SUBSURFACE MAPPING AND CROSS SECTION (cơ sở KHOA học địa CHẤT dầu KHÍ SLIDE TIẾNG ANH)

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

– 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 SECTIONs

– 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 SECTIONs

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 SECTIONs

• 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

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 SECTIONs

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

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 type of diagram illustrating three- 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 block is usually left undistorted by 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