Shaly-Sand Models on the Ogbon-1 Well

Back on the Well

The chapter closes where it opened, on the Ogbon-1 well. Most of the oil column is clean sand, and there clean Archie and every shaly-sand model give the same answer, the saturation curves lie right on top of each other. But the column carries shaly streaks too, and in those the curves part: the shaly-sand model pulls to a lower, truer water saturation while clean Archie, fooled by the clay conductance, reads too wet.

The Recovered Pay

The flag strip lights up the shaly sands where the correction earns its keep: feet of moderately dirty, porous, oil-bearing rock where clean Archie over-read the saturation and the shaly model finds the pay. Switch between Simandoux, Indonesia, and the Dual-Water free saturation and the recovered footage and the extra hydrocarbon pore shift with the model, a reminder that the choice is real money. In the clean pay the selector changes nothing, exactly as it should.

Choosing, and Closing

So the rule for a shaly-sand job is simple. In clean rock any model, even plain Archie, is fine. In the shaly streaks the model matters, and the choice follows the geology and the data: Simandoux or Indonesia when only $V_{sh}$ and $R_{sh}$ are at hand, Waxman-Smits or Dual-Water when core cation-exchange or a bound-water model pins the clay, always calibrated against a known result. With saturation now solid in clean and shaly rock alike, the course turns next to permeability, capillary pressure, and the integration that ends in net pay and a hydrocarbon volume.

References

• Worthington, P. F. (1985). The evolution of shaly-sand concepts in reservoir evaluation. The Log Analyst, 26(1).
• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.