USE OF AN EMERGENCY WELLBORE TO DETERMINE THE ATTITUDE ELEMENTS OF BEDS IN THE SUBTHRUST ZONE OF THE ASTASHEVSKAYA AREA

L. A. Pukhlyakov, I. Z. Karamaev, B. A. Nikulin

(Presented by Professor A. V. Aksarin)

The Astashevskaya area is located in the southeastern part of the Bashkir ASSR, near the contact between the Russian Platform and the folded zone of the Urals. The upper part of the section in this area is composed of terrigenous deposits of the so-called Zilair series, which is Famennian in age. The rocks of this series are thrust over various horizons of the Carboniferous system.

During the last several years, six structural and three deep exploratory wells were laid out in the Astashevskaya area. The latter entered the subthrust zone; however, for a long time it was not possible to say anything about the structure of this zone on the basis of the data obtained, since the geological sections of the drilled wells could not be correlated with one another.

In the spring of 1967, while drilling well No. 5 in this area, owing to an accident at a depth of about 2800 m, part of its borehole was left, and from a depth of 2400 m drilling of a second borehole was begun. Thus, a certain rock interval was penetrated twice by this well. The sections of the individual boreholes of this well correlated well. The following beds stood out especially clearly.

1. Dark-gray limestone with a brownish tint, fine-grained, with a network of cracks filled with calcite. This bed was intersected by the first borehole in the interval 2670–2692 m, and by the second in the interval 2656–2670 m.

2. Dark-gray, almost black argillite, micaceous, with numerous cracks filled with calcite. This bed was encountered by the first borehole in the interval 2692–2702 m and by the second borehole in the interval 2670–2680 m.

3. Dark-gray fine-grained limestone. It was encountered by the first borehole in the interval 2702–2720 m and by the second borehole in the interval 2680–2694 m.

4. Dark-gray argillite with slickensides and interbeds of micaceous marls. It was encountered by the first borehole in the interval 2720–2750 m and by the second borehole in the interval 2694–2720 m.

5. Brownish-gray limestone with cracks filled with calcite. Its top was encountered by the first borehole at a depth of 2750 m and by the second at a depth of 2720 m.

By geologists of the Isyangulovo exploration-drilling office of the trust “Bashneftegazrazvedka,” I. Z. Karamaev and B. A. Nikulin, it was

it was decided to use these data to determine the attitude elements of the bed by the method of several oblique sections, developed by senior lecturer of the Tomsk Polytechnic Institute L. A. Pukhlyakov [2]. The work was carried out in the following order.

1. With the aid of special tables [1], the horizontal projection of the branched part of well No. 5 was calculated and then constructed (Fig. 1-a); on it were plotted the points of intersection by the individual

Fig. 1. Horizontal projection of the branched part of well No. 5 of the Astashevskaya area, with plotted points of intersection by individual boreholes of some beds (a), and the directions, obtained from it, of the straight lines of the oblique section, from which the dip angle and azimuth of a group of beds were determined (b)

boreholes of the above-named rock beds (transverse ticks in Fig. 1-a). Unfortunately, most of the projection of the second borehole of this well coincided with the projection of the first, and this made the portion of the figure under consideration not entirely clear. However, the projections of the straight lines connecting the points of intersection are nevertheless visible here (see the dashed lines in Fig. 1-a). Their lengths proved to be as follows: for the top of the Okremneloye limestone, $s_1 = 7.7$ m; for its bottom, $s_2 = 7.2$ m; for the top of the argillite with slickensides and interbeds of micaceous marls, $s_3 = 8.2$ m; for the top of the brownish-gray limestone, $s_4 = 8.3$ m (Table 1, col. 7).

1. With the aid of special tables [1], the increments of the depth corrections for curvature over individual measurement intervals, $\Delta z'$, were determined. The geometric meaning of this quantity can be expressed by the relation

\[ \Delta z' = \Delta l(1-\cos \Theta), \tag{1} \]

where $\Delta l$ is the length of the measurement interval and $\Theta$ is the zenith angle at the measurement point.

