ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 7 docx

Cross
Cross
-
-
section diagrams in
section diagrams in
3
3
Dimentions
Dimentions
Fence Diagrams
Fence Diagrams
In petroleum exploration, the development of a prospect is a
In petroleum exploration, the development of a prospect is a
three
three
-
-
dimensional problem. One of the drawbacks of cross
dimensional problem. One of the drawbacks of cross
sections is that they are limited to two dimensions. How can
sections is that they are limited to two dimensions. How can
we illustrate the three
we illustrate the three
-
-
dimensional geology of an area?
dimensional geology of an area?
The most common type of diagram showing geological
The most common type of diagram showing geological
relationships in three dimensions is a fence diagram (

relationships in three dimensions is a fence diagram (
Figure
Figure
). Fence diagrams consist of a three
). Fence diagrams consist of a three
-
-
dimensional
dimensional
network of geological cross sections drawn in two
network of geological cross sections drawn in two
dimensions. The diagram is built on a map base which is
dimensions. The diagram is built on a map base which is
seen in plan view.
seen in plan view.
Figure 01
Figure 01
Fence Diagrams
Fence Diagrams
In constructing a fence diagram, the plane of the map base
In constructing a fence diagram, the plane of the map base
corresponds to the chosen datum plane, and the well location
corresponds to the chosen datum plane, and the well location
on the map is taken to be the point where the well intersects
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
the datum plane. The wells are hung on the datum and then
individual cross section panels are filled in, beginning at the
individual cross section panels are filled in, beginning at the
front of the diagram.

front of the diagram.
In fence diagrams, one must consider the overall orientation of
In fence diagrams, one must consider the overall orientation of
the figure, bearing in mind that panels oriented parallel to the
the figure, bearing in mind that panels oriented parallel to the
viewing direction will appear only as straight lines. Such
viewing direction will appear only as straight lines. Such
panels are usually omitted from the diagram. If important
panels are usually omitted from the diagram. If important
panels are not displayed to the best advantage, the viewing
panels are not displayed to the best advantage, the viewing
direction can be changed so that the features of interest are
direction can be changed so that the features of interest are
presented in the most effective way.
presented in the most effective way.
Fence Diagrams
Fence Diagrams
Front panels sometimes obscure parts of rear panels
Front panels sometimes obscure parts of rear panels
in fence diagrams. Where two panels overlap, the
in fence diagrams. Where two panels overlap, the
correlations on the front panel are drawn in with
correlations on the front panel are drawn in with
solid lines and highlighted with colors, while the
solid lines and highlighted with colors, while the
correlations on the overlapped part of the rear panel
correlations on the overlapped part of the rear panel
are simply dashed in.
are simply dashed in.
When the vertical scale is too large or the spacing of

When the vertical scale is too large or the spacing of
sections is too close, very large parts of the rear
sections is too close, very large parts of the rear
panels may be obscured (
panels may be obscured (
Figure 2
Figure 2
).
).
Figure 02
Figure 02
Fence Diagrams
Fence Diagrams
Usually, this problem can be minimized if the vertical scale
Usually, this problem can be minimized if the vertical scale
and the panel spacing are carefully chosen. However, it is
and the panel spacing are carefully chosen. However, it is
not always possible to change the scale or spacing without
not always possible to change the scale or spacing without
losing important details illustrated by the diagram. In such a
losing important details illustrated by the diagram. In such a
case, one can construct an isometric projection of the fence
case, one can construct an isometric projection of the fence
diagram (
diagram (
Figure 4
Figure 4
, o
, o
nly lines parallel to the North

nly lines parallel to the North
-
-
South
South
and East
and East
-
-
West axes are undistorted
West axes are undistorted
).
).
The map base in an isometric projection is shown as if it
The map base in an isometric projection is shown as if it
was turned at an angle and tilted toward the viewer. The
was turned at an angle and tilted toward the viewer. The
conversion is actually a transformation from orthogonal to
conversion is actually a transformation from orthogonal to
nonorthogonal
nonorthogonal
axes. A rectangular map base will become a
axes. A rectangular map base will become a
parallelogram in an isometric projection (
parallelogram in an isometric projection (
Figure 3
Figure 3
and
and
Figure 4

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
Figure 03
Figure 04
Figure 04
Block Diagrams
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

Figure 05
Block Diagrams
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 06
Figure 07
Figure 07
Figure 08
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 098
Figure 098
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.