INTENSITY OF STRUCTURES AND SOME REGULARITIES IN THE DISTRIBUTION OF OIL AND GAS POOLS UNDER THE CONDITIONS OF TOMSK OBLAST

G. I. TISHCHENKO, V. N. ROSTOVTSEV

Local structures of the third order, which serve as traps for oil and gas pools, are the result of differentiated movements. The intensity of such uplifts is expressed quantitatively by the ratio of the amplitude (for the horizon with the maximum amplitude) to the area of the structure along the same horizon within a closed contour [4].

The term “intensity of structures” was first proposed by V. S. Lazarev. A number of researchers (G. P. Evseev, N. M. Kruglikov) identify the intensity of structures as one of the principal criteria for evaluating oil- and gas-bearing zones [4].

Under the conditions of Tomsk Oblast, as throughout Western Siberia, inherited-type structures predominate, i.e., the surface of the basement is characterized by the maximum amplitude. In the present work, structures with maximum amplitude along horizon P were analyzed, as the most traceable horizon and therefore one that makes it possible to use a larger amount of material.

The main objective of the study was to reveal regularities in the distribution of local structures according to the degree of their intensity, as well as oil and gas pools of horizon Yu-1 confined to these traps.

The calculated values of structure intensity were divided into intervals of 0.5 m/km², and on their basis the weighted-average oil reserves of horizon Yu-1, the average values of the specific surface of fractures in rocks of the Bazhenov + Georgievka formations, and the number of structures with commercial and noncommercial reserves, as well as barren structures, were calculated. Then the following were plotted: a graph of the distribution of oil reserves as a function of structure intensity; a graph of the distribution of the specific surface of fractures for structures of different intensity; and a graph of the distribution of different types of structures according to the values of their intensity. Data on the specific surface of fractures were used from the published works of K. I. Mikulenko and G. G. Shemin [2].

The distribution curve for the first type of structures shows that half of the fields are confined to structures with an intensity of up to \(1.0\ \text{m/km}^2\) (58% of all structures). The remaining uplifts belong to structures with an intensity of \(1.0\text{--}3.0\ \text{m/km}^2\). Structures of higher intensity contain only two pools in Jurassic deposits. Attention should be paid to the considerable percentage of structures with pools in the intensity zone up to \(1\ \text{m/km}^2\). The largest reserves are also concentrated in this zone. The second curve characterizes a somewhat different distribution. If the maximum point of the first curve corresponded to an intensity value of \(0.75\ \text{m/km}^2\), here its highest value is associated with an intensity of \(1.25\). In addition, 70% of all structures fall within the zone \(1.5\text{--}3\ \text{m/km}^2\), and 1% of structures are confined to an intensity from 3 to \(4\ \text{m/km}^2\).

Thus, on the basis of these two curves, one can outline a zone of structural intensity unfavorable for accumulations of oil and gas. These intensity values begin somewhere at \(1.5\ \text{m/km}^2\) and higher. As for structures that proved to be empty when drilled, the following picture emerges here: 66% of all structures fall within the intensity zone \(1.5\text{--}5\ \text{m/km}^2\). Their somewhat inexplicably increased number in the interval \(0\text{--}0.5\ \text{m/km}^2\) is observed, but apparently other geological factors are of greater importance here in the formation of the pool.

We attempted to explain the above-described distribution of structures with the aid of the fracturing of the clayey seal, i.e., from the point of view of the possibility of preserving the pool. It is known that, given a favorable lithological composition of seals and sufficient thicknesses, vertical migration is provided by disjunctives—rupture disturbances—and by fracturing of rocks. The role of fracturing in these processes is significant because of the wide areal distribution of fractures [2]. Up to intensity values equal to \(2\ \text{m/km}^2\), the magnitudes of the specific surface of fractures increase. Thereafter these values decrease, and at an intensity of \(3\ \text{m/km}^2\) the specific surface of fractures decreases sharply. Moreover, the intensity interval from \(2\ \text{m/km}^2\) and higher accounts for 62% of empty structures, 25% with noncommercial inflows, and 27% with commercial pools.

A visual inspection of the fracturing values for structures with commercial pools (the data are not given in the paper), with noncommercial pools, and empty structures shows that the number of its minimum values falls on type III structures. Types I and II are characterized by rather high fracturing values. Such a variant cannot be accidental and must have some explanation. V. S. Lazarev distinguished three types of structures according to the degree of their intensity: \(0\text{--}1\ \text{m/km}^2\)—plicative; \(1\text{--}3\ \text{m/km}^2\)—intermediate (plicative-disjunctive); and greater than \(3\ \text{m/km}^2\)—disjunctive structures [4].

The decrease in the fracturing values of the seal with increasing structural intensity is apparently connected precisely with the fact that the rupture processes taking place relieve tectonic stress from all parts of the structure. The formation of fractures is arrested far from the rupture site. Therefore, the specific surface of fractures measured here is insignificant, although the monolithic character of the seal is disturbed at some place in the structure.

Structures located in the intensity zone up to \(1\ \text{m/km}^2\) contain significant oil reserves. This is a zone where the specific surface of fractures has not yet reached its maximum value.

At the same time, in the intensity zone from 1 to 2 m/km², the specific surface area of fractures reaches its highest values, and then there is a decline in the curve reflecting the distribution of fracturing as a function of structural intensity. Confirming the idea that, in this interval, the preservation of the pool is disturbed, oil reserves decrease here. Structures located in the intensity interval greater than 3 m/km² generally have pools not exceeding, on average, 3 million tons.

As for gas-condensate fields, the largest of them, Luginetskoye and Myldzhinskoye, are confined to structures whose intensity is 0.3–0.8 m/km². The fields of medium reserves—Ostaninskoye, North Vasyugan, and Kazanskoye—are also confined to structures with an intensity of no more than 1 m/km².

On the basis of the foregoing, it may be concluded that the most favorable conditions for the preservation of oil and gas pools are possessed by structures with an intensity, along horizon IIa, of no more than 2 m/km².

For large and medium, especially gas, pools, structures with an intensity of up to 1 m/km² are more favorable.

REFERENCES

1. Belousov V. V. Basic Problems of Geotectonics. Moscow, Gosgeoltekhizdat, 1962.

2. Mikulenko K. I., Shemin G. G. Fracturing of Jurassic and Lower Cretaceous rocks of the Ob–Irtysh interfluve. Transactions of SNIIGGiMS, issue 132. Novosibirsk, 1972.

3. Ryzhov P. A., Gudkov V. M. Application of Mathematical Statistics in Mineral Exploration. Moscow, Nedra Publishing House, 1966.

4. Conditions of Formation and Patterns in the Distribution of Oil and Gas Fields. Moscow, Nedra Publishing House, 1967.