Why Resistivity Matters

What Conducts

Reach the resistivity log and the question changes from how much pore space to what is in it. Rock grains are insulators, and so are oil and gas; the only thing in a clean formation that carries an electric current is the salty brine in the pores. So a rock's resistivity is set by how much connected brine it holds, which makes resistivity the one log that responds directly to hydrocarbons, the log that finds pay.

Archie's Second Law

Archie made it quantitative. Start from the wet-rock resistivity $R_o = F,R_w$, the resistivity the rock would have if it were 100 percent water (the formation factor $F$ from the next section times the water resistivity $R_w$). Replace some of that water with hydrocarbon and the true resistivity rises:

The saturation exponent $n$ is about 2, so halving the water saturation roughly quadruples the resistivity.

The Resistivity Kick

That steep rise is why pay jumps off a log. A wet sand might read a couple of ohm-metres; the same sand at 20 percent water can read fifty or more. The resistivity log shows a hydrocarbon zone as a dramatic high-resistivity kick against the conductive wet rock around it. Reading that kick is the start of the saturation work; turning Rt into an actual $S_w$ is Archie's equation, the heart of the next chapter. First, though, we need the formation factor, the deep resistivity Rt, and the right tool to measure it, the rest of this chapter.

References

• Archie, G. E. (1942). The electrical resistivity log as an aid in determining some reservoir characteristics. Transactions of the AIME, 146(1).
• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.