Reducing Dry Holes

The following article is taken from SCA's 4th Quarter GeoLOGIC newsletter.  Click here for the full publication. 

Reducing Dry Holes
by Daniel J. Tearpock and Robert C. Shoup

Every year, our industry loses hundreds of millions of dollars on dry holes. Many of those dry holes are the result of interpretations and maps that are incorrect. As such, many of those dry holes could have been avoided by critically reviewing the final prospect maps and data used, using the “Quick Look Techniques” developed by Subsurface Consultants & Associates, LLC, before the wells were drilled.

One of the more common mistakes we see when reviewing maps is two or more faults connected incorrectly as one. When this is done, any traps associated with the fault pattern are incorrectly interpreted and mapped; dry holes or uneconomic wells waiting to happen. There are several Quick Look Techniques you can use to ensure that faults have been interpreted and mapped correctly. In this article, we will discuss one of the more powerful QLTs: implied fault strike.

Before discussing implied fault strike, we first need to review fault traces. 

Figure 1 (above) shows the fault surface map for Fault A. Note that the strike of Fault A is north to south with a slight westward curvature.

Figure 2 (above) shows a structure map of a producing reservoir. The map surface and the fault surface have been integrated so that the trace of Fault A on the final map has been positioned correctly and the width of the fault gap has been properly defined. Note that the orientation of the fault trace of Fault A is north to south with a slight eastward curvature. Since the fault trace on the completed map is the intersection of the fault surface with the horizon surface, the fault trace will not have the same orientation as the fault surface. With steeply dipping beds, the orientation of the fault trace can be almost ninety degrees to the strike of the fault surface.

Figure 3 shows a 3D perspective of the mapped horizon shown in Figure 2. If you examine the figure you will see for example, that the 8600’ contour for the upthrown (footwall) block of the horizon and the 8600’ contour of the downthrown (hanging wall) block of the horizon are connected by the 8600’ contour on the fault surface. This should occur for all contours mapped and if we connect all horizon contours of equal value, we should be able to generate what the fault surface looks like based on the interpretation (Figure 4, below).

You see, unfortunately today many interpreters do not interpret or map faults. This is a major flaw in their training or education. In most interpretation work, the major and potential trapping faults should be interpreted and mapped first before ever attempting to tackle the horizons. This is a fundamental principle of basic geoscience interpretation.

In the review of many prospects, the presenters will often not have a fault surface map to review or have not even interpreted the fault to such a degree that a map can be made. So they have not followed the fundamental principles of good geoscience interpretation. Therefore it is left up to the reviewer to retrogeoscience the completed map to see if the fault picture being presented is reasonable or even possible in our three dimensional world. And thus is the prospect geologically valid in three dimensional space.

Looking at Figure 5a, we see a structure map of a faulted horizon. 

Note that the fault trace exhibits a strong bend. Is the fault trace properly mapped, or have two faults been incorrectly mapped as one? Applying the concept of implied strike, we can see when contours of equal value are connected (red lines, Figure 5a). The implied strike of the fault surface is east-west. So what we have done is to generate an implied fault surface from the completed map. This is often very surprising to geoscientists that one’s work can be checked or verified in this way. So without seeing the supposed fault that was used for this map, we can generate an implied fault surface. In this case, when we overlay the fault surface map with the horizon map (Figure 5b, below) we can see that the fault trace on the map fits fairly well with the integration of the fault surface with the horizon surface and therefore we can conclude that the map is reasonable in three dimensional space.

This is a quick method to evaluate one aspect of an interpretation and map. If the fault interpretation is unreasonable or impossible based on this “Quick Look Technique”, then there is significant question as to the reliability of the interpreted structure map. Often there is limited time to review a prospect, or a developed field map for that matter. Therefore such “Quick Look Techniques” are very applicable in doing one’s forensic geoscience to evaluate the validity of interpretations and maps.

Now take a moment to look at Figure 6a. 

A well has been proposed to test the downthrown trap which is downdip of a producing field. The trapping fault has a pronounced bend. When major bends are seen on fault traces on structure maps a red flag should go up. The question that needs to be resolved is, ‘Has the interpreter implied that the fault surface is making this major bend, or is the fault trace making this bend due to the integration of the fault and the horizon?” There is a significant difference in which of these is correct.

So our question is, “Is the trapping fault properly mapped and it is the trace that is bending on this horizon or could two faults have been interpreted incorrectly to connect as one and then mapped as one fault due to the misinterpretation?” Let us apply the Implied Strike Technique to this fault. Looking at Figure 6b (below), the two red lines are the implied strike of the 14,900’ contour that intersects the fault footwall and hanging wall traces.

The sharp bend in the implied strike suggests that the trapping fault is interpreted incorrectly as the 14,900’ contours cross at the * location. Most likely the interpreter connected two faults incorrectly as one. This significantly increases the risk of this prospect. In fact there may not be a prospect here at all and instead it is a dry hole waiting to happen.

The application of the implied strike technique, along with other QLTs applied prior to drilling, may save your company millions of dollars of dry hole costs.

Article references:

Tearpock, D.J., Bischke, R.E., and Brewton, J.L., 1994, Quick Look Techniques for Prospect Evaluation, SOG Press La., 286 p.                

Tearpock, D.J., and Bischke, R.E., 2003, Applied Subsurface Geological Mapping With Structural Methods, 2nd Edition, Prentice-Hall, N.J., 822 p.

Course materials for “Applied Subsurface Geological Mapping”, instructor D.J. Tearpock, presenting organization Subsurface Consultants & Associates, LLC

Course materials for “QAQC Skills in Subsurface Mapping”, instructor D.J. Tearpock, presenting organization Subsurface Consultants & Associates, LLC