Structure Contour Maps

• Show contours connecting points of equal elevation on a geologic surface…

– Fault– Contact– Bed– Etc.

• Show elevation of the top of some geologic feature.

Structure Contour Maps

• Drawn with a specific contour interval

• Trend of structure at any point is parallel to the contour at that point– Strike is tangent to contour– Dip is perpendicular to contour

• Rules of thumb for spacing of contours is same as for topography.

• Cross sections drawn using structure contours show shape and depth (elevation) of layer

isopachs,

• A contour that connects points of equal thickness. Commonly, the isopachs, or contours that make up an isopach map, display the stratigraphic thickness of a rock unit as opposed to the true vertical thickness. Isopachs are true stratigraphic thicknesses; i.e., perpendicular to bedding surfaces.

• 1. Contour the data shown

• 2. Interpolate between nearby points. Avoid extremely long-distance interpolations.

• 3. Sometimes it pays to treat the data as a series of three-point problems.

• 4. Once you have a clear mental picture of the structure, construct smooth contours to fit the data.

Some Rules for Construction

• Structure contours must still be parallel to the strike of a structure at every point.

• Keep the contours as simple as possible consistent with the data.

• Keep the contours smooth. Do not show abrupt changes in curvature or spacing unless you have sound geologic reasons to do so.

• Interpolate only between nearby points • If the structure is only gently curved, you may find it useful to

approximate the structure as a series of plane segments at first. For each group of three data points, construct structure contours using the three-point method. Make sure the triangles are as nearly equilateral as possible. Once the contours are constructed, draw the final contours as smoothly as possible using the construction as a guide.

• The diagram below shows the three shale dome groups.

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Traps Associated with Shale Domes

• The Encino field of San Patricio and Refugio counties, Texas has small gas and oil production in rollover rim traps. The cross section below shows this feature.

Pinchout and truncation example

Upturned and trucation traps

• . Find the strike and dip of the sandstone layer from the map at left. Outcrops are in brown.

• 2. For pairs of points at the same elevation we can draw structure contours directly. The more points on a line, the better.

• 3. In other areas we can use the three-point method to find structure contours.

• 4. Find dip by drawing a cross-section or by trigonometry.

Fold type, axial plane trace and sections

• This concept of contour lines also works under ground. Just like rocks above ground can be seen to be bent, forming anticlines (domes) or synclines (saddles), these structures extend beneath ground. Therefore, if the geologist understands what the structure of the layers of rock underground look like (either by well data or geophysical evidence such as seismic reflections), he or she can draw a contour map which represents this structure

Structure Contour Maps

• Drawn with a specific contour interval

• Trend of structure at any point is parallel to the contour at that point– Strike is tangent to contour– Dip is perpendicular to contour

• Rules of thumb for spacing of contours is same as for topography.

• Cross sections drawn using structure contours show shape and depth (elevation) of layer

Structure Contour Maps

Structure Contour Maps

Structure Contour Maps

Structure Contour Maps

• A contour that connects points of equal thickness. Commonly, the isopachs, or contours that make up an isopach map, display the stratigraphic thickness of a rock unit as opposed to the true vertical thickness. Isopachs are true stratigraphic thicknesses; i.e., perpendicular to bedding surfaces.

• The rainbow contours correspond to thickness of overburden sediment on the ocean floor and the increasingly pink colors represent greater sediment thickness. For a larger map of the isopach contours showing data points and core locations, click

How to Contour A Map

• Geologists make all kinds of contour maps.  Contouring is not hard, but it takes lots of practice and patience to make a nice-looking map. You start by "spotting", or drawing,  all the wells on a map.  In a large part of the United States, the Township and Range System is used to spot wells.

• Next, you look at the electric logs for each well.  We are going to make an "isopach" (eye-so-pak) map.  An isopach map is a map that shows the thickness of something.  In this case, it is the thickness of a sandstone formation.  

• Count the number of feet of sand in the zone you are interested in.  Then, put the number of feet of sand below each well spot.

• Start contouring with the highest values.  Use a "contour interval" that is slightly less than the biggest values.  In this case, the first line drawn is the 40-foot contour. 

• As you draw your line, look carefully when you pass between two wells.  Try to use your eye like a "ruler", and position the line at the proper distance between the two wells.  Notice the well with a footage of "38".  Since 38 is very close to 40 (the contour we are drawing, you should "pull" the 40-foot contour over close to the 38-foot line, like I did here.

• If there is a "40" on the map (there is on this one), you will draw the 40-foot line right through that well

• Now draw the 30-foot contour. "Eyeball" the map and be sure to leave the proper amount of room for the remaining contour lines.  Use a pencil, because you have to erase a lot!!

• Finish the map by drawing the 20-foot, 10-foot, and zero contour lines.  Label the "contour interval" you used at the bottom of the map.  In this case, the contour interval is 10 feet.