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
- Hand over a range with its assumptions 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 your depth surface and the contact, inside closure, capped by how thick the reservoir actually is; that cap matters, because 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, and you have stock tank oil initially in place.
In the workstation
Open the Map viewport in VOLUMES. Set the contact, and the red line on the map is where your depth surface crosses it; everything updates as you move it. The workstation quotes three numbers, low, mid and high, built by moving each input across the range an ordinary appraisal would carry. Watch how far apart they sit, from a mid case that looked precise to three significant figures. STOIIP is a product of six uncertain quantities, and uncertainties in a product compound rather than cancel.
Then go back one stage, change which wells feed the velocity fit, and watch the volume move. 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.
The handover
Open HANDOVER and read what the workstation has assembled: the survey and how you loaded it, the tie and its correlation, the surfaces, the model, the range. If you loaded the Ogbon transmittal, it also grades your horizon against the reference interpretation and tells you how much of the fault population you found. Download it. What you hand over is a range, the assumptions behind it, and the two or three things that would most change it if somebody measured them: here, the velocity model away from the wells, the contact, and whether amplitude really tracks reservoir quality. Everything else in the calculation is arithmetic.