ON THE QUESTION OF THE RATIONAL PLACEMENT OF EXPLORATORY BOREHOLES IN THE EXPLORATION OF COAL DEPOSITS IN KUZBASS

V. Ya. Koudelny

(Presented by the departments of mineral deposits, mineralogy and geology, and exploration of mineral deposits)

Industry is developing ever newer districts of Kuzbass. Prospecting and exploration work carried out in the Kemerovo, Tom–Usinsk, and Mrassk districts, and the subsequent mining operations at the Berezovsk, Biryulinsk, Olzherassk, and Tomsk deposits, made it possible to study more fully the features of the geological structure of the structures known as the Biryulinsk and Main monoclines. Comparison of exploration results with mining data in the mine fields within these structures made it possible to determine the reliability of exploration data and to resolve, on a better substantiated basis, the question of the expedient location and density of the exploration grid.

Under Kuzbass conditions, the principal factor determining the method of exploration is the degree of tectonic disturbance of the deposit. In each specific case, the arrangement of exploratory workings is determined by the combination of various forms of plicative and disjunctive disturbances, their type, size, number, and so forth. From this point of view, deposits situated within monoclines are no exception.

At the first stages of studying the deposits within the Biryulinsk and Main monoclines, an impression arose of the exceptional simplicity of the structure of their tectonic forms. This gave E. M. Pakh and E. M. Senderson [6] grounds to distinguish an independent type of deposits—gentle monoclines with two subtypes: Biryulinsk and Usinsk. They also indicated that such deposits should be explored by lines of boreholes, and recommendations were given for choosing the distances between lines in order to obtain reserves of specified categories.

Subsequent mining operations in the fields of the mines “Berezovskaya 1,” “Tom–Usinskaya 1–2,” “Tom–Usinskaya 5–6,” “Tomskaya,” and several others showed that geological exploration work had determined the general structure of the mine fields with sufficient accuracy. At the same time, many details of the tectonics either proved to have been completely undetected or received an incorrect interpretation, which substantially complicated the mining of many sections. This was convincingly shown by R. G. Kolomin, using the materials of the “Berezovskaya-1” mine as an example [2].

The structural features of the fields of the Krokhalevsk, Biryulinsk, and Berezovsk mines were shown by us in describing certain regularities

tectonic structure of the Prikuznetsk-Alatau subzone of monoclinal occurrence of rocks [3]. A similar picture is also observed in the fields of the mines of the Main Monocline in the Tom-Usa district of Kuzbass.

The coal-bearing deposits of the Kolchugino and Balakhonka series, composing the Main Monocline, occur in the form of a gentle arc, with its concavity facing inward toward Kuzbass. The average azimuths of dip of the coal seams vary from 290° in the northeastern half of the arc to 325° in its southwestern part. The angles of dip within the entire monocline remain comparatively persistent, gradually decreasing toward the southwest from 30–35° to 5–10°. In individual areas, in places where flexures and additional folds are developed, they change even over short distances.

The homoclinal structure of the Main Monocline is rather substantially complicated by a number of additional folds and faults of various morphology and orders. Gentle waviness of the coal seams is widely developed. The orientation of the additional folds in almost all cases proves to be diagonal with respect to the strike of the monocline. Their axes plunge at an angle of 3–5° along an azimuth of 0–30°.

Fault dislocations are represented mainly by three systems. The displacers of the most numerous system have a gentle (up to 15°) dip to the east-northeast. The displacers of the second system dip to the northwest at angles of 10–15°. The orientation of the axial lines of folds and of the lines of intersection of seams by displacers in the plane of the seams proves to be the same.

The third system of faults is represented by transverse steeply dipping strike-slip faults. The lines of intersection of the seams by the displacers of this system coincide with the azimuths of dip of the coal-bearing deposits and turn out to be parallel to the exploration lines laid out across the strike of the rocks of the Main Monocline. All these features of tectonics created the distinctive character of the monocline structures, the insufficient study of which was the cause of discrepancies between exploration materials and mining data.

