GENERAL PRINCIPLES OF THE METHODOLOGY FOR EXPLORATION OF MINE FIELDS IN THE KARAGANDA COAL BASIN

M. T. SAMSONOV

General information on the coal-bearing deposits of the Karaganda Basin*

The sedimentary formations composing the Karaganda hard-coal basin occupy an area of up to 3000 km² (Fig. 1). They are represented by a thick sequence of Upper Devonian (400 m), Carboniferous (4500 m), and Jurassic (600 m) continental-lagoonal sediments. Within the Carboniferous coal-bearing sequence, 7 conformably occurring series are distinguished, belonging predominantly to the Lower Carboniferous (Tournaisian, Visean, Namurian) and, possibly, to the very base of the Middle Carboniferous:

1) the Akkuduk, or coal-barren, series, 1000 m thick;

2) the Ashlyarik coal-bearing series, 700 m thick, containing up to 26 coal seams and interbeds, of which 16 have workable thickness;

3) the Karaganda coal-bearing series, 700–900 m thick, containing up to 35 coal seams and interbeds, of which 20 seams have workable thickness;

4) the Supra-Karaganda series, containing no coal seams of workable thickness; thickness of the series 500–650 m;

5) the Dolinka series, containing low-ash coking coals in the amount of 20 seams and interbeds, of which 11 have workable thickness; thickness of the series 560 m;

6) the Tentek, or Supra-Dolinka, coal-bearing series, 600 m thick, containing 17 seams of workable thickness;

7) the Shakhan coal-barren series, with variegated coloration of the rocks, 300–350 m thick.

Above them occur Jurassic deposits up to 600 m thick, represented by a sequence of loose conglomerates, pebble beds, sandstones, and shales with seams of brown coals.

The Carboniferous and Jurassic deposits are overlain by Tertiary deposits—clays and sands, the total thickness of which reaches 80 m.

Quaternary deposits are developed mainly in the western part of the basin—the Churubai-Nura district, where they reach a thickness of 18 m. They are represented by water-bearing alluvium of the Churubai-Nura and Sokur rivers.

Structurally, the Karaganda Basin as a whole is a graben, complex in structure and asymmetric in form, bounded on the west, south, and east by major tectonic faults that brought the sedimentary coal-bearing strata of the Car—

Fig. 1. Schematic geological map of the Karaganda Basin.
1 — Mesozoic deposits (for the section); 2 — boundary of the distribution of Mesozoic deposits; 3 — Tentek and Shakhan formations; 4 — Dolinka Formation; 5 — supra-Karaganda Formation; 6 — Karaganda Formation; 7 — Ashlyarik Formation; 8 — Akkuduk Formation; 9 — Terektin, Kassin, Posidoniya beds; 10 — calcareous and sulfiferous beds; 11 — Devonian conglomerate-sandstone sequence; 12 — sandstones, shales, tuffs; 13 — porphyrites and their tuffs; 14 — agglomerates; 15 — albitophyres and their tuffs; 16 — Silurian deposits (porphyrites, tuffs, sandstones, shales); 17 — Upper Paleozoic granite-porphyries; 18 — principal lines of tectonic dislocations; brachysynclines of the Dolinka Formation: I — Dubovskaya, II — Dolinskaya, III — Tentekskaya, IV — Koptakskaya, V — Karagorskaya, VI — Samarskaya.

zone with Silurian and Devonian rocks. The nature of the tectonic structure of the basin is shown in Fig. 1.

As a result of differentiated tectonic movements and erosion, not all of the listed formations have been preserved in individual areas of the basin. Each of them is characterized by its own specific features, which, together with other factors, must be taken into account in organizing exploration.

Owing to the limited scope of the present work, we shall confine ourselves to consideration of questions connected with coal exploration only in the Dolinka Formation, which is distributed chiefly in the western part of the basin—in the new Churubai-Nura district, which in its conditions differs considerably from the developed areas of the basin and requires a special approach to exploration.

Geological features of the Dolinka Formation and certain correlation criteria used in exploration practice

The deposits of the Dolinka Formation in the Karaganda basin and beyond its limits do not have continuous areal distribution, forming separate, isolated structures of the brachysynclinal type—the Dubovka, Dolinka, Tentek, Kolpak, Karagoga, and Samara structures (Fig. 1). These folds are usually bounded by large tectonic disturbances of meridional strike. In the Churubai-Nura district they are almost always overlain by a relatively thick sequence of younger formations—Tertiary and Quaternary deposits—as a result of which the mine fields here have a concealed character.

All exploration work in the Dolinka Formation can therefore be carried out only from the surface by boreholes. The exceptions are small exposures in the western part of the Tentek area, where it was possible to drive several mine workings and to sample the coal seams from them. This largely determines the exploration methodology at all its stages: prospecting, preliminary, detailed, stripping, and operational exploration.

