Pre-stack is the way to go

With the typical questions we need to answer today there is really no choice but to use pre-stack data. Of course it is only one of the elements you need for a thorough interpretation, whether you are dealing with rank exploration or development. The following discussion will explain why.

To start with you must request the right data. Whether you are reprocessing your seismic or buying it off the shelf, insist that the gathers and angle stacks are provided with the normal stacked data. The gathers should be migrated and, depending on the data range and fold, three angle stacks are best. To obtain the best balance among the angle stacks ask for the same fold in each, particularly in the target zone. Check that your final stacks, including the angle stacks, are created from the gathers that you receive. Sometimes the gathers are output at a later time and processed separately and scaled differently.

Let’s leave the seismic for now and focus on another key source of data for your interpretation – well data. If you are fortunate enough to have a few wells, they can provide significant constraints on the rock properties in the target zone. If there is an opportunity to influence the logs acquired in the wells insist on a full suite that includes compressional and shear transit time and density along with other logs. Consider the questions you have to answer and which logs may best describe the rock properties of the zone of interest. For example, you may wish to learn about the anisotropy (dipole shear) in the formation or map fractures (high resolution resistivity).

So let’s assume you have a complete log suite. Of course your trusty petrophysicist should bless the logs. This is an important data quality step and should be done before analysis. The next order of business is to tie the well or wells to the seismic. If you have a poor tie, then either or both the well and seismic may need more work. With a good tie in one or more wells the phase of the seismic can be established, so that interpretation can proceed on a correctly displayed dataset. Most people use zero phase data as their base volume.

The initial well tie is done in 1D. The next step is to match the seismic gathers with the model gathers from the well. In this step the amplitude scaling with offset or angle can be tested and corrected, if needed. Unless your processor used well data in offset scaling, it will most likely have to be adjusted. This is critical if one is to attempt to extract rock property information from the seismic gathers. Don’t forget to scale the angle stacks at the same time.

Now you are ready for some analysis. Forward modelling of gathers using the well data can demonstrate whether the property variation you seek can be identified. Some of the key rock properties that can be detected are variations in porosity, cementation, pore fluid, and lithology. They are not all independent or unique, so it is important to apply other geologic constraints to make the problem tractable.

Your software should enable you to calculate the amplitude of zero offset (intercept) and the change in amplitude with offset (gradient) on a target reflector. If there are measurable changes in intercept and gradient that can be related to something of interest, then create an AVO class volume from the angle stacks and use it to scan the data for the character change you desire. Use a program that links the class volume to the gathers, then inspect the gathers to verify the anomaly. Does it make geologic sense if you map it? Are there other ways to explain this observation? This is where experience in many different geologic settings is valuable.

Once the data quality has been verified and you have done some of the quick tests above, it makes sense to create pre-stack inversion volumes to obtain a better measure of uncertainty. Rock property volumes can then be computed based on relationships from the well data and rock property databases. These can more clearly define the lead, prospect, or area for development and help you determine the volume and assess the risk of drilling. You’ll wonder why you ever drilled a well without it.

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Q&A:

Marc, your article emphasizes the point that there is no alternative except to use pre-stack seismic data. There is also a growing realization within our industry that pre-stack seismic has been underexploited and far more value can be extracted from it. Could you enunciate point-wise the advantages of doing so?

Think about it from the rock physics perspective. You can only extract acoustic impedance contrasts and absolute values from stacked data. With pre-stack data one has the opportunity to also compute Poisson's ratio (or other pairs of elastic properties). In some cases with high quality data and good constraints one may also be able to extract density as well. With two or three elastic parameters one has far more control on the prediction of reservoir properties. With exploration and development today requiring more information about the subsurface in order to make meaningful economic decisions, it is essential to extract as much information as possible.

In view of these advantages that you have listed, why are oil companies not exploiting the full potential of pre-stack seismic data? Put another way, what do you think are the challenges of using pre-stack data in our interpretation and analysis?

Cost is always a key concern in exploration or development. My guess is that management may first try the least expensive option that will meet their need. This may be to use stacked data. Experienced operators are more likely to immediately pursue pre-stack analysis based on a better understanding of what is needed to describe the subsurface environment in their specific situation.

In the present scenario, do you think the pre-stack data is accessible on the desktops of mainstream seismic interpreters or it is still confined to the processing domain?

I believe it is more common today for interpreters to have access to angle stacks, but probably not CDP gathers. I am not sure that most have the tools to do more than a qualitative analysis themselves, but in many companies in-house experts are available, or they may contract with service companies to process these to produce quantitative products such as elastic inversions. These latter products which are generated from pre-stack data can then be included in the overall interpretation process.

As you state, it would be a big advantage to give the interpreters the ability to examine the pre-stack data and let them recover the information that may have been lost in the stacking process. While it sounds interesting, and may be true for smaller land seismic data volumes, do you think the compute power and storage capacity is available for handling pre-stack data for large and complex offshore plays?

Absolutely. The more forward-looking companies, including service companies, have been computing or offering elastic inversion products for some time and have expanded their compute power as needed. Mostly they use angle stacks rather than the gathers directly because of the improvement in signal to noise from the stacking and also the reduction in data volume.

Do you really think the access of pre-stack data on workstations will benefit the interpreters in adding value to the interpretation of the seismic data? Even for the post-stack seismic data, many interpreters indulge in the practice of generating attributes by simply pressing buttons and using the default values in the parameter windows that show up. Such practices in no way result in optimized generation of attributes. Hence the question.

For the first part of your question just consider the shale or unconventional plays. Recent papers have indicated that high-quality, pre-stack data will be necessary in order to compute the subtle variations in reservoir properties necessary to identify sweet spots. Interpreters in these plays may be directing a team of experts in order to bring all the skills necessary to solve the problem.

Your second comment suggests that we are not training our interpreters well. In the end their goal should be a geologically consistent interpretation of the subsurface with a discussion of the uncertainty involved. To successfully do this they must understand the tools available to them and when to use them, including when to seek outside assistance.

Could you tell us where this is headed? Suppose we see large scale integration of pre-stack data processing and interpretation in the next few years – what do you think comes next?

I do see that pre-stack processing will become routine as will inclusion of anisotropic correction (VTI) in all of our processing. What's next? Attenuation is addressed occasionally today, but maybe we will learn how to use it in detail on a cycle by cycle basis to tell us more about subtle variations in reservoir properties. There are already examples of very special acquisitions (and necessary processing) to extract azimuthal properties and other detailed elastic properties and structures. You will see more of these.

On a different note, Marc, we all have a driving goal or goals in life. What has it been for you all along?

Doing geophysics or earth science is something I enjoy, but it is only a way of expressing my overall goals. 1) I like to figure out how things work and solve problems. It is natural to apply these goals to figuring out what is beneath our feet, but these drivers are with me in my everyday life as well. 2) Enjoyment of the outdoors is what first moved me toward geophysics from physics. Over the years I have been able to participate in some great field trips that have reinforced my love of nature and I still make an effort to get outdoors as much as possible. 3) In a variety of ways I try to make this world a better place. Since I like to teach, I visit middle schools (12 year olds) and talk about earth science (with demos) to encourage the students to pursue science and engineering, to have them meet a real scientist and to enrich the program offered by the school.

Finally, what message would you have for young entrants to our profession?

Try to learn from all of the people you encounter – even those you might disagree with. It is possible that they may enable you to expand your perspective. Also be sure to consider giving something back and in some way help others pursue their own goals.