Resistivity on the Ogbon-1 Well

The Pay Lights Up

Everything the chapter built now goes on the shared Ogbon-1 well. The deep resistivity kicks high through the oil column and collapses below the oil-water contact, where the brine takes over. That single curve splits the well into pay above and water below, the resistivity log doing exactly the job it was made for, finding the hydrocarbon.

The Wet-Rock Baseline

To read it quantitatively, overlay the wet-rock resistivity $R_o = F,R_w$, computed foot by foot from the porosity (the formation factor) and the water resistivity. $R_o$ is the resistivity the rock would have if it were fully wet, so wherever the measured $R_t$ rides above $R_o$, the rock is holding hydrocarbon, and the size of the gap is the saturation. In the figure the deep resistivity sits well above the $R_o$ overlay through the oil, then drops onto it below the contact.

The Resistivity Ratio

That comparison is Archie's equation rearranged. Since $R_t = R_o/S_w^{,n}$, the water saturation reads straight off the two resistivities:

Run it down the well and the quicklook recovers the saturation, about 19 percent in the pay sand, the same number the well carries. With the formation factor, a trustworthy $R_t$, the porosity, and now $R_w$ and $V_{sh}$ from earlier chapters all assembled, the resistivity chapter has laid the last stone. The next chapter makes the rearrangement official: Archie, water saturation, and the Pickett plot.

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.