Abstract - Empirical methods for estimating the sealing capacities of faults in an exploration setting involve calibration of a fault seal parameter against known hydrocarbon column heights. Fault seal parameters are typically some function of fault throw and the shale content of the faulted sequence (e.g. Shale Gouge Ratio, Clay Smear Potential). Fault seal capacities predicted by these methods are rarely incorporated into hydrocarbon migration models, although there are many advantages in doing so. Whether or not migration modelling incorporating predicted fault seal capacities can replicate the location, size and hydrocarbon phase of known accumulations provides a very stringent test of fault seal prediction techniques. To achieve a match between model and actual column heights not only requires that model fault seal properties are correct for known fault-bound accumulations, but also demands that the network of migration routes and fill-spill pathways allows hydrocarbon to access these accumulations.
In this study we incorporate fault seal capacities into a migration model of the Oseberg South area, Viking Graben, an area which contains several known fault seal dependent oil accumulations. The migration modelling is based on a ray tracing method which assumes that hydrocarbons migrate along the top of a permeable carrier bed and upwards along the steepest dip governed by buoyancy. Shale content is defined for the whole of the model area by interpolating between wells. Fault seal capacities are calculated as a function of Shale Gouge Ratio (SGR, i.e. the percentage shale in the sequence moved past a point on a fault) for all faults in the model area. A Monte Carlo approach is used, whereby several thousand realisations of fault seal capacities are generated by varying three parameters which together define the relationship between SGR and seal capacity. Each realisation is ranked by the accuracy of the match it achieves to the known oil-water contacts. For the Oseberg South area, the best match is achieved for realisations in which the onset of fault seal occurs at SGR values in the range 18 to 22%. The best-fit SGR to seal capacity relationships are very similar to the relationship defined by the standard SGR calibration approach. The similarity between the results of these two independent calibration methods increases confidence in seal prediction in this area.
Abstract of talk given to:
Structurally Complex Reservoirs, Geological Society of London, February 2006.