Cutoff Sensitivity
Learning objectives
- Build a sensitivity curve of volume against a cutoff
- Distinguish a flat (safe) cutoff from a steep (consequential) one
- Explain why a steep cutoff must be anchored to flow data
- See how one cutoff's curve shifts when another moves
How Much Does the Cutoff Matter?
A cutoff is a judgment call, and the honest question is how much the answer hangs on it. Sweep one cutoff across its plausible range and plot the hydrocarbon pore thickness it leaves, the net pay weighted by porosity and oil fraction, and the slope of that curve is the sensitivity the reserves estimate inherits.
Flat and Steep
Where the curve is flat, the cutoff hardly matters and a defensible value can be picked with confidence. Where it is steep, a single porosity unit or a few saturation points swing the volume by a large fraction. The two cutoffs are not equally consequential: in a rock where porosity and water saturation are anti-correlated, the saturation ceiling does the real work, while the porosity floor is nearly inert because the pay is already high-porosity. Switching the swept cutoff shows it at once.
Argue the Steep Ones
A consequential cutoff must be argued and anchored, ideally to a permeability or a capillary-pressure threshold where the rock genuinely stops flowing, not to a round number chosen for neatness. The sensitivity curve is what you hand the reserves auditor: it shows exactly how exposed the volume is to the one input that is a choice rather than a measurement.
References
- Worthington, P. F. (2008). The application of cutoffs in integrated reservoir studies. SPE Reservoir Evaluation & Engineering, 11(6).
- Cosentino, L. (2001). Integrated Reservoir Studies. Editions Technip.