Exploration work in the mine fields of both monoclines reveals, with insufficient precision, the additional folding and waviness of the coal seams. Fault dislocations in many cases were incorrectly correlated, being classified as longitudinal thrust faults (after M. A. Usov), while such forms of rupture as transverse strike-slip faults were not established at all. In the course of exploration it proved possible to establish only those fault dislocations that had displacement amplitudes greater than 8–10 m. Smaller faults were recorded only in those cases when, directly in boreholes, duplication of coal seams or their disappearance from the section was discovered, as a consequence of the formation of gaping. Sometimes small fault dislocations were noted by zones of crushing of rock and coal. Various criteria were used to establish such zones.

It is known that the requirements for the reliability of materials obtained in carrying out exploration work are continuously increasing. This fully applies also to the study of the tectonics of areas being explored. Still greater importance must be attached to the study of tectonics. This has been convincingly shown by K. V. Mironov [5].

The raising of requirements for the completeness and reliability of structural constructions and, at the same time, the presence of facts of incorrect or insufficient decoding of tectonic structures in the mine fields of the Biryulinskaya and Main Monoclines compelled us to undertake an analysis of

causes of discrepancies between exploration results and operating data.

The causes of discrepancies may be explained either by an incorrect choice of technical means of exploration, or may be connected with the location and density of exploratory intersections. Finally, they may arise from poor-quality geological documentation of exploratory workings and boreholes and, consequently, poor-quality preparation of summary geological documents.

The last reason was not immediately confirmed. Exploration work within both monoclines was carried out by experienced teams of coal explorers, using a variety of methods and techniques for obtaining the fullest and most reliable possible geological and other information from mine workings and boreholes.

In carrying out exploration work, quite modern technical means were used. The principal means of exploring coal deposits in Kuzbass are exploratory boreholes. The technical equipment of drilling operations is being rapidly improved. The reliability of geological information obtained from boreholes is substantially increased through the use of various types of logging, which from a control method has become an important method for exploring coal deposits. Where necessary, in order to solve a number of problems, heavy mine workings such as inclines and adits were used. In the exploration of some mine fields, small mine workings—test pits and shallow shafts—were sunk in insufficient volume, but this cannot explain the discrepancies in the interpretation of elements of the geological structure that are observed when comparing exploration and operating materials.

The principal cause of these discrepancies, in our opinion, should be sought in the density of the exploration grid, more precisely, in the mutual arrangement of exploratory intersections. Exploration of mine fields within the monoclines under consideration was carried out, for the most part, before 1961, that is, before the appearance of the new instruction of the State Commission on Mineral Reserves (GKZ) [1].

Quite recently, the instructions of the All-Union Commission on Mineral Reserves (VKZ) (1942 and 1954) provided generalized groupings of coal deposits and recommendations on the density of the exploration grid. Practice has shown that these groupings are too schematic to serve as a sufficient basis for the density of the exploration grid required for reliably establishing the tectonics and other conditions of occurrence of coal seams. In Kuzbass, its own grouping of coal deposits by structural and other features has been created, generalizing geological materials on individual deposits of the basin, as well as practical experience in exploration and operation [6].

Exploration of the mine fields under consideration was carried out by core-drilling boreholes, placed along exploration lines across the strike of the coal-bearing deposits. The distances between exploration lines were within 500–750 m, and between boreholes on the lines varied from 100 to 900 m. Subsequent supplementary exploration did not introduce substantial changes into ideas about the tectonics of the mine fields. Supplementary exploration was usually carried out either by drilling boreholes along already existing exploration lines, or along intermediate lines located between the lines of detailed exploration.

Comparison of the materials of detailed exploration and supplementary exploration shows that the usual densification of the exploration grid is not accompanied by substantial refinements of the tectonics of mine fields. The latter were obtained only by preparatory and development mine workings. The comparatively small dimensions of additional folds and fault displacements

faults, the distinctive orientation of the axes of folds and the lines of truncation of seams by displacers, as well as the combination of disjunctive disturbances of different types, creates conditions under which the correlation of structural elements along exploration lines spaced apart at distances exceeding the dimensions of the structures along strike becomes multivariant.