It has been established that the Dolinka area of the Churubai-Nura district and the mine fields located within it (44, 44-bis, 45, 45-bis, 43, 43-bis, 42, 42-bis, etc.) have a block structure, since they are dissected by faults with intersecting directions. In an area with a total surface of 74 km², 78 tectonic disturbances with amplitudes exceeding 15–25 m have been found, of which 18 have an intersecting direction; as a result, the area is divided into 55 independent blocks.

The characteristic types of tectonic disturbances developed in the deposits of the Dolinka Formation are shown in Fig. 2. They are represented by upthrows, normal faults, and thrusts.

Data on tectonic disturbance obtained in the course of exploration by drilling are extremely incomplete. Disjunctive disturbances with amplitudes of less than 15–25 m, as a rule, are not detected by drilling, which is a shortcoming of the latter as a method.

At the same time, in other areas of the Karaganda basin (the Saran area, the southern part of the Churubai-Nura district), as well as in the Kuzbass, it has been established that comparatively large disjunctives are always accompanied by a multitude of smaller disjunctives. Such a regularity is already being established at the No. 2 (22/18) mine under construction in the Dolinka area, which therefore may be assigned to complex structures.

The Dolinka Formation contains up to 20 coal seams and interbeds, of which 11 have workable thickness. In section the latter are distributed unevenly, forming three closely spaced groups:

Fig. 2. Types of tectonic faults developed in the deposits of the Dolinskaya suite

1. Mine field No. 5, VIII exploration line
   Block structure

2. Mine field No. 44-bis, VI exploration line
   Complex imbricate-block structure

3. Mine field No. 44-bis, 43, XXXIV exploration line
   Complex imbricate-block structure

4. Mine field No. 46, 43-bis, XXX exploration line
   Imbricate upthrow–thrust structure

5. Mines Nos. 2, 5, IX–I exploration lines
   Wedge-shaped upthrow horst

6. Mine field No. 22-bis, II exploration line
   Complex wedge-shaped block structure of graben–ridge character
   Nadkaraganda suite

7. Mine field No. 44-bis, I exploration line
   Complex upthrow-block structure

8. Mine field No. 44-bis, IV-b exploration line
   Complex upthrow-block structure

9. Mine field No. 44-bis, V exploration line
   Imbricate upthrow structure

10. Mines Nos. 2, 5, XIV and I exploration lines
    Upthrow–thrust structure

11. Mine field No. 45, VII exploration line
    Complex normal-fault–thrust structure
    Thrust

12. Mine fields Nos. 43-bis, 45-bis, 46, XXXIII exploration line
    Complex upthrow–graben structure

13. Mine field No. 44-bis, VIII-b exploration line
    Complex block structure

14. Mine field No. 44-bis, VII exploration line
    Complex imbricate-block structure

15. Mine fields Nos. 139, 140, 141, XVII exploration line
    Normal fault inherited and developed in Mesozoic deposits

16. Mine fields Nos. 139, 140, 141, XVIII exploration line
    Normal fault inherited and developed in Mesozoic deposits
    Nadkaraganda suite

1. Seams D₁—D₅ constitute the lower coal-bearing horizon, with a total thickness of 80—85 m in the Dolinka area and 145 m in the Dubovka area.

2. Seams D₆—D₈ represent the middle coal-bearing horizon. These seams are separated from the lower horizon by a barren rock sequence 193 m thick in the Dubovka area and 154 m thick in the Dolinka area. The thickness of the sequence containing seams D₆—D₈ is 50—70 m.

3. Seams D₉—D₁₁ constitute the third—the upper—coal-bearing horizon, located higher in the section, above seam D₈, by 80—97 m. The thickness of this part of the section, including seams D₉—D₁₁, is 36—62 m.

Above seam D₁₁, at 65 m in the Tentek area and at 90 m in the Dolinka area, there occurs a horizon of crystalline tuff 2 m thick, which some geologists regard as the boundary between the Dolinka and Tentek formations. We have taken the upper boundary of the Dolinka formation along the floor of seam T₁, which already belongs to the Tentek formation.

When considering sections of the Dolinka formation, it is evident that individual lithological horizons are marked by sharp variability and poor persistence, chiefly because sandstones often pinch out along the strike or gradually pass into siltstones or argillites. The distinguished groups of coal seams (coal-bearing horizons) and the barren sequences between them, however, occupy a fairly distinct stratigraphic position, thereby reflecting the presence of cyclicity in the deposition of sediments of the mesofacial facies complex.

