Porosity Log Crossplots
The density/neutron log overlay can be "read" rapidly for lithologies, even after limited practice. The pattern recognition skills involved are rather like reading a melody from sheet music. Neutron-density crossplots provide an alternative method for lithology-porosity interpretation and have several advantages. More precise estimates of true porosity can be made and the location of zones on the crossplot gives indications of the quantitative composition of mineral components in mixed lithologies. If zone points tend to be clustered, this is an additional indication of the presence of distinctive facies, which may be related to depositional and/or diagenetic controls.
There are two major disadvantages. Depth information is lost on the crossplot, while this is a basic feature of the log overlay. Also, neutron/density log overlays are a standard blue-line log convention which are available at the well-site for immediate interpretation. Crossplots must either be prepared laboriously by hand, or generated from computer processing of digital log tapes.
The axes of the crossplot correspond to the two porosity logs (see Fig. 32). Because the theoretical values for pure sandstones, limestones and dolomites can be computed over any range of porosity, these lithology lines can be drawn on the crossplot as boundary endmembers. The "true"(actual volumetric) porosity and mineral composition of any zone are found by interpolation.