This is a well log used in the oil and gas industry. You might have encountered this acronym
RNL in your log evaluation software or any (ASCII) log files provided by the logging company. The abreviatin stands for
'Real-Time CNP (Limestone Matrix)'. This log is very common in the industry. The values of this log represent the porosity derived from a neutron logging measurement, assuming that the rock matrix material is calcite.
Concluding, this is a measure of the porosity as measured with an neutron logging device. In general this is a measurement of the apparent porosity. This can be explained as being the initial estimate of porosity, normally from logs, not corrected for the (often large) effects of clays, gas, etc. Such estimates should only be used for qualitative interpretation of clean, gas-free intervals.
Furthermore, this can be explained by looking at the volume fraction. In other words, the ratio of the volumes occupied by two components, or by one component to the total volume of the system.This log usually has the unit
'Porosity'. This is the pore volume per unit gross volume. Porosity is determined from measurements on cores or interpreted from logs.
Data for this log is produced by a tool that uses a neutron logging technique and can be used to investigate the porosity of the formation. Similar logs are listed below: