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.

Q&A:

By saying ‘the geophysical problem is ever more complicated’, you make an interesting point. Could you elaborate on this?

In the past many of the largest fields were discovered using the most basic geophysical methods (Low fold 2D seismic, post stack time migration etc.) to image thick reservoirs or features that were so large they may be expressed on the surface. Not to take anything away from these discoveries, but nowadays we are dealing with thin beds, azimuthal variations, low permeability, stratigraphic variations over short distances, extremely complicated structural features in remote areas… the list goes on. We have moved from “simple” Acoustic Imaging to Rock Physics and Geomechanics to understand the subsurface.

The challenge to generate prospects in complicated areas may not be met with traditional interpretation techniques. How do you think such challenges can be overcome and what needs to be done?

To expand on what I said above, we need to get away from saying “we have always done it this way”. The technologies in logging for shear waves, seismic recording, seismic processing, and attribute analysis has progressed so much in the over the years. It is almost unbelievable. Just look at how far microseismic has come along. Explorers not only need to embrace these technologies, but they have to understand them as well. Training budgets need to be increased and people have to make the effort to incorporate them into their workflow.

I like the statement you make by saying ‘seismic attribute analysis is still not well understood by most explorationists’. I do come across interpreters who make use of seismic attributes without realizing what those attributes really mean or what their limitations are, what those attributes give them, or what information could possibly be extracted from them. How do you think this situation could change?

With the increased sophistication of interpretation workstations, we have in some ways become Button Pushers. We select an attribute from a list of many and wait to see if the output matches our interpretation of the geology. There are many “Geoscientists” that havn’t taken a Geophysics course in years, if at all. The seismic signal is complicated. It has many dimensions… phase, amplitude, frequency… all changing with offsets and azimuths. Without an understanding of what the seismic attribute calculation is doing, one can misinterpret the results. All too often the use of seismic attribute analysis is downplayed not because it “doesn’t work”, but because the output is not well understood. Attribute analysis is seen as some Black Art, even though the attributes are derived from the same signal as what is being mapped in a structural sense. Explorers need to spend the time to learn what the workstations are doing.

Seismic data are non-unique in that many different interpretations could come from them. How do you think the uncertainty in interpretation could be reduced? Do you think assigning the same data to many different groups of geoscientists could address the problem? And is it doable?

This is in some ways the corollary of #3. I have seen many times attributes presented in meetings that do not match the interpretation. Funny enough, the first thing in the bin are the attribute maps! Interpretations are non-unique... and they could be incorrect. A map generated from an attribute extracted along a seismic horizon pick depends on having the proper interpretation. Additionally, is the extraction centered around a window? How big is the window? Are you calculating RMS, Mean, or Minimum value? Explorers need to (1) understand the attribute calculation (2) understand how to generate the proper maps. We have an area here in Malaysia where the team interprets 4 attributes simultaneously to reduce the error in their interpretations: Seismic stack for structure, Near stack for Coal, Acoustic Impedance for sand probability, and far stack for hydrocarbon potential. By using all of the available data, ambiguities in interpretations should be reduced.

One way of reducing uncertainty in seismic interpretation could be by integrating different types and kinds of data. By types here I refer to the data that are derived from the seismic data such as attributes, etc., and by kind I refer to data from boreholes, rock physics, etc., or different disciplines. How much do think these steps help in reducing the uncertainty?

I am a firm believer in using all of the available data. The trick to seismic attributes is to calibrate them to different types of data whenever possible. Modeling is essential. Because many seismic attributes are not well understood, their usefulness need to be proven to managers. The title of my chapter was “Don’t rely on preconceived notions”. Many of these notions come from what I discussed in the text above. Calibrating to the well data (logs, models, production), is the only way we can reduce the uncertainty and gain confidence in what the attributes are telling us.

Due to the uncertainty that is inherent in the seismic data (acquisition, processing, etc.) being interpreted, it is a good idea to bring attention to it, but it would be really beneficial to demonstrate this with case studies. Once this is demonstrated on real data examples, others will start following in on them. Do you agree? How do you think this could be made practical?

