Porosity: The Space in the Rock

What Porosity Is

A rock is grains plus the space between them. Porosity is the fraction of the rock that is pore space rather than solid grain:

where $V_{\text{bulk}}$ is the total volume and $V_{\text{pore}}$ is the volume of the voids. It is the first number every evaluation needs, because it sets how much fluid the rock can possibly hold. A clean, loosely packed sandstone might be 35 percent pore; a tightly cemented one only 5 percent.

The figure is a grain pack you can rebuild. Drag the sliders and watch the pore space, and the porosity, respond.

Total versus Effective

Not all pore space is useful. Total porosity counts every void, including pores sealed off by cement and the tiny pores inside clay that hold water by capillarity. Effective porosity counts only the connected pore space that can actually store and flow producible fluid. The difference is what the figure shows: as cement grows on the grains it both fills pores and seals some off, and the isolated pore (gray) drops out of the effective total (blue). Logs that respond to fluids in connected pores read closer to effective porosity, and the distinction matters most in shaly and cemented rocks.

What Controls It

Three things move porosity. Sorting: a well-sorted rock of uniform grains packs open, while a poorly sorted mix lets small grains fill the pores between large ones. Packing and compaction: burial squeezes grains together and collapses pore space. Cementation: minerals precipitate in the pores, filling and sealing them. Typical clean sandstones run 0.10 to 0.35; carbonates vary far more widely, from near zero in tight limestone to 0.30 or more where dissolution has created secondary pores.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Tiab, D. and Donaldson, E. (2015). Petrophysics, 4th ed. Gulf Professional Publishing.