Andreas,
As a calculation of the average content of
shale / clay in a faultrock at a given point, SGR is a valid calculation for
all bed thicknesses and throw values. However, it does assume that all units that
have slipped past the point of interest have contributed material to the
faultrock at the same rate. In sandstone shale sequences, sand-sand
juxtapositions should only have zero SGR values where the sand body has 0% Vcl
and the displacement is not enough to offset the bed from itself.
Which SGR value threshold yields a good
seal will be field specific and where possible should be calibrated from
production and pressure data. I think the 15-20% + threshold is a rule of thumb,
based on empirical data, that’s used for cases in the absence of other
data (i.e. exploration).
However, as you probably know, irrespective
of the SGR values, other factors may be more dominant with respect to fault
seal potential. For example, cements deposited from fluids flowing along the
fault, other deformation mechanisms (e.g. cataclasis and grain size reduction in
sandstones) and the influence of the in-situ stresses acting on the fault.
Hope this helps.
Regards
Tim
*******************************************************
Tim Wynn Ph.D.
Reservoir Geologist & Geomechanics Engineer
TRACS International
Falcon House
AB10 6XU
e-mail: [log in to unmask]
Tel: +44(0)1224 321213
Fax: +44(0)1224 321214
Web: www.tracs.com
*******************************************************
From:
Tectonics & structural geology discussion list
[mailto:[log in to unmask]] On
Behalf Of Andreas Frischbutter
Sent: 02 November 2004 15:31
To: [log in to unmask]
Subject: Shale Gouge Ratio
Hello everybody,
currently
I am working on fault sealing analysis and want to incorporate the SGR-Method
introduced by Yielding (1997).
Thereby
I came across some things I do have problems to understand.
To
calculate the SGR you apply the equation:
SGR=
Sh (%) / t x 100
where: Sh (%) is: net-percentage of shale over the section
of interest
t
: fault throw/heave
According
to Yielding the threshold between sealing- and non-sealing faults lies at a SGR
of 15-20 %
So
if the SGR gets towards higher values the faults are suposed to have sealing
capacity.
The
SGR, according to the equation, is triggered on the one hand side by the net
shale content and on the other hand by the fault throw/heave.
If
I increase the fault throw the SGR becomes less (fault leakege) , whereas if I
apply little throw, the SGR ratio is supposed to get high, thus the fault
should have sealing capacity.
Looking
at the reservoir juxtaposition on an Allan Map, at the example I am working on,
it appears that with little displacement I still have two sandbodys juxtaposed,
which contradicts the SGR value, where the fault should be actually sealing.
My
question is now: Did I get anything wrong in the understanding of the method
and is there a limitation for example:
The
throw must be at least higher than the thickness of the thinnest sand body in
order to be able to apply the algorithm.
Or
is there a certain range of SGR values, like: they are only meaningfull between
10 and 70 %.
Thank's
in advance for your support
Cheers
Andreas
Structural Geologist
ETG
Wintershall AG
Friedrich Ebert Strasse 160
34112
office phone: 0049-(0)561-301 2384