Depositional Environments and Reservoir Architecture

Geometry Is Inherited from Deposition

The shape of a reservoir is not random. It is inherited from the environment that deposited the sediment. A meandering river leaves narrow, sinuous sand ribbons in a mud floodplain; a shoreline leaves broad, sheet-like sand belts parallel to the coast; a deep-water fan stacks lobes at the foot of the slope. That geometry decides almost everything downstream, above all how the good rock connects. A handful of isolated channels and a continuous sheet can hold the same volume of sand and yet behave completely differently when you try to drain them.

Net-to-Gross and Connectivity

The most important consequence is connectivity. As the net-to-gross (the sand fraction) rises, the sand bodies eventually touch and form a network that fluids can flow through. The net-to-gross at which this happens, the percolation threshold, depends on the geometry: sheet-like sands connect at a low sand fraction, while ribbon-like channels stay isolated until the sand fraction is high. The widget lets you pick an environment and raise the net-to-gross; watch when the reservoir switches from compartmentalized to connected, and notice how different the threshold is for channels versus sheets.

The Conceptual Model

Before any grid is built, the geologist writes down a conceptual model: a statement of what kind of reservoir this is, which depositional environment, which way the good rock runs, how continuous it is, and where the barriers sit. Every later modeling choice, the variogram, the object shapes, the trends, is an attempt to honor that conceptual model in numbers. Get the concept wrong and the most sophisticated geostatistics will faithfully reproduce the wrong reservoir. That is why deposition comes first.

References

• Galloway, W. and Hobday, D. (1996). Terrigenous Clastic Depositional Systems. Springer.
• Ringrose, P. and Bentley, M. (2015). Reservoir Model Design. Springer.