The thick and persistent seam D₆ is an excellent marker horizon for the middle group of workable seams. As regards the entire Dolinka formation, its marker features are: the presence of low-ash clarain-vitrain coals; the predominance of fine-grained rocks composing the formation; the mottled texture of the argillites; a thick bed of channel sandstones with coal and rock detritus of the coal-bearing sequence above coal seam D₅; an interbed of marls with a “cone-in-cone” texture and numerous horizons of siderite concretions; an increased content of thin-bedded rocks with horizontal and oblique bedding, clearly expressed through accumulations of plant detritus arranged along the bedding; and the presence of 10—11 faunal horizons, four of which are represented mainly by phyllopods.

To characterize the persistence of the coal seams, statistical calculations were made of the total number of boreholes—points of intersection of each seam in the Dolinka area, which has an area of 74 km². Separately counted were the points of intersection of seams with nonworkable thickness and those that did not encounter coal seams at all.

In order that the statistical and graphical data obtained should acquire a definite system, the author used the geological-industrial classification proposed by E. O. Pogrebitsky (1956), in which the stability of coal seams is evaluated according to the preservation of the thickness class over specified areas. In addition, for an objective assessment of seam persistence within individual mine fields and over the Dolinka area as a whole, the author applied the persistence coefficient “Kₚ,” which shows the percentage ratio of the number of points of intersection of a seam of workable thickness to the total number of points of intersection of that seam within the area (Table 1).

Data on the areas of washout of coal seams by mine fields of the Dolinka area are given in Tables 2 and 3. In Table 2, calculations are made of the areas of complete washout of seams, and in Table 3—of partial washout, in which the thickness proves to be less than workable.

Table 1

Table 2

Table 3

Fig. 3. Methods for exploration of tectonic faults by boreholes.
1 — Quaternary sands; 2 — Tertiary clays; 3 — coal seams, with a solid line indicating those explored in detail and a dashed line indicating those insufficiently explored; 4 — lower boundary of oxidized coals; 5 — tectonic faults.

A definite dependence is established between “\(K_p\)”—the coefficient of stability—and the category of stability. The “very stable” category (Table 1) includes seams for which \(K_p\) is not less than 90%; “stable,” 90–80%; “relatively stable,” 80–70%; “unstable,” 70–40%; and “of random working thickness,” 40–20% and lower.

The coal seams of the Dolinskaya suite have a comparatively simple structure, with the inclusion of thin partings of sandy-clayey rocks, usually in the lower or middle part of the seam. Coal represented by a significant content of vitrinized components, characterized by clean varieties, as well as by small lenses of essentially structureless vitrain, with endocleavage cracks, is more brittle under friction and crushing.

In the course of drilling, vitrinized components, or vitrinite with index VI according to the new nomenclature approved by the All-Union Conference of Coal Petrographers of the Institute of Mining of the Academy of Sciences of the USSR (1956), are destroyed and washed out by the flushing fluid more quickly than the other components composing the coal seam, as compared with rock partings. As a result, the core brought up from boreholes is always, to some extent, depleted owing to losses of the purest and highest-quality components of the coal, and no longer represents the true characteristics of the seam either in thickness, in its structure, or in quality. In particular, the thickness of coal seams during drilling is understated on average by 0.15–0.2 m in comparison with logging data and with the actual data from sketches of coal seams in mines. The ash yield from borehole cores is 3–4% higher in absolute value than from samples taken in mine workings.

These data are based on a limited number of samples and observations in mine workings; therefore they cannot serve as material for a definitive conclusion about the indicated discrepancies for the seams of the Dolinskaya suite. However, when correcting the results of drilling by logging, the figures cited will be close to optimal. Therefore the quality of drilling work, with the appropriate density of the exploration grid and core recovery within the range of 75–90%, ensures classification of reserves in the higher categories.

The broad and uneven distribution, in terms of thickness, of Tertiary clays, modern water-bearing alluvial deposits, as well as the dissection of the areas where the Dolinskaya suite is developed by the Sokur, Churubai-Nura, Tentek, and Nura rivers, which produce extensive flooding during spring high-water periods, characterize the Churubai-Nura district and the Dubovka and Samara areas as complex in hydrogeological terms.

The Tertiary clays lying directly on the eroded Carboniferous surface in the Churubai-Nura district may serve as a natural aquiclude against the penetration of alluvial and river waters into the mine workings of mines under construction and in operation if the amplitude of collapse (subsidence) of the worked-out space does not exceed the thickness of the Tertiary clays, i.e., if there are no gaping breaks in continuity within the mass of the Tertiary clays. Therefore, the study of the thickness of the Tertiary clays and of their physicomechanical properties within each mine field is an integral part of exploration work, especially at the stages of detailed and stripping exploration.

