Gross Rock Volume and Contacts

The Rock That Holds Oil

Volumetrics starts with gross rock volume (GRV): the volume of reservoir rock inside the trap above the hydrocarbon contact. It is fixed by two surfaces, the top of the structure and the oil-water contact below it, together with the lateral closure of the trap.

Filling to the Spill Point

Lowering the contact thickens the oil column and grows the GRV, but only so far. Every trap has a spill point, the lowest contour that still closes; fill below it and oil leaks updip rather than accumulating. A trap filled to its spill point holds its maximum, and a deeper contact adds nothing. Whether a trap is full to spill or only partly filled is a first-order control on volume.

Structure Times Contact

GRV is the product of structure and contact: a bigger or higher-relief trap holds more, and a deeper contact fills more of it, up to spill. Mapping the top surface from seismic and finding the contact from wells or pressures are therefore the two measurements that set the gross volume before any rock property enters.