Methodology and Some Results of Studying Fracturing in Cores from Exploration Wells

V. V. Nikolaev

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

The results of many years of work by geologists of the East Kazakhstan Geological Administration, carried out under the direction of N. A. Gribsko, laid the foundation for a new area of geological exploration service—core metering, which is an important reserve for improving the quality and economic efficiency of geological investigations [3].

The introduction of the advanced experience of Altai geologists into the practice of geological exploration is proceeding extremely slowly. Serial production of instruments for extracting oriented cores has still not been established. The reason for this is apparently the considerable time expenditure required to obtain oriented samples. In this respect, the Krelius automatic core orienter [2] compares favorably with the core scope of N. A. Gribsko, M. I. Kazantsev, and A. S. Penkov. It is worth noting that the use of oriented core in the Altai deposits, as a rule, does not go beyond determining elements of rock attitude. The methodology of core metering has essentially not been developed.

A most important component in well documentation is the study of rock fracturing. Many geologists, apparently, are not familiar with methods for determining the orientation of structural elements from unoriented core [4, 5]. Researchers at VNIGRI, for example, believe that unoriented core has little practical value for studying fracture orientation [6].

Taking into account the importance of the problem under consideration, we studied the research experience, tested the possibility of studying fracturing from unoriented core, and developed a research methodology for the conditions of Kuzbass.

The adopted method for measuring fracture orientation does not differ in principle from the method of I. A. Ocheretenko [4]. For the measurements, an azimuthal angle-measuring template was used, consisting of a square with a circle cut out according to the diameter of the core. The circumference was numbered every 5° clockwise. With this template, the conditional azimuths of fracture dip were measured, that is, the angles in the plane perpendicular to the core axis between the dip directions of bedding and of fractures.

Measurement of fracture orientation is carried out as follows. The template is placed on the core so that its top faces the azimuthal scale. The zero of the template is set opposite

generatrix of the core that connects the lowest points at which bedding emerges on the core surface. Then the lowest points at which fractures emerge are transferred along generatrices to the scale of the template, where their conditional dip azimuth is read. The dip angles of bedding and fractures are measured with a geological compass with the core axis in a vertical position.

The quality of the measurements is determined by the correctness with which the core is placed in the boxes. It is not difficult to see that the conditional orientation of fractures, determined in the case of incorrect orientation of the core relative to its top and bottom, will in a number of cases be measured incorrectly. If the visible dip angles of all fractures and the dip azimuths of longitudinal fractures remain the same for the two positions of the core, then the conditional dip azimuths of transverse oblique fractures differ by 180°, and of diagonal fractures by an amount from 0° to ±180°. Instruction, systematic checking of correct placement, and control placement of the core, together with the study of indicators of normal and overturned bedding of the strata, guarantee reliable measurements of fracture orientation.

The adopted methodology presupposes the presence in the cores of clearly expressed bedding of the rocks, with visible dip angles greater than 5–10°; a sufficiently complete recovery of core in columns with an undisturbed rock structure; logging by the methods of current, SP, GK, GGK, PS, inclinometry, caliper logging, and resistivity measurement; determination of the elements of rock bedding; and timely documentation of the core.

The objectives of the study are:

a) study of fracturing as a function of lithology, depth of occurrence, and stratigraphic and structural position;

b) delineation, in well sections and over the area, of zones of increased and intense fracturing;

c) comparative analysis of the data obtained by lithologic types, stratigraphic subdivisions, and structures;

d) identification of regularities in the development of fracturing and its relationship to particular structural elements.

In structural documentation, special studies and measurements are carried out, for which the following sequence may be recommended.

1. Reconnaissance study of the section (lithology, fracturing, microfolds, microfractures, slip surfaces and other forms of microtectonics, preliminary identification of fracture systems, study of their genetic and morphological features, identification and study of zones of increased fracturing, crushing, and breccias; attention is paid to changes in the dip angles of bedding).

2. Detailed bed-by-bed study and description of fracturing and other elements of microtectonics.

a) measurement of the orientation of all fractures visible in the core and of the dip angle of bedding;

b) identification of fracture systems and their detailed description;

c) visual assessment of the intensity of fracturing;

d) description of microstructural forms (cleavage, microfolds, microfractures) and measurement of their orientation;

e) lithologic-facies description of the bed, with indication of the features of normal or overturned bedding;

f) sampling and photographing of cores.