Stages and Methods of Exploration

As a result of many years of practice in organizing and carrying out geological exploration work in the basin, stages, or more rational phases, of this work have been defined. These include: prospecting exploration, pre-

preliminary exploration, detailed exploration, development exploration, and operational exploration.

The nature of the work performed and the industrial significance of the materials obtained at the various stages of exploration are determined by the tasks assigned to them.

Prospecting Exploration

The task of the prospecting stage of exploration work includes establishing the character of the stratigraphic section, the presence of coal seams, their thickness, structure, depth of occurrence, and the possible areas of distribution of coal-bearing deposits. At the prospecting stage, exploration lines are spaced at a distance of 2000 m. The depth of the boreholes is usually designed with allowance for penetrating the thickness of the strata along the exploration line.

The data obtained at the prospecting stage make it possible to estimate coal reserves in category C₂ and at the same time serve as the basis for designing preliminary exploration and, in cases of especially favorable conditions, detailed exploration. Prospecting materials are also used to supplement data for a geological map at a scale of 1:50000.

Preliminary Exploration

In the course of carrying out the entire complex of exploration work on most mine fields of the Dolinka suite, the stage of preliminary exploration is not always clearly separated in time from the subsequent stage. The completion of preliminary exploration was very often overlapped by the beginning of detailed exploration. The results of work performed at the preliminary-exploration stage were summarized and used in the geological report only after the completion of detailed exploration. This kind of “neglect” of the preliminary-exploration stage resulted from the fact that the time limits for performing work on the industrial-economic assessment of the prospects of the new Churubai-Nura district were extremely compressed.

Exploration work on one or on a group of adjacent areas, usually carried out by a large number of drilling rigs, solved the tasks of two stages simultaneously—preliminary and detailed. The deployment of large-scale exploration work, with 50–80 rigs simultaneously operating on the areas of 2–3 adjacent mine fields, was to some extent justified also by the fact that, in addition to the gain in time in assessing the prospects of the new coal-bearing district, it was economically advantageous for the organization of production as well.

At the same time, the “urgent” character of the exploration work and the “underestimation” of the staged nature of exploration created very great difficulties in the timely and high-quality processing of primary geological materials and in using them for the purposes of rationally directing exploration work.

At present, as earlier under the conditions indicated above, at the preliminary-exploration stage the structure of mine fields is determined; the thickness, quality, and structure of coal seams are established; the stratigraphic and lithological section is refined; and the relationships of Carboniferous deposits with Tertiary clays and modern water-bearing alluvial rocks overlying the Carboniferous here are clarified. All geological conclusions and constructions are made on the basis of lines drilled in detail across the strike, spaced at a distance of 1000 m.

Detailed Exploration

In the practice of exploration of the Dolinka suite there were no cases of negative results at the preliminary stage. Therefore, in the course of detailed

...exploration in areas is carried out by increasing the density of the exploration grids to 500 m.

In tectonically complex parts of areas, for the purpose of tracing individual disturbances or clarifying their character, the exploration grid is made denser by means of intermediate lines of boreholes. The character of the disturbances and the methods of exploring them by boreholes are illustrated in Fig. 3.

The objectives of the detailed-exploration stage are strictly regulated by the instructions of the GKZ, as well as by the regulations of the Ministry of the Coal Industry of the USSR. For the conditions of the Karaganda basin, which belongs to groups II and III, at the detailed-exploration stage the ratio of explored balance reserves of categories A₂ + B must be ensured at not less than 50% of the total reserves of the area.

At the same time, at the detailed-exploration stage, a number of applied questions of geology are studied, such as: the physical and mechanical properties of the enclosing rocks, especially at the elevations where ventilation horizons are to be laid; the depth of the coal oxidation zone; the gas-bearing capacity of coal seams; the presence of free silica in the enclosing rocks for establishing the silicosis hazard during operation; and other questions.

The reliability of the geological materials obtained is ensured by the compilation of geological maps and sections at a scale of 1:5000.

Mine-Opening Exploration

Between the phase of the design assignment, prepared on the basis of the geological materials of the detailed-exploration stage, and the beginning of mine construction, there always arises in the basin the need to carry out additional exploration work with the aim of more completely detailing the geological structure over the area of the first extraction blocks. At the sites where the cage and skip shafts of the mine are to be sunk, so-called shaft boreholes are drilled, and at the sites where ventilation shafts are to be sunk, control boreholes are drilled. Regardless of the depth of the shafts, the boreholes are drilled with the aim of intersecting all seams at the given point of the mine field.

To determine the conditions for driving horizontal and inclined mine workings within the first extraction blocks, the exploration-borehole grid is made denser. New exploration lines are laid out between the lines of detailed exploration.

