Populate the Facies

Rock Type Comes First

With the skeleton built, fill it with rock. Facies, the rock types such as clean sand, shaly sand, and shale, are the first-order control on where the oil is and how it flows, so they are modeled before any continuous property. Ogbon Field is a sand body concentrated in a mid-column interval, encased in shale.

Net and Gross

The net rock is the reservoir-quality sand that actually holds and flows the oil; the rest is non-net. Their ratio is the net-to-gross, a first-order control on volume. Switch to net rock in the figure to see only the cells that count, and coarsen the grid to watch the same geology block-average into broader bodies while the net-to-gross stays put.

The Scaffold for Properties

Every continuous property is later modeled separately within each facies, so the facies model is the scaffold the rest of the build hangs on. A porosity drawn without regard to facies would blur a porous sand into a tight shale and invent rock that is not there.