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Don’t rely on preconceived notions

I am amazed that geophysics works. When you think about the seismic experiment, there are so many assumptions and technologies involved. Couple this with the fact that the earth is a filter with endless complexities and non-unique characteristics, and it is incredible that we find hydrocarbons at all.

However, the days of finding large reserves under simple bumps are gone. Today’s exploration plays are plagued with intense structures, stratigraphic variability, anisotropy, high pressures, low permeabilities, and deep targets. The geophysical problem is ever more complicated.

Traditional interpretation techniques are insufficient in these areas. It’s not about mapping the wiggle, it’s about what causes the wiggle to look that way. It’s about rock physics. A seismic signal has three properties: amplitude, phase, and frequency. Seismic attribute analysis is an evaluation of one or more of these properties in some detail.

The tricky thing with seismic attribute analysis is that it is still not well understood by most explorationists. Perceptions about the techniques involved are typically based on old technologies, tales of failure, or conjecture.

It is interesting to me that seismic interpreters routinely pick seismic wiggles and use these correlations to confidently map structures and reservoirs. However, when the same data are inverted and displayed in colour, hands go up and cries of ‘non-uniqueness’ fill the room. Let’s be clear: seismic is non-unique. Many interpretations can be made on the same dataset without proper controls.

Generating and interpreting attributes is similar to solving a mathematical problem. If you follow the general rules and make valid assumptions, you will arrive at a realistic solution. The concept is not unlike learning mathematics in school. Remember BEDMAS, a method taught for solving algebraic equations? BEDMAS is an acronym to help remember the order of operations used for solving these equations: Brackets, Exponents, Division, Multiplication, Addition, and Subtraction. Unfamiliar and difficult problems can be solved if you follow the guidelines. However, if you deviate from the workflow, skip a step, or make a poor assumption, the result is adversely affected. 

Failure in seismic attribute problems occurs for similar reasons. Skipped steps, poor assumptions, lack of diligence in processing or well correlations, and ineffective conditioning of input data all contribute to unsatisfactory results.

It is also important to communicate the results effectively. Geologists discuss a formation’s porosity as a measured entity. Geophysicists may derive a porosity using seismic attributes, but it will likely be over the thickness of the reservoir. The two will not be exactly the same, and they should not be presented as the same. Generally one is specific, the other is relative. You need to know what you are solving for.

The learning you should take from this is: When using seismic attributes, don’t rely on historical or preconceived notions. Evaluate the problem for yourself, follow the proper rules with diligence, and present the results in a way people understand. If you do this, you will find that geophysics works and can be used to solve the most difficult of problems. 

Computational geology

Advice for a prospective geologist

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