Well logging interpretation of the amount of hydrocarbon in the pore space of a formation, relies on the fluid being oil.
Gas is light compared to oil causing density logging (gamma ray emitting sensors) based measurements to produce anomalous signals.
By properly combining the two erroneous answers from density and neutron logging, it is possible to arrive at a more accurate porosity than would be possible by interpreting each of the measurements separately.
A popular method of obtaining a formation porosity estimate is based on the simultaneous use of neutron and density logs.
Under normal logging conditions, the porosity estimates obtained from these tools agree, when plotted on an appropriate lithology and fluid scale.
Log interpreters often find it difficult to accurately estimate the true formation porosity from these two curves.
A main complication in deriving an accurate porosity in the presence of shallow invasion comes from the fact that the neutron and density logging devices typically have different DOIs.
However, cross plot techniques rely on combinations of the neutron and density data that can be tuned to a particular invasion front depth.
For example, the commonly used Root mean square (RMS) equation for gas reservoirs: φformation = ((φDensity2 +φNeutron2)/2 )0.5 (1) gives accurate porosity estimates for every shallow invasion of approximately 1 inch (2.5 cm), but can be up to 5 p.u.
However, as previously stated, in a partially invaded gas information, there can be a large error in the determination of the true porosity using either the density or neutron measurement.