The stratigraphic model in the modeling workflow

The stratigraphic model is used at various stages of reservoir modeling, either as input data for subsequent modeling steps, or optionally to guide interpretations.

Construction of the structural model and the 3D grid

Creating a stratigraphic model is required if you want to continue with the next steps in creating a 3D geological model. All 3D geological models, whether they are Fluid Models, 3D Grid or 3D Meshes, are based on a structural model, which in turn uses the stratigraphic model to determine the vertical and lateral relationships of the zones.

For each event in the stratigraphic model, a surface will be constructed, based on interpolation of the underlying data. The sequential order of construction, together with construction settings (conformable/proportional to surface above/below) is key to the final layout of the internal zonation of the structural model because each surface is constructed relative to a surface that was previously constructed. Within the set of rules that determine the order of surface construction (explained in Editing the 3D structure zonation) two factors are directly related to the stratigraphic model:

Of both factors, the underlying data representation is dominant, with surfaces based on ‘dense’ data representations (i.e. tri-meshes, 2D grids, point sets and polyline sets) always being constructed before surfaces based on ‘sparse’ data representations (i.e. markers). In the application, this is called geometric prioritization, which follows the logic that dense data, typically derived from seismic interpretations, is of higher quality and more continuous than ‘sparse’ data derived from well log interpretations (i.e. markers). Surfaces based on ‘dense’ data are therefore constructed first and constrain the surfaces based on ‘sparse’ data. Within the group of events with ‘dense’ data, first stratigraphic Level 1 surfaces will be constructed, then Level 2, etc. Only when all ‘dense’ data surfaces are constructed, ‘sparse’ data surfaces come into play, following the same hierarchical logic. The combination of geometric prioritization and stratigraphic order, induced by the position of the events in the stratigraphic model, ensures that the internal zonation of the 3D geological model is geologically realistic.

Guiding interpretations and enabling early QC

Once a stratigraphic model is created you can further extend and fine-tune it by adding or removing zones and assigning events to its zone boundaries. By doing so, you can use the stratigraphic model to guide the interpretations and help perform QC steps in an early stage.

Seismic interpretation  The stratigraphic model can contain any combination of a priori knowledge and interpreted events, coming from various disciplines. The stratigraphic model places these events into the context of other data and disciplines, which enhances the internal consistency of all model data.

Structural modeling  Structural modeling can be performed in stages, with higher order units (e.g. seismic-resolution scale) modeled first, allowing QC steps to be performed at an early stage. Different stratigraphic and fault models can be combined to create multiple structural model scenarios.

Summary

The stratigraphic model brings together data interpretations of various disciplines. The hierarchical design places these interpretations in a geologically realistic context. It can be used to guide interpretations and help perform QC steps. Once finalized it forms the framework of the structural model building sequence and fluid model definition.