Map techniques for folds
Both contouring and stereographic projection are powerful techniques for the analysis of folds. When contouring folded surfaces, you may have to try out more than one possible solution. Map patterns in folded rocks may show V-shapes that represent true fold hinges, and other V-shapes that represent the effect of valleys and ridges intersecting with planar surfaces. Try to identify any V-shapes that cannot be attributed to topography; these are likely to be outcrops of fold hinges. Fold hinge outcrops lie on fold axial traces. These are lines on the map that divide the map into different fold limbs. Try to work on one limb at a time when contouring. Remember, there may be more than one possible solution, but the simplest solution, geologically, is usually the best.
If a fold is cylindrical, then all the planes in the folded surface contain the same line. In principle, if we find the intersection of any two planes, we can define the fold axis. The plane perpendicular to the fold axis is the profile plane. We can also use the stereographic projection to find the inter-limb angle, if we have measurements of the orientation of the folded surface on the limbs at the inflection points.
Do the questions in any order, so as to use the various materials when they are available. (Samples for question 1 may not be available outside the lab hours.)
1.* Look at the samples of folded rocks displayed in the lab. Choose one sample in each group, and make a labelled diagram of the fold in profile view. Make your diagram large (fill a whole page). Your diagram should be a scientific illustration, not a work of art! Use simple clear lines to show boundaries of layers within the sample. If there are too many layers to show precisely, use a dashed ornament to show the form for the layer traces. Label as many invariant features as you can.
2.* Why would you not label variant features of these folds?
3. Construct a topographic profile and cross-section through Map 1. Proceed as follows. Mark with colours the various surfaces that separate the units. Look for places where V-shapes in the traces cannot be explained by valleys and ridges in the topography, and lightly circle possible locations for fold hinge points. Draw structure contours for each surface, on each fold limb, using lead pencil. Use coloured numbers on the contours, to show which contour corresponds to which surface. Use these to construct the geology on the cross-section.
4. Describe the folds as completely as you can using the terms in the previous sections of this manual. (Note that the units are shown in their correct stratigraphic order in the legend.)
5. Examine the map of the Great Cavern Petroleum prospect.
a) Draw structure contours on the fault surface.
b) Contour the top surface of the Great Cavern Limestone. In the southeast of the map, use the intersections of the outcrop trace with the contours. Elsewhere, use the elevations marked against each of the 26 dry oil wells. Make your contours as smooth as possible, consistent with the data provided. (Make the contours as smooth as possible in 3 dimensions too: this means that the spacing of contours should be as even as possible on each fold limb.) Mark the position of any fold hinges, and draw hinge lines. Remember that top limestone structure contours will be cut off by fault structure contours of the same elevation.
c) *Calculate the plunge and trend of the easternmost fold in the area. Keep a note of your answer as you will need it in lab 5.
d) *Construct cross-section A-B
e) Construct cross-section C-D
f) On the map, and on cross-section C-D, show the potential maximum size of an oil reservoir that might have been missed by the existing wells. (Note that in porous units like the Great Cavern Limestone, oil usually rises to the highest point in the reservoir rock; the base of an oil reservoir is typically a horizontal oil-water contact.) Draw the maximum extent of the potential reservoir on the map and the cross section. Suggest a spot for well 27 on the map and cross section which would maximize the chance for an oil discovery.
g) *Make an estimate of the maximum potential volume of the reservoir, if the Great Cavern Limestone has 10% porosity. To do this, estimate the maximum possible area of the reservoir in square metres. Also calculate its maximum vertical thickness. Then, by approximating the shape of the reservoir as a cone, use the equation for the volume of a cone (one third base area times height) to figure out the approximate volume of the reservoir, and from this, a rough estimate of the maximum volume of petroleum it might contain.
(For a more realistic estimate of the potential resource it would be necessary to take into account the potential presence of other fluids in the reservoir, the effects of pressure, and the proportion of the fluids that could be economically extracted.)
h) *Research: find out the conversion factor between cubic metres and barrels, and express your answer in barrels!