Monday, 12 November 2012

Isopach maps vs isochore maps

Thickness maps are a fundamental tool in structural geology. They represent thickness variations and thickness trends of a given unit. Isochore and isopach maps are two different types of thickness map. To make it clear from the beginning:

  • An isopach is a line that connects points of equal true thickness (i.e. measured perpendicular to bedding), whereas...
  • an isochore is a line that connects points of equal vertical thickness

Fig. 1.- A constant thickness layer is drilled at different angles by three wells
Figure 1 depicts a perfect cilindrical fold, where the western flank dips more than the eastern flank. Well 1 cuts the yellow bed at a higher angle than well 3, and therefore, the vertical thickness found in well 1 is larger than in well 3. Well 2 cuts the bed where is horizontal, and therefore the vertical thickness equals the real thickness. The real thickness of the yellow bed is constant: 500 m.

If we would do an isopach map of the yellow unit, it would show a constant value of 500 m, because this unit doesn't show any thickness (true) variation... It would be a pretty boring map :-). But it would be telling us quite a lot already; for starting, we could think we are dealing with a parallel fold formed by flexural flow (for example).

Fig. 2-. Isochore map for the yellow unit in figure 1.
Figure 2 shows the isochore map -contours for equal vertical thickness- for the yellow bed depicted in figure 1. Red colour indicates minimum vertical thickness, whilst purple shows maximum value. You can see that it shows what you can see in the cross section: we would measure increasing vertical thickness as we move away from the hinge of the fold, as the dip of the bed increases and therefore we would cut the yellow bed at a higher angles.

We can see that the difference between an isopach map and an isochore map is quite obvious and simple to understand. Unfortunately, some people interchange both terms, and too many times we can see isochore maps refered to as "isopach maps"

If we would get the map from figure 2, and somebody would tell us that this is a isopach map over and anticline, we could wrongly conclude that the real thickness around the hinge line of the fold is smaller than in the flanks, and we would probably think that we have in front of us as growth anticline, for example. 

Another error may come from working with seismic surveys and not realising that if you take a surface representing a top unit, and you substract another surface representing a bottom unit, the result is a isochore map, not a isopach map.

Hence... how many structures have been wrongly studied, how many wells haven't reached a target because somebody used wrong a simple word? Better not even to know it! And this is considering that everytime we read a vertical thickness in a well log the well was actually vertical and not deviated at all. If you take a surface representing a top unit, and you substract another surface representing a bottom unit, the result is a isochore map, not a isopach map.

If you have any question, just ask in the comments. If you have any suggestion for a new article, just let me know. You are also welcomed to write some contribution!


Matt Hall said...

Nice post — I think 'isopach' is one of the most often misused words in exploration geoscience. And that wouldn't necessarily be a huge problem, if I didn't suspect that it reveals a basic misunderstanding of what has been mapped. So, thanks for the nice tutorial.

I've tried in the past to correct an isochore map to an isopach map by multiplying by the cosine of the dip. This sounds easy enough, but it doesn't make for a perfect correction, because it's not always obvious how to compute the dip (upper surface? Lower? Average?), and it's messy around faults. Best to correct at the wells, then interpolate.


Jorge said...

You mention a very good issue... If we consider vertical thickness in an isochore map, we can only project a given value to a single point in the surface.
But, as you say, the problem comes with real thickness. Where do we project that value? Vertically from the top? from the bottom? Middle way?
Thanks for the comments, is very welcome!

Scott said...

Excellent description of a basic and critical facet of geology as it relates to mapping analysis.

barasa hazarika said...

Thanks a lot for a clear explanation. My problem is I am not able to visualize the isopach map. The way its drawn is fine with me but how to see the contours whether projected on surface or direct subsurface projection. Please help me understand.

Juan Pablo Velasco said...

Isochore Map is the term adapted in my reports since 2006. This after I read a similar article differentiating Isochore vs Isopach.
I have a question though, if the upper and lower bounds are non-parallel, is it safe to say that one cannot create an isopach map of the structure, because we would either be measuring a point perpendicular to the upper or lower bound, but not to both.

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