Gas Effect and Crossover

Two Logs, Opposite Errors

Gas does something no liquid does: it fools the two porosity logs in opposite directions. The neutron is a hydrogen counter, and gas has a low hydrogen index, so a gas-filled pore looks nearly empty of hydrogen and the neutron under-reads porosity, $\phi_N \approx \phi,[,S_{xo} + (1-S_{xo}),HI_{gas},]$ with $HI_{gas}$ small. The density sees the opposite: gas is very light (about 0.2 g/cc), so the bulk density is too low and the density porosity (computed for a liquid) over-reads. One log too low, the other too high.

The Crossover

Plot the two as porosity on a shared scale that makes them overlay in clean liquid rock, and the gas zone announces itself: the density curve swings to the high-porosity side, the neutron to the low side, and the two cross. The shaded gap between them is the neutron-density crossover, one of the most trusted gas indicators in the whole log suite. Raise the gas saturation in the figure and the crossover opens wider; drop it to zero and the curves snap back together, exactly as they do in the water sand below.

Reading Through the Gas

The crossover is a flag, not a porosity. The neutron is too low and the density too high, so the true porosity sits between the two curves; a common estimate splits the difference (closer to the density in a strong gas effect). The size of the crossover grows with gas saturation, so it doubles as a rough richness indicator. Read it for what it is: the cleanest yes-or-no gas signal the porosity logs can give, sitting right on top of the porosity you came for.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Ellis, D. and Singer, J. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.