A good top seal is ductile. It is capable of accommodating large strains without brittle failure and loss of top seal integrity. Being able to predict ductility and strain are important in risking seal.
Seal ductility changes with both confining pressure and compaction. A shale with a density of 1.2 g/cm3 is more ductile than a shale at 14,000 feet with a density of 2.6 g/cm3. The shale at 14,000 feet was once at 10,000 feet and it's ductility was different; so was the seal risk.
The following figure shows how both density and confining pressure control shale ductility. Ductility , in percent, has been contoured in color with the most ductile shales in purple and the most brittle shales in red. Shales in the purple field did not undergo brittle failure, even at high strains. This relationship is important because it means that ductility (and brittle failure) can be inferred from seal density (and sonic velocity). Equally important is the fact that paleo-density and paleo-ductility can be inferred as shales undergo progressive subsidence and compaction with time. A shale seal will begin the in the lower left of this diagram and move towards the upper right depending upon burial history and overpressure development. Predicting strain magnitude and strain history is the second part of the story.
Risking seal failure, due to either deformation or overpressure, requires understanding mechanical properties and how they change. The SEALS course and the Research Reports provide a fundamental basis for risking seal.
Data compiled from numerous published sources. from Skerlec, G.M.,(2005) , Risking Fault Seal and Tops Seal, SEALS Course Manual, ©Seals International,2005. See also Skerlec, G.M. (1999), Evaluating Top and Fault Seal, in Beaumont, E.A. and Foster, N.H. (eds.), Exploration for Oil and Gas Traps, Treatise of Petroleum Geology, Handbook of Petroleum Geology, American Association of Petroleum Geology). Skerlec, G.M., 2005, Mechanical Properties of Seals, SEALS International Research Report.