Structural constructions are also difficult along exploration lines. Disjunctive disturbances have often been interpreted as additional folds. Individual intersections of thrusts were correlated as steep reverse faults. Transverse shifts were not even recorded by exploration. Gentle disjunctive disturbances with different directions of dip of the displacer planes were correlated with one another, because of which a false impression arose of the plicative character of some thrusts.

An analysis of the causes that led to the incorrect interpretation of structures, and a study of the regularities of the tectonic structure of the mine fields of the monoclines, have led us to the conclusion that the discrepancies between the results of exploration and operation are caused not by insufficient density of the exploration grid.

In recent years, in the exploration of coal deposits, a transition has been made almost everywhere to a denser network of exploration workings. Naturally, this is associated with an increase in the costs of exploration of mine fields. It seems to us that densification of the exploration network will not, under all conditions, ensure a corresponding increase in the completeness of the study of tectonics. It has already been noted above that supplementary exploration of mine fields within monoclines, carried out by densifying the exploration network, did not introduce any substantial refinements of the structures. A more complete study of the tectonics of coal deposits of this type can be achieved with a different arrangement of exploration boreholes.

It is known that exploration of mineral deposits is reduced to the creation of a system of sections, the comparison of which makes it possible to study many features of the geological structure and to solve practical problems. To obtain sections, exploration workings are arranged either along exploration lines or in a grid.

Under the conditions of the deposits under consideration, the arrangement of exploration boreholes according to a regular geometric grid is inexpedient. Taking into account the inclined occurrence of the coal seams and the linearity of the structures, the arrangement of boreholes along exploration lines should be recognized as entirely justified. This is also confirmed by the entire experience of exploration of deposits with linear folds complicated by one or another number of tectonic breaks.

In the mine fields of the Biryulinskaya and Glavnaya monoclines, exploration was carried out by successive densification of exploration lines. In the transition from one stage of exploration to another, the distance between lines was essentially reduced by half. This achieved a uniform arrangement of exploration boreholes, but, in our opinion, this also proved to be the cause of many shortcomings.

Densification of exploration lines by a factor of two in the transition from one stage to another is carried out without sufficient grounds for doing so. A uniform arrangement of exploration intersections with varying complexity of individual parts of mine fields naturally leads to violation of the principle of equal reliability [4]. For the exploration of more complex areas, a denser arrangement of exploration intersections is also required.

With a uniform arrangement of exploration boreholes to the full depth of exploration (350—400 m) and insufficient tracing of the sea—

of coal seams at the outcrops by means of mapping boreholes and shallow mine workings, a weak degree of study of the near-surface parts of the coal seams is observed. The consequence of this is the well-known situation in which the share of reserves of the higher categories sharply increases toward the lower horizons. On the horizons of first-priority mining, the percentage of reserves of categories A+B proves to be very low.

Clarification of the position of the outcrops of coal seams beneath the overburden and the raising of reserve categories on the upper two horizons are often the main tasks of stripping exploration. Meanwhile, these tasks should be solved at the stage of detailed exploration.

As was shown above, under the existing arrangement of exploratory boreholes there is insufficient study and incorrect interpretation of additional folds and tectonic ruptures. It seems to us that these shortcomings can be avoided if, in exploring deposits of this type, the arrangement of exploratory boreholes along converged—or, more precisely, paired—exploration lines is used. The essence of the proposed method is as follows.

At the stage of preliminary exploration, one or two exploration lines should not be single, as is now the case, but should be accompanied by parallel lines located at a comparatively short distance from the main lines. Mutual correlation of sections along paired lines will make it possible to obtain reliable geological material that more fully characterizes the types and degree of manifestation of plicative and disjunctive disturbances.

The use of paired lines in the general case should not be accompanied by an increase in the number of boreholes at this stage of exploration. This can be achieved by increasing the distances between individual exploration lines.

At the stage of detailed exploration, the number of paired lines increases. This occurs, first of all, through the laying out of new exploration lines near those preliminary-exploration lines along which more complex tectonics are noted or elements of multivariance occur. Analysis and joint comparison of the materials from paired and single exploration lines will make it possible to concretize the regularities of the tectonic structure both of individual sectors and of the entire mine field as a whole.

