книги / Геология нефти и газа

Fig. 5

In horizontal occurrences, absolute values of the inter-strata boundaries are approximately the same, therefore in geologic maps they either coincide with horizontal contour lines or are parallel to them. Each underlying stratum is older than the stratum overlying it. Ancient layers crop out to the ground surface in the lowest depression areas, and the youngest layers, in elevated ones. The true thickness, in case of a horizontal occurrence, is determined as the difference between the marks of the top and the base of the layer.

For horizontal occurrences, the most rational direction of a geologic crosssection will be the line passing through the lowest and highest points in the terrain. In this case, the main information for construction of a cross-section below the terrain level is yielded by drill wells.

STRATIGRAPHIC UNCONFORMITIES

If each overlying stratum or a stratum series lays on the underlying layers without depositional breaks, which reflects discontinuity of the deposition process, it is called conformity or regular bedding. If this sequence is disrupted, and for a more or less long-term period no deposition are formed (e.g. due to elevation of the bottom of a former basin above the sea level), unconformity is developed. Such unconformities are called stratigraphic.

Along different criteria, stratigraphic unconformities can be subdivided into several types.

Based on their angles, unconformities may be: parallel, angular (fig. 5), and geographic.

A parallel unconformity is signified by a break in the layers positioned in parallel.

An angular unconformity is signified by a break between two stratum series with different inclination angles. Such unconformities are usually clearly reflected both in natural vertical cross-sections and in geologic maps. In the both cases, the surface of the unconformity

separates unconformable suites, cuts different formations of the ancient suite at an angle, and runs more or less in parallel to the boundaries between individual formations of the younger suite. The value of the unconformity angle can vary in a wide range and change abruptly in different zones.

A geographic unconformity is an angular unconformity with an angle of less than 1о. It can be identified only when studying vast territories.

From the viewpoint of distinctness of the unconformity surface, there may be obvious unconformities with distinct unconformity surfaces, and hidden unconformities, with undefined positions of unconformity surfaces.

In terms of the spread area, unconformities may be regional (spread over vast territories) and local.

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Fig. 6

INCLINED OCCURRENCE OF LAYERS

Inclined occurrence of strata is a result of tectonic dislocations. If the strata are inclined to one side at a constant angle, it is conventionally called a monoclinal structure. The inclination angle may range from several degrees to 90 degrees (vertical occurrence). If this angle exceeds 90 degrees, it is called overturned occurrence.

In case of an inclined occurrence, the direction and inclination angle of the strata are measured. Their position in space is characterized by occurrence elements: strike line, dip line, and dip angle (fig. 6).

The strike line (a-a) is the line of the surface crossing with the horizontal plane. An infinite number of strike lines may be drawn on the stratum surface: all of them will be horizontal and

parallel to each other, and will differ only in their absolute marks.

The dip line (b-b) is the vector perpendicular to the strike line, lying on the stratum surface and directed towards the direction of its inclination.

The dip angle (α) is the angle between the dip line and its projection

on the horizontal plane. Their position of the dip line in space is determined by the azimuth and angle of its inclination.

The azimuth of a given direction is the right vectorial angle contained by the northward direction of the true meridian and the given direction. Hence, the dip azimuth will be the right vectorial angle between the projection of the dip line on the horizontal plane and the northward direction of the meridian.

The strike line is horizontal, therefore it has two directions. To measure the elements of rock occurrence, a mining compass is used.

Lecture 3

STRUCTURE OF A BED OUTCROPPONG FROM

OCCURRENCE ELEMENTS

The position of the top or base of a stratum on the surface, i.e. its geologic boundary depends on its dip angle and the terrain profile.

To show an inclined occurrence stratum on a map, a so-called horizontal equivalent is used. The horizontal equivalent is the projection of the interval of the stratum dip line, which is contained between two strike lines drawn on the base or the top of the stratum, on the horizontal plane.

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Fig. 7

The value of the horizontal equivalent is determined as follows.

The vertical cross-section is plotted along the direction of the stratum dip line to the scale of the map (fig. 7). In such a cross-section, stratum inclination angles (α) will correspond to the true dip angle. The line that shows the stratum (M-H) can be its top or base. We cross this stratum with several horizontal planes at equal intervals. The distance between the planes (h) is chosen depending on the cross-section of the contours on the topographic base. The

value of the horizontal equivalent will be interval (a).

On the topographic base, the dip line and the stratum strike line, which is perpendicular to the dip line, are drawn through point M, at which the occurrence elements have been measured. The strike line will have the absolute mark corresponding to the mark of the point of measuring the stratum occurrence elements. The strike line is broken into the intervals equal to the value of the

horizontal equivalent, and a series of strike lines parallel to the line drawn through point M is drawn through the obtained points. The absolute marks for each of these strike line will be decreasing consequently in the direction of the stratum subsidence by the value of the cross-section of the terrain contours, and increasing by the same value, in the direction of the stratum rise. The next operation is to find all the points, at which strike lines cross the terrain contours with the corresponding terrain contours. These points are connected with a smooth curve that will correspond to the exposure of the geologic boundary registered at point A.

DETERMINATION OF OCCURRENCE ELEMENTS BASED ON A STRATUM OUTBREAK USING THE HORIZONTAL EQUIVALENT

				By	using	horizontal
			equivalents, it is easy to solve
			the inverse problem to the
			previous		one and	determine
			stratum occurrence			elements
			on the contoured map based
			on	the	stratum	outbreak
			(fig. 8). To solve this prob-
	Fig. 8
			lem,	we	should select two

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Fig. 9

points (A and B) in the map, at which the stratum outbreak crosses the same terrain contour. These points are connected with a straight line that will be the strike line of the stratum, i.e. it is horizontal and lies on the surface of the stratum. The height of the strike line is equal to the mark of points A and B, i.e. 90 m. Then, two new points (C and D) are found, at which the line of the stratum outburst crosses the next underlying (or overlying) contour. They are also connected with a straight line which will be another strike line having the height equal to the mark of these points (80 m). Perpendicularly to the strike lines, the dip line is drawn, which will show the direction of stratum subsidence from the strike line with a greater mark towards the strike line with a smaller mark. After the strike and dip lines have been constructed, a protractor is used to measure their azimuths. To find the stratum dip angle on the scale of the map, the difference of elevations of the drawn strike lines is calculated. On one of them, this value is laid off from the crossing with the dip line (m-n interval), and the found poin is connected with the point, at which the dip line crosses the other contour. The obtained angle (φ) is the required dip angle.

STRATAL TRIANGLES

One can easily see one unique feature of the shape of the stratum outburst line in the map showing the stratum outburst (fig. 9: a – plan, b – cross-section).

It forms comparatively well-pronounced angles at the lowest and highest points of the terrain. The vertex of the angle situated at the lowest point of the terrain is directed down-dip, and that at the highest point, updip to the stratum. If we imagine that the arms of these angles are connected by straight lines, we will obtain triangles which are called stratal triangles. They make it

possible to determine easily the direction of strata subsidence in those cases, when there are no terrain contours on the topographic base of the geologic map. Strata are inclined in the direction, where the vertex of the angle formed by the stratum outburst line is directed at the lowest terrain point (in the valley), and in the direction reverse to the direction of the angle vertex, at the highest point (at the divide). The value of the angle indicating the dip direction in stratal triangles may differ. It depends on the strata inclination and the form of the terrain. For vertical subsidence, the stratum outburst will look as a straight line. For steep occurrences, the angle will be broad, and as the stratum inclination decreases, it will become sharper. For the same stratum inclination, its outburst in a steep terrain will have a sharper angle compared to shallow terrains.

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Fig. 10

NORMAL AND OVERTURNED BEDDING

Two basically different types of stratum occurrences may exist when the strata are inclined: normal and overturned (Fig. 10). In case of the normal occurrence (A), the top of a stratum is higher than its base, and when the occurrence is overturned (B), the base is higher than the top. As the strata are turned, up to the moment when the angle of their inclination equals 90 degrees, their occurrence will be normal, i.e. their top will be situated hypsometrically over the base, and younger strata will be overlying older ones. When the angle becomes greater, the strata will be in an inverted (overturned) occurrence, and older strata will be overlying young ones.

When analyzing obliquely occurring strata, it is very important to identify the character of their occurrence. Underestimating the possibility of an overturned occurrence can lead to errors in determining the locations of the top and base of the stratigraphic horizon, in characterizing the stratigraphic sequence of the strata in the cross-section, in calculating the thicknesses, and in performing

tectonic plotting. An overturned occurrence can be distinguished from a normal one by a set of characteristic indicators:

1)If the data about faunal characterization of the rocks are sufficient, an overturned occurrence can be easily identified by seeing that younger stratigraphic horizons are overlapped by older ones;

2)Frequently, the position of the top and the base can be found with a fair degree of confidence by observing the distribution of fragmentary materials in laminated series. In aquatic environment, sharp changes between undisturbed and moving conditions are manifested, in the cross-section, by accumulation of coarse sediments: sands or coarse gravels. If such sediments are deposed on the surface of a fine-grained sediment (clays or siltstone), traces of intraformational erosion appear at the interface between coarse-grained and fine-grained rocks.