1. The total value of the depth correction for curvature, $\sum \Delta z'$, was determined for each point of intersection; for this the formula used was

\[ \sum \Delta z' = \Delta z'_1 + \Delta z'_2 + \ldots + \Delta z'_n . \tag{2} \]

The values obtained were substituted into the formula for determining the absolute elevations of the points of intersection,

\[ z = H - (l - \sum \Delta z'), \tag{3} \]

where \(H\) is the absolute elevation of the wellheads and \(l\) is the depth of the points of intersection in measurements along the borehole. The calculation results are given in Table 1 (cols. 3 and 5), and also in Fig. 1-a.

Table 1

Processing of data for the group of beds of the subthrust zone, penetrated by two boreholes of well No. 5 in the Astashevskaya area

1. The dip angles of the lines lying in the planes under study, or of the lines of the oblique section \(\alpha_k\), were determined; for this purpose the formula used was

\[ \tg \alpha_k = \frac{z_1 - z_2}{S}, \tag{4} \]

where \(z_1\) and \(z_2\) are the absolute elevations of a certain bed in the first and second boreholes of the well, respectively.

As a result of this operation, the following dip angles of the lines of the oblique section were obtained: \(\alpha_1 = 63^\circ00′\), \(\alpha_2 = 72^\circ40′\), \(\alpha_3 = 73^\circ50′\), and \(\alpha_4 = 75^\circ30′\) (Table 1, col. 8).

1. On the basis of these values of the dip angles of the lines of the oblique section, the corresponding settings \(a_k\) were determined. The calculation was carried out by the formula

\[ a_k = \frac{1}{\tg \alpha_k}. \tag{5} \]

For the line lying in the top of the silicified limestone, it proved to be \(a_1 = 0.51\) m; for the line lying in its base, \(a_2 = 0.313\) m; for the line lying in the top of the argillite with slickensides and interbeds of micaceous marls, \(a_3 = 0.291\) m; and for

line lying in the roof of the brownish-gray limestone, \(a_4 = 0.258\) m (Table 1, col. 9).

1. On the horizontal projection of the branched part of the well, the directions of dip of the oblique-section lines were determined by means of a protractor. The measurement results are given in Table 1 (col. 10).

2. On a separate drawing, from an arbitrarily chosen point, the dip directions of the oblique-section lines were drawn, and along them the corresponding offsets were plotted to scale (Fig. 1-6). Through the points thus obtained, an averaging straight line was drawn (the dashed line in Fig. 1-6), expressing the strike of the group of beds under consideration.

3. From this point, a perpendicular was drawn to this straight line (the arrow in Fig. 1-6). Its length \(a_{\text{п}}\) expresses the offset of the dip angle of all four beds considered, \(\alpha_{\text{п}}\), and therefore, when substituted into the formula

\[ \tg \alpha_{\text{п}} = \frac{1}{a_{\text{п}}}, \tag{6} \]

will give us this angle. In the present case, the magnitude of this offset proved to be \(0.125\) m; hence the tangent of the angle of interest to us is \(8.0\), and the angle \(\alpha_{\text{п}}\) itself is approximately \(83^\circ\). The dip azimuth of the group of beds studied is given by the direction of the perpendicular to the dashed straight line. In the present case it proved to be approximately \(55^\circ\).

These data enabled the geologists of the Isyangulovo exploratory-drilling office, first, to find an explanation for the fact that sections of individual wells in the Astashevskaya area proved incomparable. In fact, at dip angles close to \(90^\circ\), individual wells, as a rule, cannot intersect the same beds. In addition, they provided at least some idea of the structure of the subthrust zone of this area.

In areas with a complex tectonic structure, such as the Astashevskaya area, it is apparently advisable to make wider use of two-bottomhole wells.

LITERATURE

1. Pukhlyakov L. A. Tables for Processing Curvature of Curved and Multibottom Wells when Measuring Curvature Elements at 10 and 25 Meters. Tomsk University Press, 1966.

2. Pukhlyakov L. A. Two-bottomhole wells as a means of determining the attitude elements of beds. In: Results and Prospects of the Geometrization of Mineral Deposits. Materials for the Seminar Held January 29–31, 1969. Moscow, 1969.