LAB 3
CONTOUR MAPPING
Objective
To learn how to construct contour maps with and without faults.
Background
Fundamentals
Oil and gas accumulations occur under favorable structural and/or stratigraphic
conditions. It is therefore of value to be able to use existing data points to map the
structure and hence indicate the potential location for hydrocarbon accumulation. In
Figure 1 are just a few examples of oil or gas traps.
Figure 1a. Oil accumulated in Domal structure
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Figure 1b. Structural trap resulting from faulting.
Figure 1c. Oil accumulation under an unconformity.
Figure 1d. Oil accumulation in the vicinity of a piercement-type salt dome.
A structure contour is a line of equal elevation drawn on a structural surface; such
contours are used to depict the form of the surface. In the simplest case, a structure
contour drawn on a planar surface is a straight line and is synonymous with line of strike.
On curviplanar surfaces, they appear as curved lines which are everywhere tangent to the
strike (See Figure 1a). As an imaginary line connecting points of equal elevation structure
contours are partially analogous to topographic contours. The visualization of the features
portrayed by both types of contours follows the same rules. In addition, however,
structure contours have several unique properties: the surface represented by the contours
may overhang, or it may be broken by faults. In certain circumstances it may also be
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useful to use an inclined rather than horizontal datum plane. Structure contours are
particularly important in showing the structures where the dip angles are small, and
for showing the structure of large areas.
The available factual information concerning the configuration of a structural surface
usually takes the form of a series of points of known elevation. The construction of a
structure contour map from this raw data involves two general steps: interpolation and
interpretation.
Interpolation
The choice of the contour interval depends on the amount of relief present on the surface,
the map scale, and on the spacing of the data points and the accuracy of their location and
elevation. Intermediate elevation points are then established which correspond to this
chosen interval, and tentative contours drawn through them. The location of these
intermediate points may be found by eye, especially if only a very few contours must be
interpreted, with special devices, or by simple construction.
The procedures to contour a map can be taken as follows:
1. Connect three adjacent elevation points with straight lines (for example, points A, B,
and C of Figure 2 (a), which are taken from the map of Figure 3 (a).
2. By assuming that these three points define a planar portion of the surface, the location
of the intermediate points can be found easily (Figure 2 (b)).
Figure 2 Interpolation of intermediate elevation points.
a.
b.
c.
Draw a line perpendicular to AB, through point B; this established two sides
of a right triangle.
The elevation difference between points A and B is 21 m (= 318 - 297).
Using any convenient scale, draw the hypotenuse of this triangle 21 units
long.
Along this hypotenuse count off the vertical distances to even multiples of
the contour Interval. Here the interval is 10 m, so that points 3 and 13 units
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d.
along this line represents 300 and 310 m contours. The location of these
points are found by dropping perpendiculars back to line AB.
Repeat for the two other sides of the triangle ABC, and draw in the tentative
contours as straight lines (Figure 2 (a)).
These intermediate points and tentative contour segments are only a first approximation
of the form of the structural surface. For a curviplanar surface, curvilinear contours must,
of course, be used. The next step is to draw these lines. An examination of the map may
reveal advantageous places to start drawing, for example, in areas where close spacing of
data points gives greater control, or where the steep slopes require a number of contours
to be interpolated, or in areas of the highest or lowest elevations. Working in bands
curved lines are drawn which agree with the known elevations. In the absence of other
information, the contours should be as smooth as possible, and with a spacing which is as
nearly equal as possible. In general this will require that the contours do not pass exactly
through the interpolated elevations, but this is understandable since the assumption which
determined their location is false.
The contour patterns should progressively evolve during the work. It will be found that
altering the position of one contour line requires that several adjacent ones must also be
shifted. The final pattern of this stage of objective contouring is the one which technically
accounts for the known elevations, but introduces no features not demanded by them. The
map will be easier to read if certain contours are shown with a heavy line, such as every
fifth one, and if some or perhaps all are labeled with their value (Figure 3 (b)).
Figure 3 Method of contouring. (a) A map showing the elevation data. (b) The contoured
equivalent.
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Interpretation
A map can be contoured so that all of the technical requirements just described are fully
satisfied, yet fail to convey the probable structural conditions. Such a map is shown in A
of Figure 4. There are no technical errors in the contouring of this map, but it fails to give
a consistent picture of the structure. On the west side of the map, the strike is east and
west, but the dip varies from very low in the north to steep in the central portion and back
to low in the south. In the central part of the area there is no consistency in the structural
features in that contours are pinched together in some places and widely spaced at others.
The east side shows a constantly changing dip and strike. Although it is quite possible for
such structural conditions to exist, it is not probable.
