Page 144 - Well Logging and Formation Evaluation

134 Well Logging and Formation Evaluation

density optimizes your position, provided that the values you use are
still supportable by any core or regional data. I would personally not
recommend using a density/neutron crossplot approach even if it gave
a better equity case. However, this method is widely used in some oil
companies, and if the other company proposes it (and it favors your
equity position), then you might not choose to protest too strongly against
the method being adopted. As with net/gross, how the mapping of
porosity is performed may also have an impact on equity and needs to be
considered.

Saturation

As discussed above, it is recommended to agree early on whether a
resistivity-derived or saturation/height approach should be used. If an
Archie model is adopted, then you can try to optimize your equity through
the choice of R w, m, and n. As before, any values proposed should be
supportable by either core or log data.
If appropriate, a shaly sand model may also be applied. However, the
more complicated the methodology becomes, the harder it will be to reach
an agreement with other companies as to the parameters that should be
used. If it is agreed to use a saturation/height function approach, then the
parameters defining the curve that optimize equity can be proposed in a
similar way as with Archie. Since the proper methodology for averaging
cap curves is not always well understood by petrophysicists in oil com-
panies, by proposing a function up front, supported by the available
cap-curve data, one has a good chance of getting it accepted.

Fluid Contacts

Since the reservoir shape will usually not be symmetric with respect to
the boundary, the position of the contacts will often have a large effect on
equity. Note that if a saturation/height function is being used, the FWL
should be used as the cutoff for volumetric determination. If a conven-
tional Archie approach is being used, then the hydrocarbon/water contact
(HWC) is more appropriate. For reservoirs with a gas cap, moving the
GOC up or down will either favor or disfavor one company.
Where contacts are clearly observable on the logs, there is not much
room for debate. However, where this is not the case, then there may be
a wide range of possible contacts, depending on assumptions made with
regard to the formation pressures, spill point, bubble point, etc. As before,