Cutoffs: Net Reservoir and Net Pay
Learning objectives
- Define net reservoir and net pay through cutoffs
- Apply the shale, porosity, and saturation cutoffs as a cascade
- Read net pay as the high-porosity, low-Sw, clean corner
- Recognize cutoffs as judgment calls that move the volume
From Curves to a Verdict
The course has produced a shelf of continuous curves, shale volume, porosity, water saturation, permeability, lithology. Volumetrics needs a verdict: which rock counts. Each curve becomes a cutoff that sorts the rock in two steps. Net reservoir is rock clean enough and porous enough to flow at all, so it passes a shale-volume ceiling and a porosity floor. Net pay is the subset that is also oily enough to produce, so it additionally passes a water-saturation ceiling.
The Cascade
Run as a cascade, the gross interval narrows to net reservoir, then to net pay. On a porosity-against-saturation crossplot the pay is the upper-left corner, high porosity and low water, with the shaly and tight points dropped out entirely. The cascade is the bridge from the interpretation logs to a number a reserves report can carry.
Judgment, Not Physics
The cutoff values are not handed down by nature. A porosity floor of 8 versus 10 porosity units, a saturation ceiling of 50 versus 60 percent, each shifts the boundary and the pay it encloses. A good cutoff is defensible, documented, and applied consistently across the field, often anchored to a permeability or a capillary-pressure threshold below which the rock will not flow. The next sections turn this net pay into thickness, then into a hydrocarbon volume.
References
- Worthington, P. F. and Cosentino, L. (2005). The role of cut-offs in integrated reservoir studies. SPE Reservoir Evaluation & Engineering, 8(4).
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.