3)In the stratum overlying the surface of an intraformational fault, one can frequently see lentils and irregular accumulations of large-grained rocks, e.g. conglomerations in sandstone rocks linked to washouts or pockets on the unconformity surface. If the occurrence is normal, lentils and accumulations of

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Fig. 11

large-grained materials are situated over the erosion surface, and if the occurrence is overturned, below it.

4) In some cases, the mode of occurrence can be indicated by cross bedding. Oblique layers are usually cut off sharply at the top and connect smoothly to the contact base.

5) Frequently, close studies of the contacts between the strata make it possible to discern penetration of the rocks from the overlying stratum into the underlying layer. Usually, the reason for this is the emergence of cracks in the underlying layer, which are caused by densification or desiccation of the sediment and into which the sediment of the overlying stratum drifts.

6) In solidified lavas, the hardened zone in the top is several times thicker than at the base. On the surface of lava flows, a special, pillow-type jointing form is developed. To determine the top and bottom surfaces of the spread of lava, one can accumulations of gas bubbles or vugs, which are sometimes filled with secondary minerals, near the top.

Lecture 4

FOLDED MODES OF STRATUM OCCURRENCE

1. Folds and Elements of Folds

Folds are undulating (or wave-like) bends in bedded formations, which are formed by plastic deformations of the rocks. An aggregation of folds is a folded

structure (folding).

Two main fold types are distinguished: anticline and syncline folds.

Anticline folds (anticlines) are the bends, in the centre of which the oldest, as compared to their edges, rocks are situated (fig. 11). In syncline folds (synclines), the central parts are composed of younger rocks as compared with the rocks composing their edges. The following elements are distinguished in the fold:

a) curve: the part of the fold at the point where the strata bend (cross-hatched in the Figure);

b)core: the oldest (in the anticline) or youngest (in the syncline) rocks;

c)limbs: parts of the fold adjacent to the hinge (in an anticline and a syncline adjacent to each other, one limb is common);

d)axial plane: the surface passing through the lines of bending of the strata that make up the fold;

e)hinge: the line of the axial plane’s crossing the top or base of each of the strata that make up the fold;

f)axis: the line of the axial plane’s crossing the terrain.

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2. Morphological Classification of Folds

In terms of the ratio between fold limbs and the shape of the hinge, the classification is as follows:

a)Usual, or normal folds: limbs dip in opposite directions;

b)Isocline folds: limbs are

parallel;				a	b	c	d
c)	Fan-shaped	folds:	limbs
resemble a fan, and the curve is						Fig. 12
rounded.	The cores	in the	crestal

zones can become squeezed.
		d) Flat-topped, or box folds: the curve is wide and flat, and the limbs are
steep (fig. 12).
		In terms of the axial plane and limbs, folds can be:
		a) Symmetrical, or upright: the axial plane is vertical, and the both limbs
dip symmetrically;
		b) Inclined: the axial		plane is inclined, and the limbs are asymmetric;
		c) Overturned: inclined folds, in which one limb is tucked under, and the
strata in it are in an overturned occurrence;
								d)	Recumbent	folds:
						the axial surface is situated
						horizontally;
	a	b	c	d
								e)	Overfolds: the	axial
						surface has gone beyond the
			Fig. 13
						horizontal position and acqu-
						ired a backward inclination.
In these cases, anticline curves look as syncline ones (fig. 13).
Monoclines. Monoclines are a special type of fold
forms. They are elbow bends in bedded formations
(fig. 14). Usually, they are expressed by an oblique
position of strata in a narrow area while their general
occurrence is nearly horizontal. In vertical cross-
									Fig.14
sections, monoclines are characterized by:

–upper limb (AB);

–lower limb (CD);

–joining limb (BD);

–the dip angle of the joining limb (α); and

–the vertical amplitude of the joining limb (h).

