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to fluid flow during production from a reservoir. If we can assume that faults are not in
themselves hydrocarbon seals, then hydrocarbons can flow across the fault wherever
permeable layers are juxtaposed across it. This can be analysed by drawing sections
along the fault plane, showing the layers intersecting the fault on both the upthrown and
the downthrown side (Allan, 1989 ) . From this juxtaposition diagram, the spillpoint of
a structure can be determined as the shallowest depth at which hydrocarbon migration
across the fault is possible. This procedure is not as simple as it sounds. Many faults
are not simple single discontinuities, but are complex zones consisting of a series of
interconnected fault segments (Knipe et al. , 1998) . Often, a large single fault would
be judged an effective barrier to fluid flow from the juxtaposition diagrams, but the
equivalent ensemble of small-displacement faults might not be. Very careful mapping
of the faults is then required, using amplitude, coherency and dip maps, together with
review of vertical sections, to establish the fault pattern. This is particularly difficult to
do for the small faults that grade into seismic noise.
A complication is that the fault plane itself may be an effective hydrocarbon seal,
even though permeable strata are juxtaposed across it. This can be the result of smearing
of clay along the fault plane during displacement along the fault. Various methods for
predicting the presence of clay smear have been summarised by Foxford et al. (1998) .
These may be based on:
(i) the percentage of shale or mudstone layers in the faulted sequence,
(ii) the percentage of shale in the sequence that was moved past any point on the fault
surface,
(iii) the along-fault distance in the slip direction of a point on the fault surface from a
potential shale source layer, and the thickness of the layer.
Foxford et al. used the second of these approaches (the shale gouge ratio, SGR), and
found that an SGR of less than 20% was characteristic of fault zones that did not
contain shale gouge in their particular study. Similar cutoff values have been found in
other studies. As the SGR increases above this level, the fault plane becomes a more
effective seal, able to hold a longer hydrocarbon column over geological time or sustain
a larger pressure drop across it on a field production timescale. However, the thickness
of the shaley gouge can be highly variable and unpredictable. It is therefore difficult
to use SGR in a quantitative way to determine fault permeabilities (Manzocchi et al. ,
1999) . Compilation of data from existing fields is needed to reduce these uncertainties
(Hesthammer & Fossen, 2000) .
References
Allan, U. S. (1989). Model for hydrocarbon migration and entrapment. American Association of
Petroleum Geologists Bulletin , 73 , 803-11.
 
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