Volumes, and what you actually hand over
Learning objectives
- Compute gross rock volume from a depth surface, a contact and a thickness cap
- Show how uncertainty in six inputs multiplies rather than averages
- Explain why the velocity model is usually the largest single error in a volume
- Argue for handing over a range rather than a number
Six sections of work arrive here as a single number, and the single number is the least honest thing you could hand over.
The arithmetic is the easy part. Gross rock volume is the space between the top surface and the contact, inside closure, capped by how thick the reservoir actually is. That cap matters: without it the column keeps growing down-flank and the volume inflates by a factor that looks entirely plausible until somebody asks how thick the sand is. Multiply by net-to-gross, porosity and one minus water saturation, divide by the formation volume factor, convert units, and you have stock tank oil initially in place.
Uncertainties multiply
STOIIP is a product of six uncertain quantities, so the uncertainties compound rather than cancel. Move each input across the range an ordinary appraisal would carry and the low and high cases separate by a factor of several, from a mid case that looked precise to three significant figures.
That is why a volumetric deliverable is a distribution and the mid case is a summary of it, not a result. A single number handed to someone who will make a decision with it is a claim to a precision that the inputs cannot support.
The velocity model is the biggest error, and it does not announce itself
Switch between the one-well depth map and the true structure and watch the volume move.
Something worth pausing on: the previous stage showed the one-well map overstating the closure, 69.7 metres against 32.2. Here it gives a smaller volume. Both are true. A velocity that is too high pushes the whole structure deeper, so however much relief it appears to have, less of it sits above a contact that is fixed at a particular depth.
Getting the velocity wrong does not make a prospect simply bigger or smaller. It breaks the relationship between the structure and the contact, and which way the volume moves depends on both. That is the argument for treating depth conversion as part of the interpretation rather than as a conversion applied to a finished one.
What to hand over
A range, the assumptions behind it, and the two or three things that would most change it if somebody measured them. On this prospect those are the velocity model, the contact, and whether the amplitude really is tracking porosity. Everything else in the calculation is arithmetic.