Secondary Porosity
Learning objectives
- Explain why the sonic reads mainly primary porosity
- Compute secondary porosity as the density-neutron porosity minus the sonic
- Recognize secondary porosity as vugs and fractures
- Explain why secondary porosity matters for flow in carbonates
The Fastest Path
A sound pulse is lazy: it takes the fastest route through the rock, threading the connected matrix and skirting around isolated vugs and fractures. So the sonic reads only the primary, intergranular porosity, the pore space woven through the grain framework. Vugs and fractures, however much volume they hold, barely slow the pulse, and the sonic hardly sees them. The density and neutron have no such bias: they respond to all the pore space, vugs included.
The Difference Is the Vugs
That difference in what the tools see is a measurement in disguise. The density-neutron porosity is the total; the sonic is the primary; so the gap between them is the secondary porosity
In a clean sandstone the two porosities agree and the secondary porosity is zero, all of it is intergranular. In a vuggy or fractured carbonate the density-neutron runs well above the sonic, and that gap is the vug-and-fracture pore volume.
Why It Matters
Secondary porosity is not a curiosity. In carbonates the vugs and fractures often carry most of the flow even when they are a small share of the volume, because they form fast, connected pathways through an otherwise tight matrix. A reservoir that looks marginal on its intergranular porosity can be a strong producer once the secondary system is recognised. Reading the sonic against the density-neutron is the simplest way to flag it from logs alone.
References
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
- Lucia, F. J. (2007). Carbonate Reservoir Characterization, 2nd ed. Springer.