The Neutron-Density Crossplot

Crossing the Two Logs

The density alone gives porosity only if you already know the matrix; the neutron alone has the same flaw, leaning the other way. Each carries one equation with two unknowns, lithology and porosity. Two equations fix two unknowns, so plot one log against the other. On a crossplot of neutron porosity against bulk density, each pure lithology traces its own curve: the sandstone, limestone, and dolomite lines run from the dense, zero-porosity corner up to high porosity, separated because the two tools disagree about each matrix in a characteristic way.

Reading Both at Once

Now a data point tells you two things. Which line it lands nearest is the lithology; where it sits along that line is the porosity. A clean wet sandstone falls on the sandstone line, a limestone on the limestone line, and a point between them suggests a mixture. For a quick number when the lithology is unknown, the crossplot porosity is close to the average of the neutron and density (limestone) porosities, $\phi \approx \tfrac{1}{2}(\phi_N + \phi_D)$, but the real power is reading lithology and porosity together off the chart.

The Workhorse

This is the most-used chart in petrophysics because it turns two ambiguous logs into one clean answer. It also sets a baseline: a clean liquid-filled rock sits on its lithology line. When a point pulls off the lines, the departure itself is the signal, gas lifting it one way and shale pulling it the other. Those two departures are the next two sections, and they are read against exactly this backdrop.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Schlumberger (2009). Log Interpretation Charts. Schlumberger.