What Petrophysics Asks

The Question

A well is drilled and logged, but the logs do not read porosity or oil directly. They read physics: the natural radioactivity of the rock, its bulk density, how fast sound travels through it, and how strongly it resists an electric current. Petrophysics is the craft of turning those measurements into the rock and fluid properties a development decision needs: how much pore space there is, how much of that space holds hydrocarbons, and how easily those hydrocarbons will flow.

The roadmap below is the whole course in one figure. The logging tools measure physics in the borehole; the interpreter turns those logs into the rock and fluid properties (shale volume, porosity, water saturation, permeability); then cutoffs and volumetrics decide what is worth producing. Click any stage to see what it consumes, what it produces, its governing relation, and a live number computed on the shared Ogbon-1 well that you will evaluate end to end in the capstone.

The Chain

Every evaluation follows the same chain. From the logs we estimate the shale volume, how clean the rock is, because clay confuses almost every other reading. Then porosity, the fraction of the rock that is pore space. Then water saturation, the fraction of that pore space filled with water, so its complement is the hydrocarbon. Then permeability, which decides whether those hydrocarbons can actually move. Apply cutoffs to screen out rock that is too shaly, too tight, or too wet, and what remains is net pay: the thickness worth completing, and the hydrocarbon volume in place.

Each link feeds the next, so an error early in the chain propagates to the end. A porosity that reads too high inflates both the volume in place and the permeability; a shale volume left uncorrected biases the saturation. Reading the chain in order, and checking each step against the others, is what separates a defensible evaluation from a number that merely looks reasonable.

Why It Matters

These are the very numbers the reservoir model is built from: the porosity, saturation, and permeability that fill the grid and decide the reserves. Petrophysics sits upstream of the entire reservoir-modeling workflow, which is why this course is the companion that comes before it. Get the petrophysics right and everything downstream stands on solid ground. Get it wrong and no amount of simulation can recover the truth, because the simulator can only flow the rock it is given.

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.