability in the character and degree of the lithological-physical properties of the rocks composing it over area and section. The heterogeneity of reservoir beds is caused mainly by geological factors, the result of which is a change in the lithology, petrography, and physical properties of the rocks. In characterizing heterogeneity, macro- and microheterogeneity are distinguished.

Macroheterogeneity is manifested in the section by alternation of reservoir rocks with practically impermeable rocks. At the boundary of these lithological varieties, the principal parameters of productive beds change abruptly and discontinuously.

Microheterogeneity reflects structural, textural, and other features of the structure of the distinguished rock. The reservoir properties in this case change more smoothly and continuously.

At present, various methods are used in studying geological heterogeneity; these may be combined into three groups:

a) geological-geophysical;
b) laboratory-experimental;
c) field-hydrodynamic.

Applying the first method, on the basis of logging materials with the involvement of core data, the lithological heterogeneity of the sandy beds of the Tyumen and Vasyugan suites is noted. Of all the heterogeneous beds, the least variability is observed in bed Yu₁ of the Vasyugan suite, which can possibly be explained by its marine and coastal-marine genesis.

The sandy beds of the Tyumen suite, over area and section, are replaced by separate interbeds. In some wells such extensive argillization occurs that the productive bed completely loses its capacity-filtration properties.

Such a significant change in the physico-lithological characteristics of the sandy beds of the Tyumen suite can be explained by their formation under continental conditions, when a sharp change in sedimentation conditions occurred. The absence, at the exploration stage of the field, of laboratory-experimental and field-hydrodynamic data on the character of the geological heterogeneity of productive beds makes its accounting very difficult.

Analyzing the available factual material for fields prepared for development, it is proposed to evaluate geological heterogeneity quantitatively in the following way. The entire cycle of work on quantitative accounting for geological heterogeneity is based on the results of analysis of field-geophysical materials (micro-lateral logging (MBK), micrologging, standard logging), core data, and is divided into three stages:

1. From micro-lateral logging diagrams, bed heterogeneity is taken into account with an accuracy of up to 0.2 m. In the absence of MBK diagrams, micrologging curves are used.

2. On the basis of standard electric-logging diagrams, typical sections of productive beds are identified over the area of the deposit; in the Kelloway-Oxford deposits of the Ob-Irtysh interfluve, six (10) are distinguished.

3. For each well (or group of wells) united by its own

...type of section, all the core recovered from the productive bed is examined. Core recovery very rarely reaches 100%; therefore, from the available core of each group of wells, using electric-log diagrams, a single composite section of a definite type is compiled. In the composite section of the bed thus obtained, all impermeable interbeds less than 20 cm thick are visually distinguished, and their number and total thickness are calculated. The resulting total thickness of all impermeable interbeds is then subtracted from the oil-and-gas-saturated thickness of the bed.

Taking microprobe logging data into account, the oil-and-gas-saturated thickness was determined in the most recent reserve report for the Lukinets field. However, it was not possible to use the proposed method to the end because of the small core recovery from the last drilled wells. In the future we propose to use a similar accounting of geological heterogeneity in calculating oil reserves at the Pervomaisk field.

Brief Conclusions

1. The choice of a system for arranging exploratory wells depends on many factors, the chief of which are the shape and size of the pool; with a linearly elongated shape—a profile system, and with an isometric one—an areal system.

2. The presence or absence of disjunctive disturbances has a considerable influence on the choice of exploration system. Failure to take this factor into account leads to miscalculations concerning the degree of exploration maturity of the field and the estimation of oil and gas reserves (Sredne-Vasyugan field).

3. Geological heterogeneity of productive beds has a substantial influence on the results of the work; accounting for it at the stage of field exploration is not always possible because of the sparse well grid and insufficient core recovery from productive beds.

The proposed method will help to make a quantitative assessment of it and thereby increase the reliability of such a fundamental calculation parameter as oil-and-gas-saturated thickness.

1. Oil and gas fields should be considered prepared for commercial development if the degree of their exploration corresponds to a specific area of no more than 9–13 km² per well.

REFERENCES

1. M. V. Abramovich. On the accuracy of calculating oil reserves by the volumetric method. Izv. AN AzSSR, geological series, No. 5, 1951.

2. M. M. Brenner. Problems of the economics of geological exploration for oil and gas. Sovetskaya geologiya, 1961, No. 10.

3. F. G. Gurari, K. I. Mikulenko et al. Tectonics of the Mesozoic–Cenozoic cover of the West Siberian Plate. Tr. SNIIGGiMS, issue 100, Novosibirsk, 1971.

4. M. A. Zhdanov. Oil-and-gas-field geology. Moscow, Gostoptekhizdat, 1962.

5. Instructions for applying the classification of reserves to oil and combustible-gas fields. Moscow, “Nedra,” 1971.

6. V. S. Melik-Pashaev. On methods of exploring oil fields in platform regions. Tr. VNII, issue 33, Moscow, Gostoptekhizdat, 1961.

7. A. S. Polushkin, V. P. Sanin. On the question of methods for commercial exploration of the Megionskoye field. Geologiya nefti i gaza, 1965, No. 12.

8. M. N. Soson, M. A. Zhdanov et al. Fundamentals of commercial exploration of gas pools for effective reserve preparation. Moscow, “Nedra,” 1970.

9. V. V. Stasenkov, I. M. Klimushin et al. Methods for studying geological heterogeneity of oil reservoirs. Moscow, “Nedra,” 1972.

10. Z. Ya. Serdyuk, N. P. Zapivalov et al. Lithological prerequisites for the oil-and-gas potential of Kelloway–Oxfordian deposits of the Ob–Irtysh interfluve. Tr. SNIIGGiMS, issue 137, Novosibirsk, 1971.