geol 3402 lab 04: geologic maps ii: structure contours & 3-point

GEOL 3402 Lab 04: Geologic Maps II: Structure Contours & 3-Point Problems v. 2015

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Lab 4: Geologic Maps II: Structure Contours & 3 Point Problems

Objectives:

! Construct and evaluate structure contours on various geologic features ! Calculate the orientation of a planar geologic surface using a variety of 3 point methods

Materials: tracing paper, pencils, ruler, protractor, divider, trig-function calculator Structure Contours A structure contour is an imaginary line that connects points of equal elevation (a contour) on a structural surface such as a fault, the top of a stratigraphic bed, or a buried erosional surface (unconformity). Structure contour maps can be thought of as a “topographic” map that contours some structural or sedimentological surface, just as a real topographic map contours the earth’s surface.

Figure 1 A (above) shows a shaded oblique perspective of an inclined plane with a NW strike and a 75° dip to the SW. Structure contours are drawn at 100’ intervals (dashed white lines). Figure 1B depicts the map view of the structure contours. Note how the star symbols on Figure 1B correlate with the stars as projected from the 400’ contour in Figure 1A. Structure contour maps are often used in the petroleum industry to describe sub-surface structures where drill hole data is available, but only limited or no ground surface structural data is available. Examples might include domal petroleum traps or tabular aquifers.

The data used to construct a structure contour map can be presented in terms of elevations above (or below) sea level or in terms of depths below the ground surface. You must always specify the datum plane on every structure contour map. Note that if the ground surface is not horizontal (if you are drilling on a sloping hill side, for example) and the data are provided as depth below ground surface, a correction must be applied so that all points represent depth below the same horizontal datum plane. Sea level is a common datum plane for most contour maps.


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Study Figure 2 to help visualize topographic contours, the “rule of V’s”, and structure contours. In the top block diagram (Fig. 2A), a transparent layer about 100’ thick is gently inclined to the north. Note how the trace of the planar unit “v’s up the drainage”, indicating that it dips “up the drainage”. Compare Fig. 2A and 2C to help visualize the shape of the contact.

The black solid line in Fig. 2A represents the trace of the “top” of the planar unit where it crosses the 300’ topo contour. The dashed black line connects the two localities where the planar surface intercepts the 400’ topo contour interval (Fig. 2A). Hence, these two lines represent structure contours on the inclined unit. Note that these lines are parallel to the strike of the plane – i.e., they are horizontal lines contained within an inclined plane! Yay! Using the quadrant convention, what’s the strike of the unit? Figure 2B displays a cross section drawn parallel to X-X’. Visualize the N-dipping plane in 3D. Each black dot on the topographic profile represents the point where the topography intersects the map contour interval. Figure 2C is a map view of the trace of the inclined plane on a shaded-topographic relief map (notice the “v” pattern) and structure contours drawn every 20 feet. Each white circle represents a point where the structure contour intersects the outcrop trace of the unit. Also shown is a topographic profile and the inclined bed (Fig. 2D). 3-point problem teaser… Notice that it’s possible to solve for the dip, δ, using trigonometry: Tan δ = Δ elev. (ft)/map distance (ft) – see if you can “see” this relationship in Fig. 2C and D. Cool! Figure 2. Three views of structure contours.


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Can you contour non-planar surfaces?

Non-planar structures such as folds and domes may also be contoured. In the above diagram (Fig. 3A) a south-plunging anticline is contoured in feet below the local land surface. (Do you remember what plunging means?) The left hand diagram shows an oblique perspective; Figure 3B shows the 2-D contour map. Note the plunging anticline symbol.

Structure Contour – a line of equal elevation on a continuous or projected structural surface.

Structure Datum – Stratigraphic or structural surface (sometimes called a horizon) on which the contours are drawn (e.g., the top of a reservoir, unconformity, contact, fault, etc.).

Structural Elevation – Elevation on the datum above or below sea level (or depth below Earth’s surface).

