A Review of Fault Seal Calibration Methods

Graham Yielding, Alan Roberts, Pete Bretan, Brett Freeman, Dave Quinn, and Michiel van Noorden
Badley Geoscience Ltd, Spilsby, United Kingdom

Calibration is a necessary step in the workflow for prediction of fault seal. It is required because there is no direct way to predict the hydraulic behaviour of a fault zone at the scale of a hydrocarbon trap. Therefore, over the last 20 years two general approaches have been developed:

(i) Invent a simple algorithm which attempts to capture some salient feature of the fault zone. Then look at known trap-bounding faults to see if there is any relationship between the algorithm and the presence or strength of a fault seal (field or sub-surface calibration). Clay Smear Potential, Shale Smear Factor & Shale Gouge Ratio are example algorithms.
(ii) Measure hydraulic properties of fault-zone samples (lab calibration). Then map these results onto the appropriate parts of trap-bounding faults.

The calibration results may be summarised in a probabilistic way (chance of seal or no-seal) or a deterministic way (how strong is the seal). Seal strength is typically described by Hg-air threshold pressure in the lab or static pressure differences in the subsurface (e.g. hydrocarbon buoyancy pressure). Subsurface traps are unlikely to be filled exactly to fault-seal failure, because around half the trapped column is likely to escape once this point is reached.

Different workers have variously parameterised seal strength as a linear, log-linear or stepped function of fault-zone composition. Further uncertainty is introduced when converting the calibrated seal strength to potential hydrocarbon column height, because of the variability of subsurface hydrocarbon fluids (capillary properties and density).

Using exploration and appraisal examples, some of the assumptions in this workflow are critically assessed in order to suggest uncertainty ranges that should pragmatically be applied in fault seal predictions.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery