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

Exploring the Ten Habits - Habit Three: Managing Your Time

by Bob Shoup

In SCA’s ongoing blog series highlighting, "The Ten Habits of Highly Successful Oil Finders" we are now featuring Habit Three:

Successful oil finders plan their time and their work in order to ensure accurate interpretations and maps.

You may have heard the quote “Poor planning on your part does not constitute an emergency on my part.” Unfortunately, in our industry, poor planning does result in dry holes - lots of them! One of the surest ways a company can drill a string of dry holes is to enter a rig commitment before having a portfolio of prospects that have been accurately mapped and approved for drilling. While your manager may be responsible for poor planning, you are responsible for the accuracy and quality of your maps. Rushed interpretations almost always result in poorly mapped prospects. And poorly mapped prospects are poor prospects. Your ability to deliver high quality interpretations and maps will depend on your ability to plan and manage your time.

Generally speaking, projects have deadlines. So when you begin a project you must first decide what work is needed, and when that work is needed. To do that, you, or your manager, must first begin with the end in mind. What decision is needed and what are the costs of that decision? The decision to commit hundreds of millions of dollars to a development project requires a different level of understanding than does a decision to spend tens of millions of dollars to drill a well, or hundreds of thousands of dollars to buy seismic.

As an oil finder, your responsibility is to determine how much work is needed to ensure accurate interpretations and maps.  How many wells do you need to correlate? How much seismic do you need to interpret? How much reservoir engineering data do you need to integrate? How many maps do you need to make? How complicated is the geology of the area you are working in? Are there legacy interpretations you can build on?  

Once you have determined how much work is needed to make good quality interpretations you must then determine how much time is needed to make those interpretations. Each of us work at a different pace, so know your pace and set realistic time frames and goals. Once this is done, go back to management and see if the time frame you need fits with the time frame that matches the business needs.

If your business time frame doesn’t meet your workflow time frame, then the workflow must be adjusted. Here the “Law of Diminishing Returns” can serve as a guideline in adjusting the decision point and the workflow.  The Law of Diminishing returns states that the continued application of effort or skill toward a particular project or goal tends to decline in effectiveness after a certain level of result has been achieved. In other words, at a certain point, more effort does not yield proportionately better results.

In our industry an optimal decision is made when all of the available data has been integrated into a series of final maps that are the basis for reserve estimation. Decisions made after this point are usually decisions that managers are agonizing over, or are avoiding, and time spent after the optimal decision point is usually not justified (see chart below).

Click image to enlarge

So begin with the end in mind and tailor your activities to meet deadlines without compromising quality.  So, looking at the graphic again, work with your manager to determine what work is needed to reach a “good” decision point?  Or, alternatively what do you need to do to get past the point of making a bad decision.  

One thing that can improve your efficiency as well as the quality of your interpretations is training. SCA has an exciting training line-up featuring short courses tailored to the requirements of upstream professionals.  Review our training calendar and take the next step towards ensuring your oil finding career is a successful one for many years to come. 

Exploring the Ten Habits - Habit One: Ensuring Interpretations Are Reasonable and Valid

by Bob Shoup

Bob Shoup's blog entry from earlier this year, "The Ten Habits of Highly Successful Oil Finders" sparked a lot of interest and discussion. As promised, we are now delving further into each habit, and want to invite your participation.  Read on to find out more...

Habit #1: Successful oil finders ensure that their interpretations are geologically and geometrically valid in three dimensions. Here is an investigation as to why.

At SCA, we see many prospect maps that are geologically unreasonable and often geometrically impossible. These unreasonable and impossible maps have cost our industry billions of dollars in unnecessary dry holes. These dry holes are all the more tragic when you consider the fact that all of the wells drilled on the basis of these unreasonable maps could have been avoided with the use of proper techniques. 

Figure 1

For example, one of the most common mistakes we see is the result of interpreters connecting two or more faults as one.  (Figure 1)

Figure 2

Often the two faults that are connected have throws in different directions, which we call 'screw faults'. With the exception of strike-slip faults, screw faults are a geometrically impossible interpretation. Yet we see them on many maps. They are so common that a screw fault interpretation can be seen in the user manual of one of our industry's leading 3D interpretation packages.

Any prospect that relies on a closure against a screw fault is a dry hole waiting to happen. One way to mitigate this risk is through utilization of Quick Look Techniques as developed and taught by SCA's leadership and training faculty, and also available as a printed textbook.  A classic Quick Look Technique to avoid drilling wells on prospects that rely on screw faults is to plot the vertical separation along the trapping fault (Figure 2). 

Figure 3

This technique takes less than an hour and can result in you saving your company the cost of a dry hole  (Figure 3). Overall, a pretty good investment of time.

So, now we would like to hear from you. Do you know of any dry holes that were drilled on geometrically impossible interpretations? We encourage you to share your examples (redacted as necessary to prevent release of proprietary information), so that others may benefit from your experience. If the example you send is selected for inclusion in an upcoming SCA Quick Look Techniques class, you can attend that class (or any other scheduled QLT class) at no charge.