ON ACCOUNTING FOR COAL RESERVES IN THE FLANKS OF TECTONIC FAULTS

V. Ya. Koudelnyi

(Presented by the Department of Geology and Exploration of Mineral Deposits)

As a result of a detailed study of the tectonics of the Prokopyevsk–Kiselevsk district of Kuzbass, it proved possible to establish the spatial and genetic connection between a dense network of small tectonic faults and large ones. On this basis, a conclusion was drawn regarding the low reliability of the calculated reserves of high-category coals in structurally complex areas [1]. It was proposed that, in the flanks of large tectonic faults, zones should be distinguished in which coal reserves are to be classified no higher than category $C_1$. At the same time, the question was raised of forecasting the tectonic structure of mine fields on the basis of exploration materials, taking into account the regularities revealed [2]. The necessity was also emphasized of a closer linkage between exploration geology and mine geology.

Further study of these questions in the Anzhero-Sudzhensk and Kemerovo districts of Kuzbass has confirmed the correctness of the conclusions drawn and makes it possible to specify the question of accounting for coal reserves in the flanks of large tectonic faults.

It is known that, during exploitation, at the places where seams are truncated by tectonic faults, portions of seams of varying width remain untouched. The study of such “throwaway” portions of seams shows that very often the coal here is strongly crushed, slickensided, and extremely unstable, acquiring the character of “quicksands.” As a rule, in such places its ash content sharply increases. The driving of mine workings at the points where seams are truncated by faults is associated with great difficulties because of increased rock pressure, which puts the support out of operation; as a result, the cross section of the workings cannot be maintained, and collapses are frequent. Even in the case when the coal at the truncation preserves its structure, more intensive fracturing of the coal and of the enclosing rocks is observed, and a large number of small faults appear, which also creates difficulties in carrying out mining operations.

Ultimately, all this leads to the fact that such portions are not mined, and therefore the actual coal reserves in reality prove to be smaller than those established by exploration. The latter may also be connected with the fact that on structural maps constructed on the floor or roof of coal seams for the calculation of reserves, the truncation of a seam by a dislocation is not a boundary of the workable thickness of the seam.

It has been noted that the angle between the planes of the seam and of the fault plane for concordant longitudinal faults is, in the overwhelming majority of cases, acute and does not exceed 15–20°. At the faces, in such cases, portions of seams of nonworkable thickness are observed. The width of such portions depends on the dihedral angle between the planes of the seam and the fault plane and may reach 5–10 m. At the Severnaya mine of the Kemerovo district, a case was noted in which, during the driving of a heading, the Kemerovsky seam over a distance of more than 40 m had no workable thickness, since it had been cut off from the floor side by a direct thrust fault G.

The width of the seam strip not worked at the faces, as the study of factual material shows, does not remain constant and depends on a whole series of factors. The principal ones are:

1. The amplitude of the tectonic fault. Study of materials on the mining of coal seams shows that the greater the amplitude of the fault, the greater the width of the strip left at the faces during exploitation. However, this conclusion is not indisputable, since it may fail to reflect the true situation. The point is that this factor must be considered in close connection with other factors and, in particular, with the type of fault.

2. The type of tectonic fault. A large body of factual material convincingly shows that in the flanks of discordant faults more extensive zones of crushing of rock and coal are observed, which, naturally, cannot but affect the width of the coal-seam strip left at the fault. It is interesting to note that if, in the flanks of large concordant faults, small faults are uniform and in type analogous to the large one, then in the flanks of discordant faults they are distinguished by great diversity. In this connection it is not uncommon to observe a situation in which the zones of small faults even at small discordant faults are considerably more complex and larger than at larger concordant faults.

The comparatively slight development of discordant faults in the Kemerovo district did not allow us to determine more precisely the width of the coal-seam strip left during mining. In this respect, concordant faults have been studied considerably better.

1. For concordant (as also, probably, for discordant) faults, a clear dependence is established between the width of the strip left and the angle at which the seam is intersected by the fault plane in plan, i.e., on whether the tectonic fault is diagonal, transverse, or longitudinal (Fig. 1).

On the basis of a study of the mining of coal seams at faces and from measurement data on mine-surveying plans, it has been established that for medium-sized faults (with amplitudes of several tens of meters) identified in the Kemerovo district, the width of the strip left varies from 5 to 50 m. The greatest width of this strip is observed at diagonal faults, which is associated both with geological conditions and with mining systems. Its average value proves to be 25 m. In second place are transverse faults. At seam faces cut by transverse faults, a strip of coal remains with an average width of 15 m. Finally, at longitudinal concordant faults, the average strip width is 10 m.

The figures given above are valid only when the tectonic faults are isolated. However, closely spaced faults are frequently observed. In this case the width of the strips left increases, and sometimes the coal reserves of such portions are written off entirely.

It seems to us that the time has come to raise the question of special accounting for coal reserves in the flanks of tectonic faults. Apparently,

such reserves should be assigned no higher than category C₁, should not be taken into account in determining the industrial reserves of mine fields, and should be considered only as possible reserves. Clearly, in this direction it is necessary to continue the study of factual materials, to generalize the materials of the mine geological service, and to seek improvement in forecasting when generalizing exploration materials. A detailed study of the core from exploratory boreholes should also play a significant role.

Fig. 1. Mining of coal seams near tectonic faults (a—diagonal, б—transverse, в—longitudinal):
1) truncation of the seam by a tectonic fault; 2) mined-out sections of the seam.

REFERENCES

1. Belitskii A. A., Koudelnyi V. Ya. On the reliability of calculated reserves of high categories in complexly dislocated areas of Kuzbass. Exploration and Protection of Mineral Resources, No. 6, 1958.

2. Belitskii A. A. Toward the development of a method for forecasting the faulting of mine fields in Kuzbass. Proceedings of the Tomsk Polytechnic Institute named after S. M. Kirov, vol. 99, Problems of the Geology of Kuzbass, vol. 2, Tomsk, 1959.