Fault seal probability map

Another useful tool for determining fault seal characteristics is the shale smear map. Although the fault seal probability calculation, derived from an empirical database which contains faults which were affected by shale smear, already incorporates a shale smear factor (see also Lindsay et al., 1993), the purpose of producing shale smear maps is to define the shale smear envelope for individual shale beds across fault surfaces. They can also be used as an independent check on the fault seal probability calculations. Only one shale smear map was produced in the study, for the Revfallet Fault Complex, where the greatest thickness of syn- to post-rift shale occurs. 

The basic requirement for mapping fault seals is the generation of a realistic, maximum probability map of sealing capacities along individual fault zones. This involves evaluation of the possible juxtaposition patterns within the zone as well as an assessment of the variance of fault rock properties. 

Analysis of the location and heights of potential leaky fault Juxtaposition windows which arise from variations in (i) the possible depths, geometries and locations of stratigraphic horizons and faults (ii) the difference between the cumulative throw on individual fault zones, indicated from seismic and the most likely size of the throw on the largest real fault in that zone and (iii) the sediment architecture and continuity. The end result should be a probability map of the distribution of sealed and leaky windows along the critical fault zones. 

Fault displacement is usually taken as fault throw from depth structure maps. A relation has been determined, from empirical oil and gas field data (Knott, 1993) and outcrop studies (Knott, 1994), which shows that there is a positive correlation between the probability of a fault sealing and fault throw divided by reservoir thickness (Dn). 

Fig. 7. Fault assisted top seal leakage, (a) Probability of top seal leakage. Analytical solution for shale beds of constant thickness /, in which identical faults of maximum throw are randomly dispersed. This relationship for probability of seal leakage also holds approximately for seals in which the shale layers and fault throws are each normally distributed about the same mean t. (b) Determination of the throw-cumulative frequency relationship. Faults in a volume of rock, from a map-based statistical analysis of the fault population. Adding 1 to the slope C2 simulates the addition of the third dimension (Gauthier and Lake, 1993). Here a length/Tfnjx fst o 100 1 was used, (c) Determination of the seal risk. Comparing the number of faults required for leakage with the number of faults in the trap volume determines the seal risk. In the example shown, the probability that the seal is breached lies between 50 and 90%, For points in the sealed field, the effect of increasing fault throw on the number of faults needed for breaching is illustrated.

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