The Cemented Sand That Isn't Pay
Learning objectives
- State the screening failure: a velocity-porosity transform calibrated on the wrong rock family can throw away real pay
- Anchor the numbers: a sand with 2 percent contact cement at a true porosity of 34 percent reads a brine Vp of 3.916 km/s
- Read that velocity through a friable-line transform and watch it return a porosity of 10.0 percent, a 24-point error from 2 percent of cement
- Diagnose the failure on the Part 5.6 template (the point plots far above the friable line) and calibrate the transform to the right family
A Fast Sand With Room to Spare
Screening a reservoir often comes down to a single transform: measure a velocity, read off a porosity, and keep the intervals with enough pore space to be worth completing. The transform is only as honest as the rock family it was built on. Take a clean sand at a true porosity of 34 percent, and add just 2 percent of quartz cement at the grain contacts, the difference between the critical porosity 0.36 and the true 0.34. Contact cement is the stiffest possible place to put mineral: welded exactly where the grains touch and carry load. The dry frame bulk modulus jumps to about GPa, and brine-saturated the sand runs at a P-wave velocity of km/s. That is a fast, hard-looking sand.
Now screen it. A velocity-porosity transform calibrated on the friable, uncemented line, the loose-sorting trend of Part 5.3, reads that 3.916 km/s and returns a porosity of 10.0 percent. On paper the interval is nearly tight, marginal at best, and a screening pass would cut it. Yet the rock holds 34 percent porosity. The transform is wrong by 24 porosity points, and it is wrong in the worst possible direction: it discards genuine reservoir.
Two Percent of Cement, Twenty-Four Points of Error
The error is not a rounding slip; it is a wrong model. A friable transform assumes the only way a sand gets fast is to lose porosity, sorting and compaction packing the grains tighter. Along that trend, 3.916 km/s genuinely does mean about 10 percent porosity. But there is a second, far more efficient way to stiffen a sand: cement the contacts. A trace of cement welded at the load-bearing points buys an enormous velocity increase for almost no loss of pore volume, because it works on the frame stiffness rather than on the porosity. The sand is fast because it is cemented, not because it is tight, and a transform blind to cement mistakes the one for the other.
This is why a single number, velocity to porosity, is never enough on its own. The same velocity carries two utterly different verdicts, a tight sand to be abandoned or a porous cemented sand to be completed, and only a model that knows which family the rock belongs to can choose between them.
Reading It on the Template
The diagnosis is immediate on the rock-physics template of Part 5.6. Plot the sand in velocity-porosity space against the two bounding trends, the friable line for uncemented sorting and the contact-cement line rising steeply from the critical porosity. The friable transform failed because it forced the point onto the wrong curve. The real sand plots far above the friable line and hard against the contact-cement trend, exactly where a lightly cemented, high-porosity sand belongs. That vertical distance above the friable line is the cement signature, and it is the reading the single-number transform threw away. The remedy is not a better number but a better family: calibrate the porosity transform to the contact-cement trend for this reservoir, and the 34 percent porosity comes back correctly. The lesson carries a warning label for the next section, where the rock that breaks the transform is not a sand at all but a carbonate, and the culprit is not cement but the shape of the pores.
References
- Dvorkin, J., & Nur, A. (1996). Elasticity of high-porosity sandstones: Theory for two North Sea data sets. Geophysics, 61(5), 1363-1370.
- Avseth, P., Mukerji, T., & Mavko, G. (2005). Quantitative Seismic Interpretation. Cambridge University Press.