DENSITY CHARACTERISTICS OF THE KOLCHUGINO SERIES OF DEPOSITS IN THE LENINSKY GEOLOGICAL-ECONOMIC DISTRICT OF KUZBASS

L. I. IVANCHURA

(Presented by the scientific seminar of the Department of Geophysical Exploration Methods)

In 1969, as a result of field work, we selected 1,707 rock samples from 11 boreholes in the Leninsky geological-economic district of Kuzbass.

The samples are represented mainly by siltstones and sandstones belonging to the Upper Permian deposits of the Kolchugino Series. The sandstones are usually light gray to dark, with a greenish-bluish tint, ranging from fine- to coarse-grained. The siltstones of various grain sizes have an ashen or dark-gray color.

Determination of density by the method of hydrostatic weighing on technical balances was carried out on samples at room temperature of 18–20°C and normal atmospheric pressure. The accuracy of a single density determination is ±0.02 g/cm³.

To find the most probable values, close to the true or mean values of the measured physical parameter, methods of mathematical statistics were applied.

Following the opinion of A. G. Tarkhov [2], B. V. Vikhirev [1], and other researchers, we adopted the following method for processing the measurement results. According to macroscopic determination, all rock samples were subdivided into separate lithological varieties, of which the most representative for mathematical processing are: fine-grained siltstone (556 samples), coarse-grained siltstone (408 samples), and fine-grained sandstone (542 samples).

For each lithological variety of rocks, point diagrams of the physical parameter under consideration were constructed; on the basis of these, taking into account the dependence given by Sturges, a grouping interval of 0.04 units was chosen. Using the grouping intervals, variation curves of the density distribution of the studied rocks were calculated, and statistical characteristics were determined.

Tables 1–3 present the calculation data.

Figure 1 shows the variation curves of the density distribution of the studied rock varieties. The arithmetic mean values of density for fine- and coarse-grained siltstone and for fine-grained sandstone are respectively 2.60; 2.60; and 2.57 g/cm³. The standard deviations for these rocks are respectively 0.046; 0.04; and 0.052 g/cm³.

Along with the mass determinations of the density of siltstones and sandstones, single measurements of this physical parameter were performed

Table 1

Fine-grained aleurolite

Table 2

Coarse-grained aleurolite

Table 3

Fine-grained sandstone

for coals of ranks D, G, Zh. The studies showed that the density of coals lies within the range from 1.2 to 1.4 g/cm³, i.e., is 1.5–2 times lower in relation to the enclosing sandy-clayey deposits. On the basis of the results obtained, the following conclusions may be drawn:

1. A substantial difference in density is observed between coal and the enclosing sandy-clayey rocks composing the Kolchugino series of deposits of the Leninsky geological-economic district of Kuzbass.

2. Density values decrease from aleurolites to sandstone and to coals.

3. Fine- and coarse-grained aleurolites are not distinguished from one another by their arithmetic mean values. The standard deviations for these rocks are close.

4. The difference in density by arithmetic mean values between aleurolites and fine-grained sandstone is 0.03 g/cm³.

Fig. 1. Variation curves of density distribution. 1. Fine-grained aleurolite. 2. Coarse-grained aleurolite. 3. Fine-grained sandstone. 4. Coal.

Thus, the results of laboratory investigations confirm that the density of rocks is a favorable condition that makes it possible to use this physical parameter for distinguishing coal seams in the section by those methods based on the density factor, for example, gamma—gamma logging. Separation of sandy-clayey deposits by this physical parameter is difficult, and in a number of cases impossible.

REFERENCES

1. B. V. Vikhirev. Methodology for studying distribution series of density and other physical properties of rocks. In: Exploration Geophysics, issue 23. Nedra, 1967.

2. A. G. Tarkhov. On the statistical processing of results of mass determinations of the physical properties of rock samples. In: Applied Geophysics, issue 20, Gostoptekhizdat, 1958.