Fault seal probability

Fig. 5. CSP calibration. The data for this calibration were gathered on 91 reservoirs along 10 faults in three different fields. Comparing the CSP in hydrocarbon bearing sand to sand windows with those in water bearing sand to sand windows results in a fault seal probability curve. The fault seal probability increases with increasing CSP up to a certain value above which sealing is independent of CSP.

Fault seal probability analysis is a quantitative method that allows an assessment of the risk of a fault acting either as a barrier to hydrocarbon migration, or as a trapping element within a structure. Fault seal probability is a value ranging from one to zero where a value of one is the highest probability for sealing, and zero is the lowest. This value is derived from the equation that combines the main parameters involved in the formation of fault seal. These parameters, fault displacement, connectivity, and net to gross ratio, are related to the processes of cataclasis and cementation, juxtaposition, and shale smear. The parameters, their measurement and impact on fault seal, are discussed below. 

In cases where detailed stratigraphies are unknown, or difficult to model, such as the Upper Jurassic and Lower Cretaceous, both of which contain potential reservoir sandstones, it is still possible to gain a good approximation of fault seal probability using... 

Fig. 6. Sensitivity study of fault seal probability variation with throw and stratigraphic templates.

Sensitivity studies of the impact of the stratigraphy used in calculating fault seal probabilities in the study were carried out. These indicated that for large fault throws (say several hundred metres), significant variation in fault seal probability values occurs (roughly 10-30% Fig. 6). For smaller throws, however, fault seal probability values are relatively insensitive to the stratigraphies used. 

The last variable to be included in the fault seal probability equation is the net to gross ratio. The assumptions used in the formulation of the equation are that with low net to gross ratios, say <0.4, there is a greater likelihood for sealing due to shale smear than in higher net to gross ratio successions (see also Bou-vier et al., 1989 Harding and Tuminas, 1989). 

Fault seal probabilities along the Revfallet Fault Complex are generally high to moderate with nearly... 

The graben to the east of Heidrun North, penetrated by well 6507/8-6, is almost entirely surrounded by high values of fault seal probability except at the northern end where moderate fault seal probabilities occur. Most internal faults within the graben have values at the high end of the moderate category. 

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. 

In general, in the Greater Heidrun there are significant pressure differences on a regional scale between areas bounded by the major sealing faults. Fault seal probability can thus be used to assist in the definition of possible pressure compartments. The same faults also bound the main hydrocarbon-bearing traps and prospects, particularly those that rely on fault seal for success. 

Fig. 14. Pressure difference versus average fault seal probability.

Fig. 15. Oil column height versus fault seal probability. (Note scale change on y axis.)...

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