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3. Representation of Folds in the Maps

Folds can be shown both in usual geologic maps, and in special structural maps. In geologic maps, the fold structure is shown by outbursts of the strata differing in their age and composition.

Structural maps are more frequently used to show the features of the morphology of deep-seated folds. They are plotted in isolines (bed contour lines) that show the surface of one separate layer.

Most frequently, structural maps are plotted in oil and coal geology basing on the data of drill wells.

The topographic map show all the wells, using which one can determine the position (absolute mark) of one and the same horizon of interest.

To do this, we should know the absolute mark of the well mouth (altitude) and the depth to that stratum. Their difference yields the required mark: 200 – 340 = –140.

Fig. 15

After calculating of all altitude marks, wells are connected with straight lines, which comprise a network of triangles (fig. 15). Here, one should try to make the triangles equilateral, wherever possible. Selecting the vertical distance between stratal contours, the required marks are found on the sides of the triangles, and the equal marks are connected with smooth curves, which are stratal contours.

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Lecture 5

MODES OF OCCURRENCE OF MAGMATIC

AND METAMORPHIC ROCKS

1. Effusive Rocks

Effusive rocks can be found in geologic cross-sections among formations of all geologic periods, from Archean to Quaternary. They occur in the form of overland and underwater effusions as tuffs and tuffites produced by volcano ejections. Pre-Cambrian effusive rocks have mainly undergone active metamorphism and turned into various crystalline schists, porphyroids, etc.

Volcanic activity can be of two types: eruptions of the central type, when magma eruption and extrusion occurs through a channel, which has comparable dimensions in the transverse (horizontal) cross-section; and eruptions of the linear type, when magma is erupted through a channel, in the horizontal crosssection of which the length in one of the directions exceeds the width by tens and hundreds of times.

The mode of occurrence of effusive rocks is also largely dependent on the chemical composition of the erupted lava. Basic lavas are very fluid. As a result, before they solidify, they can cover vast areas on the Earth surface. By contrast, acid lavas are significantly less fluid, and do not propagate far from the volcanic orifice.

As a rule, in eruptions of the central type on continents, a volcanic cone is formed of eruption products, volcanic ash, bombs, etc.

Acid lavas can solidify before they reach the surface.

Linear-type eruptions are most frequently associated with platforms. Usually, the lava content is basic, hence even on insignificantly inclined

terrains lava can sometimes cover extremely vast areas forming so-called trappean plateaus (East Siberia).

Underwater lava outflows occur in the conditions, which differ significantly from those on the continents. Such lavas are usually characterized by stable areal thickness along great distances, good sorting of piroclastic materials, and alternation with marine sedimentary rocks.

The age of effusive formations is determined from the age of the sedimentary rocks, in which they are confined.

2. Intrusive Rocks

Intrusive rocks are wide-spread in the Earth crust. The majority of ancient crystalline core-areas outcropped at present consist mainly of intrusive rocks. Depending on their relationships with enclosing rocks, intrusive rocks are subdivided into concordant and discordant. Main types of concordant intrusions are laccoliths, lopoliths, phacoliths, and sills.

Laccoliths are bodies whose shape resembles the cap of a mushroom, and the size (diameter) usually does not exceed 5 km. They are produced by

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penetration of magma under a significant pressure, due to which the overlying strate are usually bended by the pressure, and occur in concordance with the laccolith body (fig. 16).

Fig. 16

Laccoliths are usually formed at shallow depths (500–600 m) and, consequently, are frequently exposed due to erosion processes. Most often, they are composed of acid rocks. Such formations are common in the Crimea, the Caucasus, the Carpathians, and other regions.

Lopoliths are intrusive bowl-shaped bodies from hundreds of meters to hundreds of kilometers in diameter. This is a typical mode of occurrence of intrusions with basic, ultrabasic, and alkaline contents.

Phacoliths are intrusive bodies shaped as a saddle or a lens in plan and cross-section. Most frequently, they are used in the curve sections of anticline folds (fig. 17). Thickness of phacoliths can reach several hundreds of meters.

Sills (or nappes) are intrusive sheaths. They are intrusions shaped as sheets and occurring mainly in concordance with the bedding (fig. 18). Their thickness is from several centimeters to several hundreds of meters, and the areas they occupy exceed sometimes 1000 square km.

The rocks comprising sills can be from acid to basic ones (Siberian traps). Main types of discordant intrusions are batholiths, diapirs, nekks, dikes,

and lodes.

Fig. 17		Fig. 18

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