Preliminary processing of the results is carried out for the correct orientation of the studies and for clarifying disputed questions in the field. It includes the construction of point circular diagrams of fracturing on Wulff polar nets, and, in cases of difficulty,

in distinguishing fracture systems—and statistical-graphical processing of them.

Fracturing diagrams are constructed for intervals of the well distinguished in accordance with structural features and inclinometry data. For example, when the well intersects a disjunctive dislocation, fracturing diagrams are constructed for the rocks of the hanging wall, the footwall, and the zone of the tectonic rupture; when it intersects folds—for the limbs and the zone of the axial plane.

Separate diagrams are constructed for intervals with different bedding elements. If there are substantial changes in fracturing in rocks of different composition or in separate stratigraphic subdivisions, the intervals are distinguished with allowance for the lithologic-stratigraphic feature. A more detailed division is made according to inclinometry data. Within the selected intervals the borehole axis must be rectilinear, and the bedding elements of the rocks identical.

For processing diagrams in density isolines it is sufficient to have no fewer than 15 measurements of fractures of each system. If the measurements prove insufficient, in a number of cases it is possible to combine diagrams by intervals distinguished in accordance with lithologic-stratigraphic features or inclinometry data, provided that no changes in fracturing and in the bedding elements of the rocks are observed.

The construction of a combined fracturing diagram characterizing some part of the section with identical bedding elements, as well as determination of the true orientation of fractures when the bedding elements of the rocks are known, can be carried out by means of the Wulff equatorial net by bringing the poles of bedding of the combined diagrams (and, together with these, the poles of the fractures) to some conventional or true orientation of the bed (1, 4, 5, 6).

It is more expedient to process diagrams in density isolines with the aid of the Breich template, which is a stereographic projection of a hemisphere divided into 200 equal-area figures [1]. The template is used for counting the number of points and for drawing density isolines.

Along with a visual estimate of the intensity of fracturing, it is necessary to construct graphs of specific fracturing, which make it possible to trace the change in the intensity of fracturing in the section along the well.

Using the formula for determining the normal thickness of a bed from the apparent thickness and the apparent dip angle of bedding, one can write the expression for determining the density of fractures of a certain system in a given bed:

\[ n_i=\frac{N_i}{m_v\cdot\cos\alpha}, \]

where \(n_i\) is the number of fractures of a certain system falling on 1 dm of the normal to them (fracture density), \(N_i\) is the number of measurements of fractures of this system in the given bed, \(m_v\) is the apparent thickness of the bed in dm (or the length of the core when its recovery is incomplete), and \(\alpha\) is the mean apparent dip angle of the fractures of the given system.

The sum of the fracture densities of all systems characterizes the intensity of fracturing of the bed (its specific fracturing) and reflects the number of fractures cutting a sphere 1 dm in diameter.

The study of fracturing in core was carried out in conjunction with detailed investigations of the tectonics of the mine fields of the Kemerovo and Leninsk districts of the Kuzbass. The results of the study of fractures were used-

were used in diagnosing tectonic disturbances in borehole sections and in predicting tectonic structure.

In the area of the southern crosscut toward the Pioneer mine field, the coal-bearing deposits are represented by the Alykaevskaya subsuite of the Lower Balakhonka suite, which forms the northwestern limb of the Kemerovo syncline. The limb has a persistent southeasterly dip at angles of 65–85°. From the northwest and southeast the area is bounded by two large longitudinal disjunctive faults.

The overwhelming majority of fractures are oriented normal to the bedding or coincide with it, forming a system of bedding-parallel fractures and four systems of normal-cutting fractures (Fig. 1).

Fig. 1. Diagram of fracturing of rocks in the area of the southern crosscut toward the Pioneer mine field. Borehole No. 2832. Density isolines: 1—2—3—4—5%; 200 measurements of fracture orientation.

Transverse and longitudinal fractures are usually open and are filled partially or completely with calcite, pyrite, and sandy-clayey material. The fracture width is 0.1–5 cm and increases noticeably in the northeastern direction. The fracture walls often bear traces of significant displacements.