I totally agree. Anyone associated with seismic data usually understands the uncertainty in it. We live with it all the time. But it never fails that after a lot of time and effort has been take to produce the perfect map, a guy at the back of the room asks “What about tuning?”. It is easy to criticize. Case studies show how interpretations are made and how seismic attributes were beneficial in the outcome. Many times, they demonstrate the failure of these preconceived notions. Case studies are typically some of the most popular sessions at conferences. These presentations – both successes and failures – are great learnings that benefit everyone.

Some oil companies have asset teams comprising geophysicists, geologists and reservoir engineers working together for generating prospects. This also allows each of these individuals to educate the others about how and what they are doing as well as learn about what others are doing and how. Do you think these kinds of teams are working well? You could give us your company perspective if you like.

In my opinion, asset teams consisting of all three disciplines are crucial. Everyone needs to be on the same page and share their own insights to the reservoir model. Going back to question #5, there has to be collaboration to reduce uncertainty and drilling risk. Years ago seismic workstations were just that, a tool to map seismic data. Nowadays, these software packages incorporate all the necessary data to generate a detailed reservoir model. Uncertainties are reduced and a uniform vision of the subsurface is created. In addition, with the development of unconventional reservoirs and microseismic, the realm of the geophysicist and the engineer are becoming closely linked. Young’s Modulus and Poisson’s ratio were typically used to understand seismic amplitudes. They are also used by engineers to design wells. “Geomechanics” and “Rock Physics” are linking the disciplines more than ever.

What in your opinion are the three unsolved problems in geophysics? Pl. name them in the order of their importance.

I find the greatest problem I face is Scale. We are continuously investigating thinner and thinner reservoirs with subtle characteristics. We’re not in the Gulf of Mexico anymore. Reservoir Engineers are the first to point out the lack of vertical resolution in seismic data. That said, there has been great strides in recording technologies, processing algorithms and statistical methods that is narrowing the gap.

The second problem I see is similar to scale, but not in the vertical sense. When you mention Geomechanics to an Engineer, they think of borehole stability. Geomechanics is also involved in the far-field seismic experiment in that it relates to velocity (compaction), anisotropy, and faults/fractures. Understanding lateral and horizontal stresses and their relation or expression in the near-field (i.e. borehole) is a great problem. Not only for exploiting unconventional reservoirs, but in multi-million dollar international wells drilling down the flank of a fault.

The third problem I see is more philosophical. When I went to school much of what I learned could be explained in a pool hall. Angles of incidence, reflections, velocities, all happened in front of your eyes. It was easy to grasp and explain to your manager. Now we have progressed to the world of Azimuthal Anisotropy, VTI, HTI, Broadband recordings... the list goes on. We are becoming very specialized and understanding, let alone explaining, the concepts of geophysics are becoming more and more difficult to explain. We are in danger of isolating ourselves further from the asset team which deems everything we do “a science project”. Education and understanding has always been a problem in geophysics and solving it doesn’t seem to be getting any easier.

On a lighter note, Eric according to a survey carried out in the UK some time back, the happiest people are the children (3-8 years) and the older individuals (68+ years). What does this tell you? Even if we think we are happy at present, would you take it that we will be happier when we retire, or does it tell you something else?

I remember watching my kids grow up. When they were young they had no worries. The biggest challenge was how many colours they could use in finger painting. Everyone told them they were great and they believed them. Young kids have few self esteem issues. Then come life... school, exams, summer jobs, money, girl friends, more money. After while, there’s a lot of baggage you need to carry. After 68+ years, you start to unload the baggage. Many of the things that weighed you down in your mid-life becomes no longer important. If we are happy at present, I believe you either have shed your baggage, or are comfortable with the weight. Does that mean you are happier when you retire? Not sure. Some see that as baggage in itself. As for me, I try to be happy all the time. My wife calls that “delusional”.