Water Saturation on the Ogbon-1 Well

The Whole Chain, One Curve

This is where the chapter, and most of the course, finally comes together on the shared Ogbon-1 well. Every curve we have built feeds one continuous answer: the gamma ray gives the shale, the porosity tools give $\phi$, the resistivity gives $R_t$, and Archie turns $\phi$ and $R_t$ into a water-saturation curve foot by foot down the well, $S_w = ((a R_w)/(\phi^{m} R_t))^{1/n}$. The Sw track kicks low through the clean oil sands and swings to 100 percent below the oil-water contact, exactly mirroring the resistivity.

Cutoffs and Net Pay

A saturation curve is not yet an answer; the reservoir engineer needs to know which rock counts. Three cutoffs make that call: a shale ceiling (here $V_{sh} < 40$) throws out the dirty rock, a porosity floor ($\phi > 8$ pu) throws out the tight rock, and a saturation ceiling ($S_w < 50$) throws out the wet rock. Where all three pass, the net-pay flag lights up, and adding the flagged feet gives the net pay, about 277 ft here, at an average pay saturation near 23 percent.

The Deliverable

That continuous Sw curve and the net-pay summary are exactly what a petrophysicist hands to the reservoir engineer, and they are the bridge to the next course: net pay times porosity times the hydrocarbon saturation, summed over the field, is the hydrocarbon pore volume that feeds the volumetric estimate. Chapter 7 has taken the well from raw logs to that number. The chapters ahead sharpen the saturation in shaly sands, add permeability and capillary pressure, and assemble the full formation-evaluation workflow, but the spine, log to porosity to resistivity to Archie to net pay, is now in place.

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.