Seismic Normal Compaction Trends and Lithology

Abstract

Abstract Eaton's method, the standard method of determining pore pressure from seismic velocities, has been adapted from logging techniques. It requires the use of a normal (i.e. hydrostatic) compaction trend specific to very-low-silt shales, and also that the analysis only be performed upon relatively silt-free shales. As a practical matter, the gamma ray log is used to distinguish shales which are low enough in silt to be appropriate for comparison to the normal compaction trend. (Other methods of determining pore pressure from sonic velocities, such as Bowers (1995), also require a normal compaction trend for low-silt shales and restrict the analysis to very-low-silt shales.) The obvious challenge in applying Eaton's method to seismic velocities is that all lithologies present in the subsurface contribute to the seismic interval velocity, and not just the shales. Again, the logic for practical matters is that, since shales are the most statistically likely lithology (at least in the Tertiary GOM), then a large enough velocity analysis window will be dominated by shales in terms of the velocity response. Hence one must be careful not to use velocities with too high a vertical resolution for fear of too strongly mixing lithology velocity responses in with pore pressure velocity responses. The exact limit of useful vertical resolution is debatable. Alan Huffman, at the 2010 SEG Annual Convention, opined that velocity analysis vertical resolutions finer than 1000 feet begin to run the risk of lithology effects. As likely, the highest useful velocity resolution is probably both depth- and area-specific. But since pore pressure prediction from seismic velocities is routinely performed and found to be useful, these problems are clearly not insuperable. One of the key issues is to pick a normal compaction trend that honors the siltiness level of the local geology by adjusting the trend away from a pure shale compaction trend to that silty shale which is statistically the most likely in the region and depth of interest. As a practical matter, this can be performed by manually altering the compaction coefficient of the compaction trend until the resulting predicting pore pressure agrees with mudweights, well formation pressure tests (RFTs and MDTs), and other drilling information. Alternatively, if the compaction trend is calculated explicitly using volume of clay (VClay) as an input, then the VClay is manually altered until the predicted pore pressure agrees with the mudweights, MDTs, etc. In essence, the problem is solved by simply calibrating it away.