At places where the main mine workings intersect, or along the direction of bremsbergs, crosscuts, and others, additional boreholes are drilled outside the exploration lines in order to determine the immediate conditions for driving the workings. Some of the boreholes, in accordance with the design, are located with a view to obtaining data on the depth of weathering of the rocks and the zone of coal oxidation.

It is recommended that the mine-opening exploration stage include the entire complex of geological-exploration works carried out, as it were, in two phases: the first—in the period between the design assignment, prepared on the basis of the geological materials of detailed exploration, and the beginning of mine construction; and the second—in the period of mine construction, up to the transfer of the last to operation.

As a result of the mine-opening exploration by drilling, and also after the clarification and introduction of corrections into the data of detailed exploration of the field on the basis of mine-geology data obtained in the process of mine construction, a final geological report is compiled. The latter is submitted to the commission for acceptance of the mine into operation as a kind of geological passport of the mine, signifying at the same time the completion of mine-opening exploration.

Operational Exploration

In terms of the nature of the problems to be solved and the methods of work, operational exploration differs considerably from all the preceding stages of exploration. Its principal task is to ensure the most rational direction of development and stoping mine workings. To solve this task, not only drilling data are used, including data from previous stages of exploration, but also the mine workings themselves, which, under conditions where the deposit is of a concealed nature, represent one of the ideal cases of exploration of mine fields.

As long-standing practice has confirmed, at the stage of operational exploration the most valuable materials are those of the mine geological service, provided that the documentation of mine workings is carried out in a qualified manner and at the level required by existing instructions. Thus, for example, the deciphering of the complex tectonic structure of the Saransk area (mines Nos. 101, 104, 105, 106, 107), with a large number of washouts of coal seams, became possible only after careful analysis of the documentation of mine workings. Meanwhile, in these mine fields the density of the exploration grid had been brought to 250 m and even to 65 m between lines.

Operational exploration by drilling begins from the moment the mine is put into operation. These works are carried out immediately before extraction or during the extraction of coal. They have no design-planning principles of any kind and are, as it were, “accidental.” The need to carry them out arises depending on the specific geological conditions revealed by mine workings.

Analysis of the Experience of Exploration Work in Areas of Distribution of the Dolinskaya Suite

Taking into account the characteristics of the Dolinskaya suite and the conditions of its occurrence, we shall attempt to analyze the results of the exploration work carried out. In doing so, we shall pay special attention to the degree of exploredness as a function of the completed volume of drilling, and to the clarification of certain special questions: determination of the weathering zone, gas-bearing capacity, silica hazard, and others.

1. Volume of Drilling Work and Degree of Exploredness

In analyzing this question, the author used the methodology proposed by V. Ya. Koudelny (1957) for the mines of the Kuznetsk Basin.

The degree of exploredness is determined by the ratio

\[ \frac{A_2 + B}{A_2 + B + C_1}\ \text{in } \%. \]

The dependence of the degree of exploredness on the expenditure of core-drilling footage for various stages of exploration in mine fields Nos. 2, 5, 44, and 44-bis of the Churubai-Nurinsk mines is presented by us in the graphs (Figs. 4, 5, 6). The indicator “P” here characterizes the expenditure of footage of core boreholes per 1000 tons of explored coal reserves of high categories \((A_2 + B)\).

As can be seen from the graphs presented, during the period of prospecting and preliminary exploration, with a comparatively large increase in footage, the degree of exploredness remains low, while the indicator “P” is characterized by a large number (points Nos. 1, 4, 7, 10, Fig. 5).

At the stage of detailed exploration, an increase in drilling footage leads to a rapid rise in the degree of exploredness. At points Nos. 2, 5, 8, 11, which characterize the completion of the stage of detailed exploration, we have the most favorable ratio of footage expenditures to the growth of coal reserves in high categories.

Fig. 4

\(OY\) — total meterage by mine fields;
\(OX\) — ratio of reserves of the higher categories to total reserves (in percent).

Within the figure:

Ratio of mechanical drilling expenditures by stages of exploration

Conventional designations
Along the \(OY\) axis — total meterage by mine fields.
Along the \(OX\) axis — ratio of reserves of the higher categories to total reserves (in percent).

Fig. 5

Axis \(OY\) — expenditures of meterage for exploration of \(1000\) tons of reserves of the higher categories (in meters) — indicator \(n\);
axis \(OX\) — ratio of reserves of the higher categories to total reserves (in percent).

Within the figure:

Ratio of mechanical drilling expenditures for exploration of 1,000 tons of reserves of the higher categories.

Fig. 6

Axis \(OY\) — total meterage by mine fields;
axis \(OX\) — ratio of reserves of the higher categories to total reserves (in percent).