Taking into account the regularities obtained, the next stage of detailed exploration should be the drilling of boreholes along intermediate exploration lines. The fundamental difference between these works and those carried out up to now is that the intermediate lines are not the result of doubling the exploration grid. The locations of their placement, as well as the number of lines, are determined on the basis of the specific geological setting revealed by the preceding work.

A second distinction is that the boreholes of the intermediate lines should be predominantly shallow and should refine the geological setting in the upper two levels. They will also contribute to obtaining reserves of higher categories. The concluding phase of detailed exploration should be the drilling of single deep structural boreholes. Exploration of coal deposits with the proposed arrangement of exploration traverses should be based on a careful tectonic prognosis. The latter should be used both in selecting the sites for laying out paired exploration lines and, especially, in the placement of intermediate lines. Broad use of the method of analogy is also recommended.

Tectonic prognosis and the method of analogy, in particular, may help in solving the question of the distances between the main and auxiliary-

... exploratory lines. Their application can be illustrated by the example of exploration of mine fields of the Main Monocline.

Mining operations here have confirmed a quite regular alternation of zones with significant tectonic complications and disjointed sectors, where additional folds and faults are practically absent and where only gentle waviness is observed. The width of the complicated zones reaches 100 m. The distances between zones are in the range of 300–500 m. Proceeding from these conditions, the distances between the lines of the paired system obviously should not exceed 50 m, while the distances between paired and intermediate lines cannot be greater than 300–500 m. In each case, the distances must be refined with account taken of the specific situation.

It may appear that the proposed method of placing boreholes will entail a sharp increase in the volume of core drilling during exploration. Experimental testing of this assumption on models of explored mine fields shows that this fear has no basis. The number of boreholes generally increases. This occurs as a result of drilling shallow boreholes to evaluate the upper horizons along intermediate exploratory lines.

The number of boreholes along the main and auxiliary paired lines is not the same. Along the main lines it is greater than along the auxiliary lines, on which the forms and orientations of structures identified in the closely spaced section are refined. On paired lines, boreholes may be multilateral, which can substantially reduce the volume of core drilling. The technology and technique of drilling such boreholes have been sufficiently developed [7].

The new method of placing exploratory boreholes, in our opinion, will contribute to a more complete study of the tectonics of the areas being explored. At the same time, such structural elements (with diagonal lineation) will be identified and properly correlated which, with the usual arrangement of exploratory lines, either are not established or are interpreted incorrectly. The reliability of structural constructions can be increased by using the favorable conditions for core-metric studies and for conducting geophysical work in clusters of closely spaced boreholes.

In many cases in Kuzbass, mine fields are located within the limbs of large folds. Questions of studying their tectonics acquire primary importance, since the limbs are significantly disturbed. Exploration of such fields is still more complicated. The proposed method of placing exploratory boreholes under these conditions may prove rational and may contribute to a more complete study of the geology without increasing exploration costs.

REFERENCES

1. Instructions for Applying the Classification of Reserves to Deposits of Coals and Combustible Shales. Gosgeoltekhizdat, 1961.

2. R. G. Kolomin. Comparison of data from exploratory and mining operations at the Berezovskaya-1 mine in Kuzbass. Razvedka i okhrana nedr, No. 7, 1962.

3. V. Ya. Koudelny. Certain regularities of the tectonic structure of the Kemerovo district of Kuzbass. Izv. TPI, vol. 135, 1965.

4. V. M. Kreiter. Prospecting and Exploration of Mineral Deposits. Part 2. Gosgeoltekhizdat, 1961.

5. K. V. Mironov. More attention to the study of the tectonics of coal deposits. Razvedka i okhrana nedr, No. 9, 1966.

6. E. M. Pakh, E. M. Senderzon. Density of the detailed-exploration network and testing of coal deposits in Kuzbass. Questions of the Geology of Kuzbass, vol. 2, Izv. TPI, vol. 99, 1959.

7. S. S. Sulakshin. Regularities of Curvature and Directional Drilling of Geological-Exploration Boreholes. Nedra, 1966.