Once the data gathered and Interpreted by the team, it is time to build the geomodel. The next step is to create the surfaces delimiting the different geological units (Figure 1) Those will represent the horizons (stratigraphic modeling) and the faults if any (structural modeling). If each surface is only defined by well tops, the interpolation will likely be poorly constrained and the model will be highly uncertain far from the wells. If those surfaces were also interpreted on seismic, the interpolation will respect both the well tops and the seismic interpretation and the result will be more reliable: not certain, but at least “less” uncertain.

Structural and stratigraphic modeling involve more than just interpolating data. The modeler must choose an interpolation technique that properly mimics the geological context of the reservoir. Figure 2 illustrates this – this simple reservoir will also be used in the next sections of this chapter. The reservoir is made of a single geological unit delimited by two horizons A and B. Three wells have been drilled and picked (W1, W2 and W3). W2 was TD before reaching the horizon B. The top horizon A is easily built by interpolating the TVDSS values of the three well picks. How shall the horizon B be modeled?

Two mathematical approaches are possible. With approach 1, the TVDSS of the two well picks B are interpolated, in the same way the horizon A was modeled. With approach 2, the thickness of the unit is interpolated and then the TVDSS of the horizon B is defined as being equal to the TVDSS of horizon A minus the local thickness of the unit. As shown on Figure 2, the resulting geometry of the horizon B varies a lot depending on the technique being applied. Furthermore, the well picks alone (the data) can’t help us decide which approach should be used. Only a geologist can answer this. Based on his understanding of the geological context and on his work on the logs, the cores and the surrounding area, he might conclude that:

• Unit A was deposited above an unconformity (Unit B). The two horizons should be modeled separately (approach 1). Or…
• Unit A and Unit B are conformable one to the other. The two horizons should be modeled together (approach 2).

In case the geologist doesn’t have enough data to decide either way, then two models might be needed to capture this uncertainty: one following the approach 1 and one following the approach 2.

As illustrated with the modeling of the Teapot Dome (Figure 3), faults are also modeled as surfaces.