Within the figure:

Graphs of advisable expenditures of exploratory drilling

Further exploration works, in a comparatively small volume at the stage of development exploration, as is seen in the graphs (points Nos. 3, 6, 9, 12), not only do not yield an increase in reserves of high categories but, on the contrary, reduce the degree of exploredness by 8–10%. Such an obviously abnormal and contradictory phenomenon, as investigations show, became possible because of an incorrect assessment of the degree of exploredness according to the methodology previously applied at the stage of detailed exploration.

The correctness of this thesis is confirmed by an analysis of the actual degree of exploredness, which we carried out on the basis of exploration materials for mines Nos. 2 and 5, for seam D₄, as the most representative for the Dolinskii area (Fig. 7).

Fig. 7. Structural map of seam D₄ within the fields of mines 2 and 5 of the Dolinskii area of the Churubai-Nura district of the Karaganda basin.
1 — outcrop of coal seam D₄; 2 — tectonic disturbances; 3 — floor contours of the seam; 4 — contour of reserves of nonstandard thickness; 5 — areas excluded from the calculation; 6 — boundaries of mine fields; 7 — contours of reserves of categories A₂+B, calculated from boreholes of detailed exploration (1952); 8 — contours of reserves of categories A₂+B according to GKZ Protocol No. 1336 of 18/VIII–1956.

When the possible outlines of blocks with reserves \(A_2+B\) are plotted on a grid of boreholes spaced 500–750 m apart, drilled before 1952 for the detailed-exploration stage, but taking into account the known geological structure of the mine fields constructed from the data of later work—exposure exploration in 1956 (Fig. 7)—it is established that the actual degree of exploration at the detailed-exploration stage was considerably lower than that accepted for approval by the State Commission on Mineral Reserves.

From the graph presented (Fig. 6) it is clear that the actual degree of exploration (point “a”) for mine No. 2 was 52% as against 77% (Fig. 4, point “m”), and for mine No. 5 (Fig. 6, point “d”) it was respectively 29% as against 61% (Fig. 4, point “m”).

Only when assessing the actual degree of exploration at the detailed-exploration stage was it possible to ascertain a normal improvement in the categorization of reserves due to additional drilling work passing into the exposure-exploration stage. In this case the degree of exploration at mine No. 2 increases from 52% (Fig. 6, point “a”) to 68% (point “c”), and at mine No. 5, correspondingly, from 29% (point “d”) to 51% (point “k”).

In order to evaluate more correctly the expedient limits of expenditures at the detailed-exploration stage, we additionally determined auxiliary points “e” and “v” in Figure 6. They were found by excluding the meterage of boreholes drilled in the area of the first extraction sections, which do not affect, or affect only very slightly, the categorization of reserves under the currently known geological situation of the mine fields.

In analyzing the expenditures of exploratory drilling (Fig. 6), it was established that the degree of exploration increases substantially only within certain segments of the curves. As can be seen from the graph, in the exploration of mine No. 5, in order to obtain 29% of categories of reserves \(A_2+B\) out of the total balance reserves of 80,220 thousand tons, it was necessary to drill 23,000 linear m; correspondingly, for 47%—48,000 linear m, and for 51%—59,650 linear m of mechanical drilling. The value of coefficient “p” (the expenditures of exploratory-drilling meterage per 1,000 tons of reserves \(A_2+B\)) for point “d” on the curve for mine No. 5 (Fig. 6) is equal to 0.98 m, for point “e”—1.24 m, and for point “k”—1.44 m.

The expenditures of meterage at the detailed-exploration stage are evaluated quite differently for the adjacent mine No. 2. The graph (Fig. 6) shows that, in order to obtain 52% of categories of reserves \(A_2+B\) out of the total balance reserves of 60,668 thousand tons, 18,808 linear m of mechanical drilling were required; correspondingly, for 65%—30,000 linear m and, finally, for 70% of category \(A_2+B\)—36,000 linear m. The value of coefficient “p” for point “a” on the curve for mine No. 2 is equal to 0.54 m, for point “v”—0.84 m, and for point “c”—0.85 m.

As is evident from the materials presented, the most advantageous expenditures of exploratory-drilling meterage, with a coal-bearing coefficient of 3.0–4.7, for areas with a tectonic structure of the type of mine No. 2 will be characterized by point “a” on the graph in Fig. 6. The value of coefficient “p” here is estimated at 0.54 m. The total expenditures of exploratory-drilling meterage by the end of the detailed-exploration stage reach 18,000–20,000 linear m. The exploratory-borehole grid is characterized by limits of 500 m.

