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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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