Building the Structural Framework
Learning objectives
- Explain what the structural and stratigraphic framework is and why it is built first
- Distinguish horizons, faults, and zones, and how layering subdivides zones
- Describe corner-point grids and pillar gridding, and how they honor faults and dip
- Compare proportional, follow-top, and follow-base layering schemes
- Build a faulted, layered geocellular grid and reason about its geometry
From Geology to a Grid
A reservoir model begins as a geocellular grid: a three dimensional mesh of cells that fills the reservoir volume. Before any rock property or fluid is added, we build the grid skeleton, the structural and stratigraphic framework. Structure is the shape of the reservoir (its folds, its dip, and the faults that cut it). Stratigraphy is the layering inside it (the zones and the thin layers that record how the sediment was deposited). Get this skeleton right and everything downstream, the properties and the flow, sits on a faithful geometry. Get it wrong and no amount of careful property modeling can repair the result.
Horizons, Faults, and Zones
Three objects define the framework:
- Horizons are surfaces that mark geological boundaries, usually picked from seismic and tied to well tops. The top and base of the reservoir are horizons.
- Faults are surfaces across which the rock has been displaced. A normal fault drops one side down relative to the other; the vertical offset is the throw. Faults can seal or leak, so they matter both for the grid geometry and later for flow.
- Zones are the volumes between consecutive horizons. Each zone is then subdivided into many thin layers, which are the cells in the vertical direction.
Corner-Point Grids and Pillar Gridding
Real reservoirs are faulted and dipping, so a simple box grid will not do. The industry standard is the corner-point grid. Vertical or near vertical lines called pillars are placed on a map grid, often along the faults, and each cell is defined by eight corner points, two on each of four pillars. Because the corners slide independently along the pillars, the grid can follow a fault plane exactly and honor steep dip without distorting the cells. This is pillar gridding.
The figure below builds one. Set the structural dip, throw a normal fault across the middle, and watch the two fault blocks offset. Exaggerate the vertical scale to see the layering, peel layers to look inside, and color by zone to see the stratigraphy or by porosity to preview a property model on the same skeleton.
Layering Schemes
Within a zone, how should the layers follow the bounding horizons? The choice encodes a depositional assumption:
Proportional
The zone is split into a fixed number of layers of equal proportional thickness, parallel to neither surface. Good for conformable zones filled gradually.
Follow top
Layers parallel the top horizon and onlap or truncate against the base. Good when the top is a flooding surface and the unit built downward from it.
Follow base
Layers parallel the base horizon and are truncated by the top. Good for prograding units built upward from the base.
The scheme controls how flow barriers and high permeability streaks connect between wells, so it is a real modeling decision, not a cosmetic one.
Why the Framework Matters
The framework is the one part of the model that every later step inherits. Facies and porosity are simulated cell by cell on this grid. Volumes are summed over these cells. The flow simulator solves its equations between these cells. A grid with distorted cells, a misplaced fault, or the wrong layering will bias the volumes and corrupt the flow long before any property uncertainty matters. That is why we build it first and check it carefully.
References
- Ringrose, P. and Bentley, M. (2015). Reservoir Model Design. Springer.
- Cosentino, L. (2001). Integrated Reservoir Studies. Editions Technip.