Accurate
determinations of petrophysical properties in carbonate reservoirs
(i.e., porosity, permeability, and saturation) require detailed
knowledge of pore types. Traditional methods of estimating hydrocarbon
reserves are also sensitive to variations in pore types. However,
neutron and density wireline logging devices are assumed to indicate
total porosity. In reservoirs in which moldic or separate-vug porosity
exists, a distinction between moldic and interparticle porosity
is especially important for accurate determination of petrophysical
properties.
Conventionally,
the sonic (acoustic) log is chosen to provide information on the
volume of moldic porosity. However, because the sonic tool is commonly
considered to be of secondary importance, acoustic porosity logs
are uncommon in many data sets. Resistivity logs can also provide
valuable information on the volume of moldic porosity in much the
same way sonic logs do. Accordingly, a well-constrained methodology
for determining moldic pore volume from resistivity logs offers
great potential for improving the accuracy of reservoir-wide petrophysical
calculations.
Such
a methodology has been developed for the Fullerton Clear Fork reservoir.
By combining a bulk volume water calculation derived from the Archie
equation and a ratio water saturation computation, porosity can
be calculated exclusively from resistivity logs. Comparisons of
resistivity porosity calculations with thin-section point-count
data of pore type abundance from cores and calculations of moldic
versus interparticle pore volume made from combination porosity
logging suites (sonic, neutron, and density) indicate that this
resistivity log method is a good measure of interparticle porosity.
This technique allows the user to get a more complete picture of
the porosity and fluid distribution throughout the reservoir. |