RISK

Risking fault seal involves uncertainty; as does anything else in the business such as trap, reservoir, etc.  Quantitative fault seal analysis is dependent upon both stratigraphy and fault displacement.  In a frontier setting there is more uncertainty than in drilling the 40th well in a mature Gulf Coast field.

The importance of fault seal analysis is that it forces you to focus on what are the real risks and not some nebulous term "seal".  The real risks are:  how certain am I of the stratigraphy, and how certain am I of the fault displacement and structure.  Changes in stratigraphy, such as the presence of a thin thief sand, can dramatically affect seal risk.  Variations in stratigraphy and displacement will change the SGR and juxtaposition relationships along the fault.  Faulty log analysis is just "garbage in".

A good quantitative fault seal analysis will look at the range of probable values.  The stratigraphy may vary through a range of net/gross, sand thickness, and shale thickness.  The fault mapping may have varying degrees of uncertainty.  Given this uncertainty, fault seal analysis enables you to recognize the really critical elements in your prospect interpretation.  It may force you to spend more time interpreting the northern end of a specific fault than would otherwise be required.  It may force you to spend more time looking at the stratigraphic variation within a specific systems tract.  What is the probability of a specific sand/sand juxtaposition or JLLP?  Monte Carlo analysis is as applicable to fault seal analysis as it is to any other risk in exploration.

One of the strengths of the SEALS course is that it not only teaches you how to do quantitative fault seal analysis and deal with uncertainty, it also shows you all of the other things that can go wrong.  The factors that control seal vary from basin to basin and structural style to structural style.

In regions dominated by fracturing leakage is controlled by other factors.  Risking seal requires understanding fracture patterns, fracture density, mechanical properties, present-day stress fields, reactivation, overpressure, and fault block kinematics.  Brittle failure is more common in some structural styles, basins with late reactivation, and basins with active, present-day tectonics.

The importance of fault seal analysis is that it forces you to focus on what are the real risks and not some nebulous term "seal".