In Figure 4, B shows the same control points contoured in a manner that reveals two
plunging anticlinal noses, two synclines, and a well-defined terrace. This sheet was
contoured, not to tie the widely separated control points together in the simplest manner,
but rather to develop the forms of any geologic structures that might be suggested in the
variations in the rate of dip or changes in strike. In other words, this map bears the
unmistakable marks of geologic interpretation of the data.
A knowledge of the general character and form of structures in the region aids greatly in
correctly interpreting the subsurface structure where the well control is sparse. When the
character of folding is known, an attempt should be made to contour the scattered points
so that the features shown bear out the regional trends or tendencies. Often the subsurface
geologist is called upon to construct structure maps where little is known about the
regional trends. However, there are usually some clues in the datum elevations
themselves. A common but often erroneous assumption is that most of the higher wells
are on the highest parts of local structures, and most of the lower ones are on the lowest
points of the structures. When starting to contour the subsurface map, it is better not to be
too strictly constrained by the few scattered elevations on the sheet. In some places the
actual structural elevations probably exceed those of the highest wells and at others are
less than those of the lowest wells. As long as the technical requirements of contouring
are adhered to, the geologist has considerable license, and he should endeavor to present
a consistent and feasible picture.
Figure 4, C illustrates the method by which the map, B, in the figure was constructed. A
cursory inspection of the datum values of the wells shows that the regional strike is
roughly east and west over most of the area. A high rate of dip is shown between
elevation 1,500 and 2,100 feet on the west side and 1,650 and 2,040 feet on the east. It is
assumed that at these two localities the pairs of wells are aligned somewhere near the
direction of full dip, and that from these wells to other nearby ones, where a much lower
rate of dip is suggested, the directions are along components of the true dip. Therefore,
the contours are drawn in such a way that a consistent rate of dip is maintained. By
assuming a northwesterly strike through points 2,100 and 1,500, the points 2,415 and 980
are contoured with negligible variation in either dip or strike. A similar procedure is
followed for each locality where the distribution and relative datum elevations of the
wells provide the best control on the rate of the dip and the local direction of the strike.
These areas are then joined by extending certain contours with values nearest those of
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scattered datum points located between the “detailed" areas, as shown by the dashed lines
in the figure. These lines form the skeleton of the map, and it is a simple matter to fill in
the remaining contours.
Figure 4 Comparison of careless (A) and orderly (B) methods of contouring.
Contouring Procedures
The step-by-step procedure to contouring is given below.
1) Plot and post well data on the location map.
2) Convert all depths to elevations with respect to the chosen datum.
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Datum
The datum is the reference point chosen to indicate the subsurface geologic feature of
interest. A structural datum uses a constant elevation, typically mean sea level (MSL) as
the reference point. This datum is chosen to reflect structural changes in the formation.
A stratigraphic datum follows the top of a formation and is selected to illustrate changes
in thickness in underlying beds.
A schematic of a structural datum is shown in Figure 5.
0’
500’
KB
DF
GL
MSL
-500’
1000’
Figure 5 Schematic of structural datum
The subsea depth to a given point is the difference between the measuring point elevation
and the measured depth (MD). The measuring point elevation can be from ground level
(GL), the derrick floor (DF) or the Kelly bushing (KB). The logging company will
indicate on the log header which reference was used for the well. For example, if the KB
elevation is 500 ft and the total measured depth is 1000 ft., then the subsea depth is –500
ft.
In this first example, the measured depth was equivalent to the true vertical depth (TVD)
of the well. However in many cases, either by design or not, the MD is not equal to the
TVD. Figure 6 illustrates the problem. A directional survey is required to convert the
measured depth to true vertical depth.
KB
DF
GL
MSL
MD
TVD
Figure 6 Schematic of a deviated well
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3) Locate the high point (on structure contour maps) or the thickest part of the reservoir
(on net pay or isopach maps), block it in, and use that as a reference.
4) From the highest point, draw rays (very lightly and in pencil) from the high point
wells to a lower elevation well that is nearby on the flank of the structure (rays should
NEVER cross the high part of the structure).
5) Measure the map distance between the two wells and using the difference in elevation
between those two points, calculate a map contour factor by dividing the distance into
the difference in elevation. You then divide the map factor into the elevation
difference between the high point well and the next contour value to locate that
contour.
6) Repeat this procedure until the contours are blocked out and you have some idea as to
the form of the structure.
7) Sketch in the contours so they are symmetrical about the structure and are smooth.
8) Finally label the contours clearly, erase any stray marks and darken the lines so they
are clear and sharp.
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Exercises
Well locations and their corresponding elevations are shown in the above map. Contour
the map using a 100 ft interval.
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