Dear Milky Way,
If you use the neutron porosity conversion chart, with input limestone porosity units (as recorded) and output sandstone porosity units, you will get a higher porosity by about 4 to 5 p.u. However, if you input limestone porosity units and obtain output in limestone porosity units, they should be the same, as the limestone porosity is a linear line with one to one correlation.
In my experience, RHOB
(bulk density log) is the best log to use for deriving formation porosity. The RHOB log is less affected by clays (clay bound water) than the neutron log. Even if you use a fixed grain density of 2.65 gm/cc for both sand and shale intervals, the computed porosity is not too far off from the actual porosity. We have done extensive calibration with core data and found that the total porosity PHIT derived from the RHOB log matches the core porosity (oven dried porosity) very well. Shell, for example, is a firm believer in RHOB derived porosity. If you use the combination of neutron and density (either apparent porosity or crossplot porosity) you may overestimate the computed porosity. This is due to the fact that the neutron porosity log is very much affected by clays (clay bound water). Of course, the RHOB log has to be corrected for gas or light hydrocarbon effects, by using an appropriate method, before computing porosity.
However, if you wish to use the neutron porosity in combination with the RHOB porosity, both must be in the same lithological units. Therefore, you need to convert the neutron porosity log (which is recorded in limestone units as a standard) into sandstone porosity units. Then only you can use a suitable equation like "Gaymard's equation" or "2/3 + 1/3" averaging equation to compute the gas-corrected porosity.
As for the Rmf, Rm and Rmc values, you should use the ones which come from "Press". This just means that the mud filtrate and mud cake have been obtained from a filter press, to filter out the water from the solids. These values should be converted to downhole temperature as continuous curves, so that they can be extrapolated to the corresponding temperature of the reservoir where you are making your interpretation. If you input the Rmf, Rm and Rmc at the measured temperature (at surface) IP should extrapolate them to downhole temperature. You should also compute a continuous temperature curve using the surface temperature and bottom hole temperature at their corresponding depths. This temperature curve will then be used by IP to extrapolate the mud properties to downhole temperature. The Rmf, Rm and Rmc values need not be at the same temperature as long you correctly input the corresponding temperatures at which they are measured.
Ko Ko Kyi