NOTES:

Fig. 3


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3 Point Problems in Detail

Fig. 4A depicts three wells drilled at an elevation of 1000’ above sea level. Each well intersects an inclined fault plane at a different depth below the local ground surface. What is the orientation of the fault plane in the subsurface? Knowing how to calculate the orientation of the fault plane will allow you to then predict where the fault plane would project further into the subsurface. Thus, you will have a better understanding of the 3D geometry of rock bodies, reservoirs and other features in the un-exposed subsurface.

Step 1 (Fig. 4B): Visualize and sketch the problem.

Overlay tracing paper and precisely trace the well locations, map scale and use a ruler to connect the wells. The triangle 123 in Figure B essentially represents the fault plane, with each apex of the triangle at different depths. We need to find a line of equal elevation – i.e., a structure contour or a strike line - contained in this plane. Distinguish the shallowest and deepest wells (e.g., well 2 and well 3, respectively). Somewhere along the line 23 is point 1’ at a depth of

700’ (Fig. 4C). Once you find point 1’ on line 23, you have established the strike line fo the plane.

Note that the depth to 1’ in Figure 4C is 700’; in other words a well drilled at point 1’ on line 23 would intersect the fault plane at an elevation of 300’ above sea level.

Step 2 (Fig. 4D): How to find point 1’?

Method 1: Draw a profile plane as in D with the same scale as the map; draw a line that connects the depth to fault in wells 2 and 3. Then, locate the depth to fault in well 1 along that line (i.e., 700’, or 300’ on the profile). Project that point up to line 23 (Fig. D).

Method 2: calculate the difference between depth-to-fault between middle well and shallow well, and, the deepest well and shallow well. The ratio represents the distance along line 23 to point 1’:

Intermediate well – shallow well = 700’ – 400’ = 300’

Deep well – shallow well = 900’ – 400’ = 500’

Therefore, point 1’ is 3/5th’s of distance from well 3 along line 23. Use the map scale to measure this distance.

Figure 4A-D.

0 500 ft.

N

700’900’

400’

well 1

well 3

well 2

map surface at 1000 ‘ above sea level

vertical depth’s to inclined fault plane

A

700’900’

400’

1

3

2

1’

3 2

1000’

600’

SL

-200’

200’

400’

800’1’?

line 23

V = H

0 500 ft.

CB

700’900’

400’

1

3

2

700’

200’

100’300’400’500’600’1’

1’

700’

200’

100’300’400’500’600’3

2

V =

H

D

ft. ab

ove SL

ft. above SL

REMEMBER!vertical depth’s to inclined fault plane


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Step 3 (Fig. 4E): Draw structure contours.

Connect points 1 and 1’ and you’ve just drawn a structure contour at a depth of 700’. Complete the map by drawing a contour at 100’ intervals.

Step 4: Calculate the strike of the fault plane using either azimuthal or quadrant conventions.

Step 5 (Fig. 4F): Define dip direction & calculate the dip angle, δ.

Measure the map distance (MD) between two structure contours using the map scale. Note the vertical distance between the two structure contours (Δelev.). Using trig we know that

δ = tan-1 (Δelev /MD) = tan-1 (200’/400’) = 27°

Therefore, the orientation of the fault plane is N20°E/27°NW. Be able to visualize the fault plane in the resulting structure contour map (Fig. 4G).

Figure 4. E-G. NOTES:

700’900’

400’

1

3

2

1’900’

700’

400’

500’

600’

800’

0 500 ft.

EN

700’900’

400’

1

3

2

1’

400’

-100

’-2

00’SL10

0’200’

300’

dip

dire

ctio

n

0 500 ft.

0

500

ft.

horiz

onta

l

fold

line

F

N

δ

0 500 ft.

N

well 1

well 3

well 2900’

700’

400’

500’

600’

800’

27°

G

geol 3402 lab 04: geologic maps ii: structure contours & 3-point

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