Horizons, Faults, and Zones

The Building Blocks

The structural framework is assembled from three kinds of object, and every later step inherits them. Horizons are surfaces that mark geological boundaries, picked from seismic and tied to well tops; the top and base of the reservoir are horizons, and so are internal markers. Faults are surfaces across which the rock has been displaced. Zones are the volumes between consecutive horizons. The widget builds the picture: add horizons and watch the zones between them appear, then throw a fault to offset the whole stack.

From Surfaces to Volumes to Cells

The hierarchy runs from surfaces to volumes to cells. Two horizons bound a zone; each zone is later subdivided into many thin layers, which become the cells in the vertical direction. The number of zones is geological (it follows the stratigraphy), while the number of layers is a modeling choice (it sets the vertical resolution). Faults cut across this stack, breaking the reservoir into separate fault blocks that may or may not be in pressure communication, which matters enormously for how the field is drained.

Where the Objects Come From

Horizons and faults come mostly from the seismic interpretation, refined where wells tie them. Zones come from the stratigraphic correlation between wells. Getting these objects right, consistent with both the seismic and the wells, is the foundation: a misplaced horizon or a missed fault will bias every volume and every flow path built on top of it.