Facies Come First

Rock Type Sets the Property

Facies, the rock types such as clean sand and shale, are the first-order control on porosity and permeability. Clean sand is porous and permeable; shale is tight. A reservoir property is therefore not one population but several, one per facies, and that changes how we have to model it.

Why One Distribution Fails

Pool sand and shale porosity together and the histogram has two peaks. Fitting a single distribution drops its mean into the empty valley between them, a porosity that no real rock has, so any cell drawn from it is wrong. The widget shows that single fit missing both populations at once.

The Workflow

So facies are modeled first, as a categorical property, and then each continuous property is modeled separately within each facies, using its own histogram and variogram. The facies proportions set the net-to-gross. The rest of this chapter is the toolkit for building that facies model, from simple to powerful.