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.
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.