GIAO TRINH CHAPTER 7

Definition: A cross section is a profile showing geological features in a vertical plane through the earth There are two categories of cross sections: structural and stratigraphic.. •

Chapter 07

SUBSURFACE MAPPING

AND CROSS SECTION

UA-2011

– MECHANICAL & INTERPRETIVE CONTOURING

• STRUCTURAL CONTOUR MAP

• ISOPACH AND OTHER SUBSURFACE MAPS

– STRUCTURAL CROSS SECTIONS

– STRATIGRAPHIC CROSS SECTIONS

– CORRELATION AND GEOLOGICAL INTERPRETATION

– COMPLETING THE CROSS SECTION

• CROSS-SECTION DIAGRAMS IN 3 DIMENSIONS

– FENCE DIAGRAMS

– BLOCK DIAGRAMS

• COMPUTER-DRAWN CROSS SECTION s

– MECHANICAL & INTERPRETIVE CONTOURING

• STRUCTURAL CONTOUR MAP

• ISOPACH AND OTHER SUBSURFACE MAPS

LATERAL CORRELATION OF LOGGED DATA

• Using Data Points to Define Surfaces

Fig 01-Sharp vertical stratigraphic changes are excellent regional markers

Fig : 02

Fig : 03

……

• Using Surfaces to Delimit Units

• Contour Mapping

• Mechanical & Interpretive Contouring

Fig : 04

Fig : 05

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– STRUCTURAL CROSS SECTIONS

– STRATIGRAPHIC CROSS SECTIONS

– CORRELATION AND GEOLOGICAL INTERPRETATION

– COMPLETING THE CROSS SECTION

• CROSS-SECTION DIAGRAMS IN 3 DIMENSIONS

– FENCE DIAGRAMS

– BLOCK DIAGRAMS

• COMPUTER-DRAWN CROSS SECTION s

Definition: A cross section is a profile showing

geological features in a vertical plane through the

earth

There are two categories of cross sections:

structural and stratigraphic

Structural cross sections illustrate

present-day structural features such as dips, folds, and faults

Stratigraphic cross sections show

characteristics such as formation thicknesses,

lithologic sequences, stratigraphic correlations, facies changes, unconformities, fossil zones, and ages

Cross sections are also useful for

display purposes Diagrammatic cross sections show broad relationships

and serve to orient the audience to

the general geology of a region (Fig: 06)

Fig : 06

– STRUCTURAL CROSS SECTIONS

– STRATIGRAPHIC CROSS SECTIONS

– CORRELATION AND GEOLOGICAL INTERPRETATION

– COMPLETING THE CROSS SECTION

• CROSS-SECTION DIAGRAMS IN 3 DIMENSIONS

– FENCE DIAGRAMS

– BLOCK DIAGRAMS

• COMPUTER-DRAWN CROSS SECTION s

• Input Data

Different types of data can be used in developing a cross section The diagram may be based on outcrop information, or on subsurface data derived from wells and geophysical surveys, or it may use both outcrop and subsurface data

• Outcrop information are particularly

useful in frontier areas of petroleum

exploration, where subsurface

information is often sparse or

unavailable

• Many geological cross sections made for

petroleum exploration rely on subsurface data.

• The geologist should recorrelate all logs

to ensure accuracy and consistency.

Distortion of true thickness (t) and true dip (d) measurements

due to borehole deviation; tA= apparent thickness and dA=

apparent dip.