For areas with a tectonic structure of the type of mine No. 5, most characteristic of the greater part of the Churubai-Nura district, rational expenditures of exploratory drilling are characterized by point “e” on the graph (Fig. 6). The value of coefficient “p” here is equal to 1.24 m. The total expenditures of exploratory drilling by the end of the detailed-exploration stage of an area of the mine No. 5 type will reach 48,000–50,000 linear m. The exploratory grid, apart from the main lines spaced at 500 m, becomes somewhat denser due to boreholes

intermediate lines in tracing and determining the nature of tectonic disturbances.

The costs of exploratory drilling, expressed by the segments of curves “e”—“k” and “a”—“c” (Fig. 6), are economically inexpedient at the detailed-exploration stage. These costs can be justified only at the stage of development exploration.

Comparing the geological structure and the costs of exploratory drilling in adjacent areas of Mines Nos. 2 and 5, a perfectly logical conclusion suggests itself: it is impossible to establish any strictly defined figures for the costs of exploratory drilling over larger or smaller exploration areas.

The figures we have derived analytically may be regarded as optimal when planning monetary allocations for geological-exploration work under analogous conditions. In each specific case, the effective use of monetary allocations and volumes of exploratory drilling depends on a creative solution of the questions involved in evaluating the entire complex of exploration work in the given area.

2. Some Special Questions of Exploration

The coals of the Dolinskaya suite, possessing the property of easy disintegration, owing to the content of a large percentage of vitrinized substance, for a long time yielded a very low (35–50%) recovery of coal core. Only after improvement of the double core barrel by I. F. Kulikov, senior foreman of the Churubai-Nurinskaya geological-exploration party, and its introduction into production did core recovery when drilling the coal seams of the Dolinskaya suite begin to reach 80–95%. No firm technical standards or limits for core recovery when using this device have been established, but the achieved core recovery level of 80–90%, with obligatory logging observations, provides sufficient accuracy of the materials obtained for reserve estimation in the higher categories.

An integral part of the existing exploration methods in the basin is the extensive use of geophysical work. As a rule, all core-drilling boreholes are subjected to logging. In addition to checking the correctness of the assessment of the hypsometry of the floor and roof of coal seams in the section, logging work solves a number of other tasks.

Thus, analysis of the results of control logging fractions during exploration of the Dolinskaya suite shows that up to 90% of seam intersections have increased thickness compared with drilling data. The average increase in the thickness of seams accepted for reserve calculation according to logging, as compared with drilling data for the same seam intersections, reaches 0.27 m. The number of seams (seam intersections) missed and detected by logging amounts to up to 5% of the total number of seam intersections.

Inclinometry data show that the maximum curvature of boreholes, both in the vertical and in the horizontal planes, reaches 20–30 m in the Dolinsky area and 150 m in the Dubovsky area.

For studying coals for washability and coking properties under semi-plant conditions at the detailed- and development-exploration stages, coal seams were in some cases sampled by drilling test-pit boreholes 200–300 mm in diameter. They were laid out in clusters of from 2 to 5 boreholes in each, depending on the thickness of the seam, on the basis of obtaining a sample weight of 200 kg at depths below the oxidation zone. Sampling of coal seams by test-pit boreholes is necessitated by the great thickness of the overlying deposits.

Taking into account the totality of data from investigations of physico-mechanical

properties of rocks from core samples numbering up to 800–1000 pieces for a mine field; it has been established that the thickness of the weathering zone of the rocks of the Dolinskaya suite in the Dolinsky area ranges from 20 to 35 m from the Carboniferous surface.

The depth of the oxidation zone of the coals of the Dolinskaya suite in the northern half of the Dolinsky area is taken as, on average, 20–25 m from the Carboniferous surface, and somewhat higher—15–20 m—in the southern part of the area. Only below this level does the coal acquire normal technological properties.

In the Dubovsky area, where the Carboniferous deposits of the Dolinskaya suite are directly overlain by a thick sequence of Mesozoic deposits, the coal oxidation zone is almost absent or extends down to 1.5–2 m from the Carboniferous surface. This phenomenon is evidently explained by the great intensity of erosion of the surface of the Carboniferous rocks and by the relatively very short period between their denudation and the deposition of the covering rocks. As a result, the agents of weathering did not have time to exert any significant influence on the Carboniferous coals. Subsequently, the Mesozoic rocks, up to 250–300 m thick, lying on the Carboniferous, isolated the latter from weathering.

From the data given above it is evident that the weathering zone of the rocks and the oxidation zone of the coals have different depths. This is explained by the fact that rock and coal change differently under the action of weathering agents.

Recently, a serious attempt has been made to find the most objective methods for determining gas content and forecasting the possible gas abundance of mines in the course of exploration of mine fields. Considerable difficulties in this work are presented by sampling coal seams for gas content. It was carried out by two methods.