Diagonal fractures, observed only in the southwestern part of the area, are closed, with fine striations and with calcite or pyrite on the walls.

Bedding-parallel fractures with slickensides and striations, usually directed up the dip of the rocks, are especially numerous. Sometimes these fractures are open and completely filled with calcite.

The greatest fracturing is observed in the upper horizons down to a depth of approximately 20 m from the surface of the bedrock. Here the same fracture systems occur, but the fractures are wide, open, with iron hydroxides and limonite pseudomorphs after pyrite.

Below, approximately down to a depth of 200 m, fracturing is less intense. The fracture density of each system averages 1 fracture/dm. Partial filling of fractures, with a high specific fracturing, ensures free circulation of groundwater.

At deeper horizons, the fracturing becomes closed and hidden in character. The intensity of fracturing decreases sharply. The fracture walls are coated with pyrite.

The uniform character of the fracturing and the weak development of oblique fractures indicate an insignificant degree of tectonic disturbance of the rocks. The conditions for mining the coal seams will be favorable.

The study of fracturing in the Nikitinskii Severnyi area was carried out in deposits of the Uskat subformation of the Il’inskaya formation. Against the background of the simple structures of the Leninskii district, the area appears anomalous in the complexity of its tectonic structure and in the disturbance of the rocks. Numerous, differently oriented brachyfolds and disjunctive faults form a mosaic-block, generally synclinal, structure enclosed between two regional longitudinal disjunctives (the Urskii and Kil’chigizskii).

The fracturing of the rocks is very intense. The orientation and number of fracture systems vary depending on structural conditions. The fractures everywhere have a closed character, reflecting the conditions of intensive all-round compression of the rocks during deformation. Here, equally widespread are bedding-plane, normal, and oblique fractures (Fig. 2). In areas where transverse folds develop, transverse oblique fractures predominate.

Fig. 2. Diagram of rock fracturing in the Nikitinskii Severnyi area. Well No. 7148. Density isolines: 0.3—0.6—1.3%. 300 measurements of fracture orientation.

Significant disturbance of the rocks is observed in the upper horizons down to a depth of approximately 16 m from the surface of the bedrock, at the contacts of beds, especially in the roof and floor of coal seams, and in zones of numerous tectonic disturbances. It has been noted that these horizons, as a rule, are characterized by reduced density and by better reservoir properties in comparison with normally fractured rocks. This makes it possible to assume that zones of increased fracturing and ruptures can be identified by logging methods.

On the basis of the study of fracturing, one may conclude that the conditions for mining coal seams in many cases will be very difficult, especially in areas where plicative structures join and where they are intersected by tectonic faults. The coal reserves here can confidently be assigned to off-balance reserves.

The investigations carried out confirm the possibility of studying fracturing from core under the conditions of the Kuzbass. Check observations in mine workings attest to the reliability of the proposed method. As experience shows, these investigations can be combined with geological documentation of the core.

The results of the work have been approved by the trust “Kuzbassuglegeologiya”; the research method has been recommended for introduction into production.

LITERATURE

1. A. B. Vistelius. Structural Diagrams. Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1958.

2. E. E. Gintsburg, V. B. Shitikhin. A new instrument for oriented lifting of core from small-diameter boreholes. Exploration and Protection of Mineral Resources, No. 11, 1962.

3. N. A. Dybovskii. Coremetry—a new field in the geological exploration service. Collection of papers, enterprise No. 3, ONTI MGiON USSR. Gosgeoltekhizdat, 1959.

4. I. A. Ochretenko. A method for measuring the azimuth and dip angle of fractures in core using a transparent grid. Exploration and Protection of Mineral Resources, No. 9, 1958.

5. E. I. Patalakha. A method for determining the orientation of planar structural elements from the core of a deviated borehole. Proceedings of the Academy of Sciences of the Kazakh SSR, geological series, issue 2 (47), Alma-Ata, 1962.

6. Fractured Rocks and Their Reservoir Properties. Proceedings of VNIGRI, issue 121. Gostoptekhizdat, Leningrad, 1958.