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58   PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS





                                                   Water
                                      Oil
                                             Water







                                       A          B          C

                                   A  Oil saturation about 80 % (Productive zone)
                                   B  Oil saturation about 50 % (Transitional zone)
                                   C  Oil saturation about 10–20 % (Water–wet zone)






                                                         Oil zone



                                              Oil–water
                                              transition zone
                                  Water

                    Figure 3.2   Oil – water saturation levels in the pores of (a) productive, (b) transitional, and


               (c) water - wet zones of an ideal reservoir such as the one illustrated in the bottom half of the

               figure.  (Adapted from an illustration in Monicard  (1980) .)
               electric logs or measured in cores. As  S   w   is used to compute reserve estimates, accu-
               rate values are critically important. Before determining  S   w   to calculate reserve esti-
               mates, it is necessary to understand the kinds of data used to compute  S   w   and which
               pitfalls to avoid in the methods of computation.
                    It is sometimes stated that the four most important properties used in petrophysi-
               cal logging are porosity, permeability, resistivity, and saturation (Asquith and Gibson,
                 1982 ). Porosity, permeability, and saturation are secondary or derived rock proper-
               ties. Resistivity is a tertiary property that depends on porosity, saturation, and elec-
               trical properties of the formation fluids. Porosity, permeability, and saturation are

               measured directly from borehole cores as part of  “ complete core analysis ”  routines,

               but when cores are not available   φ  and  S   w   are determined from wireline logs. Perme-
               ability can sometimes be estimated from log values in sandstone reservoirs with
               simple and uniform intergranular porosity, but the variety of pore characteristics in
               carbonate reservoirs generally precludes using wireline log readings to estimate
               permeability. Resistivity logs provide values for  R   t  , the  “ true formation resistivity, ”
               where fluids include water and some hydrocarbons;  R   o  , the resistivity of the porous

               rock formation at 100% water saturation;  R   i  , the resistivity of the zone invaded by

               borehole fluids; and  R   xo  , the resistivity of the flushed zone surrounding the borehole.
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