1. By sealing coal core, raised by ordinary core barrels, in special vessels for all seams. The number and nature of the arrangement of sampling boreholes for gas content are determined by the projects for detailed and preliminary exploration. In order to establish the depth of the upper boundary of methane gases, the depth intervals for seam sampling were taken from 100 to 250 m.

2. By taking samples with the KG-55 pipe—a core-gas sampler of the Kazakhuglegeologiya Trust, designed by geological engineer M. A. Ermekov. A distinctive feature of this method is that it provides the possibility of direct determination of the gas content of seams. Sampling in the Dolinsky area was carried out in depth intervals of 200–355 m in order to establish the regularities of the increase in methane content with depth.

On the basis of the results of investigations of samples for gas content in the special laboratory of the Kazakhuglegeologiya Trust, taking into account a number of preceding factors, M. A. Ermekov believes that the approximate value of the expected methane abundance (in m³/t of daily output) for mine fields in the Dolinsky area will increase from 3.25–6.90 m³/t, in the depth interval 190–200 m, to 25–40 m³/t at a depth of 300 m. There are data indicating that the closely spaced occurrence of coal seams, such as D₁–D₂, D₃–D₄, and others, with a descending scheme of opening, will lead to a sharp increase in gas abundance when working the upper of the closely spaced seams.

The tendency of coal to spontaneous combustion is studied by the difference in ignition temperatures of unoxidized (reduced with benzidine) and oxidized (with perhydrol) coal. Tests have established that the lowering of the ignition temperature of Dolinsky coals lies within the range from 23 to 55°. This corresponds to the data for coals prone to spontaneous combustion.

The determination of free silicon dioxide in the enclosing rocks of the Dolinka suite, for the purpose of establishing the silica hazard of mines, was carried out on core samples in the course of exploratory drilling. Two methods were used for determining free silicon dioxide, approved by the Ministry of the Coal Industry of the USSR—the petrographic and the chemical. The research data from both methods show that all enclosing rocks of the Dolinka suite contain more than 10% free silicon dioxide; sandstones contain 13–26%, and argillites and aleurolites, on average, 23–26%.

Samples of these rocks examined by VSEGEI personnel by the method of thermal analysis showed a free silicic-acid content of no more than 10%. There is every reason to believe that the data of the thermal-analysis method will be the closest to the truth regarding the presence of free silicic acid.

The study of hydrogeological conditions is carried out by means of an entire system of different observations. However, a number of questions still remain unresolved. These include, in particular, the question of the interaction of groundwater of different stratigraphic complexes under natural conditions and during the development of mining operations, and, in connection with this, the question of possible inflows of water into mine workings; the question of the influence of groundwater on the physicomechanical properties of rocks; and others.

At present the trust, along with a detailed study of the hydrogeological conditions and physicomechanical properties of the rocks of each mine field, is carrying out a synthesis of materials for the entire district, without which the solution of many practical questions is impossible.

Conclusion

In conclusion, we shall briefly dwell on several tasks facing Kazakhstan coal prospectors.

1. Proceeding from the premises of the presence of graben structures, widely developed in the Paleozoic deposits of Central Kazakhstan, with which coal-bearing deposits are often associated, the prospects for finding new coal-bearing areas are considered more favorable.

As practice has shown, coal deposits in the form of grabens are represented in the modern relief by depressions, usually surrounded on all sides by more elevated forms of relief caused by the dense rocks of the Silurian–Devonian effusive-metamorphic complex. Depressions of this kind, on the flanks of which Lower Carboniferous limestones often crop out, occur comparatively often within Central Kazakhstan. In the coming years it is necessary to organize broad prospecting work, above all in the described depression structures, which may include the Tassuat, Esengeldin, Tokarev, Shebundin, East-Kuuchekin, Tamdyk, and a number of others.

1. The stages and methods of exploration set forth by the author should be made the basis of the activity of geological-exploration organizations. This will make it possible to provide the design and construction of mines with thoroughly explored mine fields at substantially lower expenditures of state funds on exploration, without detriment to the development of the coal industry.

2. Scientific organizations and institutes should be involved more broadly in rendering assistance in solving the urgent tasks of studying the basin and the adjoining coal deposits. Such tasks include:

a) study of the microtectonics of the basin;
b) compilation of a geological-chemical map of the basin;
c) study of the physicomechanical properties of rocks according to data from exploratory boreholes;

c) determination of the balance of water resources of the Karaganda industrial district;

d) compilation of a hydrogeological map of the basin and establishment of the influence of alluvial waters on the water content of mine workings in the Churubai-Nura district, etc.

Trust “Kazakhuglegeologiya”.