Fig : 07

Segments of the borehole which do not lie in the plane of section are dashed onto the cross sections Due to foreshortening, the dashed segments show only apparent deviation This can be

seen by comparison with the inset plan view

Fig : 08

• Line of Section

After gathering available data, the next

in cross section development step is

to choose the line of section, i.e., the line along which the cross section

would intersect the surface of the

earth (Fig 08)

Cross sections may trend perpendicular

to strike, oblique (uốn theo) to strike,

or parallel to strike (Fig 09)

– STRUCTURAL CROSS SECTIONS

– STRATIGRAPHIC CROSS SECTIONS

– CORRELATION AND GEOLOGICAL INTERPRETATION

– COMPLETING THE CROSS SECTION

• CROSS-SECTION DIAGRAMS IN 3 DIMENTIONS

– FENCE DIAGRAMS

– BLOCK DIAGRAMS

• COMPUTER-DRAWN CROSS SECTION s

The most common type of diagram showing geological relationships in three dimensions

is a fence diagram (Figure 1) Fence diagrams consist of a three-dimensional network of geological cross sections drawn

in two dimensions The diagram is built on a map base which is seen in plan view.

Figure 01

Fence Diagrams

In constructing a fence diagram, the plane

of the map base corresponds to the chosen datum plane, and the well location on the

map is taken to be the point where the well intersects the datum plane The wells are

hung on the datum and then individual

cross section panels are filled in, beginning

at the front of the diagram

In fence diagrams, one must consider the overall orientation of the figure, bearing in mind that panels oriented parallel to the viewing direction will appear only as straight lines Such panels are usually omitted from the diagram If important panels are not displayed to the best advantage, the viewing direction can be changed so that the features of interest are presented in the most effective way

Fence Diagrams

Front panels sometimes obscure parts of rear

panels in fence diagrams Where two panels

overlap, the correlations on the front panel are drawn in with solid lines and highlighted with

colors, while the correlations on the overlapped part of the rear panel are simply dashed in

When the vertical scale is too large or the spacing of sections is too close, very large parts

of the rear panels may be obscured (Figure 2)

Figure 02

Fence Diagrams

Usually, this problem can be minimized if the vertical scale and the panel spacing

are carefully chosen However, it is not

always possible to change the scale or

spacing without losing important details illustrated by the diagram In such a case, one can construct an isometric projection

of the fence diagram ( Figure 4 , only lines

parallel to the North-South and East-West axes are undistorted)

The map base in an isometric projection is shown as if it was turned at an angle and tilted toward the viewer The conversion

is actually a transformation from

orthogonal to nonorthogonal axes A

rectangular map base will become a

parallelogram in an isometric projection

( Figure 3 and Figure 4)

• All lines that were parallel to the original

horizontal and vertical axes remain

parallel to the corresponding new axes, and the scale along these lines also

remains unchanged However, lines

which were not parallel to the original

axes will be distorted Points along such lines must be transferred onto the

projection by coordinates relative to the new axes A grid system is helpful in this process

Figure 03

Block Diagrams

• Along with the fence diagram, another

dimensional geological relationships is a block diagram (Figure 5)

• This type of figure is a two-dimensional

representation of a rectangular block Two intersecting cross sections often form the sides of the block; the top of the block

shows either a mapped surface or relief on the uppermost geological boundary shown

in the cross sections The block may be

rotated to achieve the most effective

viewing angle, and it may be drawn with or without the use of perspective

Figure 05

Block Diagrams

• Isometric block diagrams do not employ

perspective opposite sides of the block

remain parallel, making the distant end of the diagram appear to be larger (Figure

6 , Figure 7, and Figure 8)

The top of this type of block diagram may

be given any degree of tilt toward the

viewer

Figure 06

Figure 08

• A large amount of tilt emphasizes features

on the top surface of the block; a smaller degree of tilt brings out details on the

sides of the block Scales along the x, y,

and z axes need not be equal

• The scale along the sides of the figure (y)

is sometimes chosen to be smaller than

the scale across the front and back edges (x), thus creating an illusion of

perspective

• Perspective can be used in constructing

block diagrams (Figure 9)

Figure 09

In perspective blocks, all parallel lines with

a component in the y direction converge to a vanishing point on the horizon

Sets of parallel lines with different orientations converge to different vanishing points

The cross section on the front face of the

perspective It is possible both to rotate the block about the z axis or to display it at different elevations relative to the horizon Again, the choice of block orientation is made to emphasize the important features

of the diagram

Computer-Drawn Cross Sections