Shale and Effective Porosity

The Shale Trap

Shale is the inverse of gas. Clay minerals hold bound water, and that water is full of hydrogen, so the neutron counts it as porosity. A shaly sand therefore reads a high neutron porosity even when little of that pore space can actually flow. On the crossplot the point leaves the clean-sand line and slides toward the shale point, which sits at high neutron and moderate density. The further toward the shale point, the more of the apparent porosity is really just clay-bound water.

Total Versus Effective

This forces a distinction we have hinted at since Chapter 1. The total porosity is all the pore space, including the water bound to the clay. The effective porosity is the connected pore space that can hold and give up hydrocarbons, with the clay-bound water removed. In a clean sand the two are equal; in a shaly sand the total stays deceptively high while the effective porosity falls away. Effective porosity, not total, is what feeds the saturation and the hydrocarbon volume.

The Shale Correction

To recover it, bring back the shale volume $V_{sh}$ from the gamma ray and strip each curve of its shale share: read the shale's own neutron and density (the shale point), then

This is where Chapter 3 and Chapter 4 finally join: the gamma ray gave $V_{sh}$, the density and neutron gave porosity, and together they give the effective porosity the saturation chapters need. With shale volume and effective porosity in hand, and Rw already from the SP, the table is set for Archie.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Crain, E. R. (2019). Crain's Petrophysical Handbook, online ed.