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Using Buoyancy Pressure Profiles to Assess Uncertainty in Fault Seal Calibration
Peter Bretan, Graham Yielding
Badley Geoscience Limited, Lincolnshire, United Kingdom
The authors are grateful to Rob Weeden, Peter Boult, and an anonymous reviewer for their thoughtful and constructive comments.
Effective fault seal model calibration is dependent on the quality of the available data. Buoyancy pressure profiles provide a method to assess the potential uncertainty involved in deriving key input data, such as Vclay (volumetric clay fraction), and in the empirical equations used to derive seal-failure criteria. For a membrane-sealing fault, seal failure occurs when the buoyancy pressure exerted by the hydrocarbon column is equal to the minimum capillary entry pressure of the fault zone. If the seal is intact, the predicted fault zone capillary entry pressure value must be higher than the buoyancy pressure and, on a buoyancy pressure profile, plot to the right of the buoyancy pressure trend line. Predicted values that are lower than the buoyancy pressure indicate either that the fault is leaking or that one or more of the input data are in error.
Buoyancy pressure profiles are used postmortem to identify which data analysis techniques and seal-failure criteria best predict the observed hydrocarbon contacts in a given area. They can also be used to verify the threshold shale gouge ratio values that represent the onset of fault sealing.
The effect of varying key input parameters can be rapidly checked using buoyancy pressure profiles without resorting to more time-consuming probabilistic approaches. The analysis can identify which data preparation techniques provide appropriate estimates of the fault zone capillary entry pressure, relative to the observed buoyancy pressure, thereby significantly reducing potential uncertainty in the fault seal calibration.
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