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Simulating Facies using a bivariate Truncation Diagram
The PGS method may be summarized in three steps: 1. Define a Truncation Diagram (also called the lithotype rules); 2. Simulate two Gaussian random variables according to the given variograms; 3. Assign to the simulation grid cells the facies corresponding to the area on the Truncation Diagram to which the Gaussian variables are leading.
Imposing non-stationarity via locally varying fractions
Fully integrated in Facies Modeling, the PGS method supports most of the common features of built-in methods, such as upscaled cells, region conditioning, multi-realization and non-stationarity. Regarding the latter, the PGS method allows to use Probability Properties as target fractions, for one or all the facies codes. The Truncation Diagram is then locally adjusted to the proportion trends.
Use the drawing toolbox of the Truncation Diagram editor
The Truncation Diagram Editor offers a complete and user-friendly way to create truncation diagrams manually. Add horizontal and vertical limits, move or remove limits, and assign different facies codes to specific zones, with just a few clicks. The Undo / Redo and Reset actions are also very convenient. Created Truncation Diagrams can be saved and edited unlimitedly
Building a Truncation Diagram from hard data
In case the rules of the Truncation Diagram (TD) are not known, it is possible to use an exclusive feature of the Truncation Diagram Editor to build the TD directly from hard data. Simply select a facies property as well as two continuous properties independently correlated to the facies data. Then initialize the corresponding TD, with just one click.
Adjusting the Truncation Diagram to target fractions
Another very convenient feature of the Truncation Diagram Editor consists in the possibility to adjust the Truncation Diagram (TD), by automatically moving its limits, to fit at best the specified target fractions. If the TD was initially built from hard data, areas with greater density of data points will change less than those with lower density.
Visualizing a Truncation Diagram and its statistics
New Truncation Diagrams (TDs) are stored in the Input pane of Petrel and can be edited or visualized afterward. When opening a TD in a Map window, it is possible to evaluate its structure visually. The Settings window of the TD gives further information and offers a Statistics tab displaying a preview of the diagram, its facies fractions as well as other noteworthy figures.
Using locally varying variogram range and orientation
Another exclusive feature of PGS is the ability to use non-stationary variograms. The user can specify locally varying range (major, minor and vertical) and orientation (azimuth and dip) from a Property, a Surface or a Surface Attribute. Each generated gaussian field is then influenced locally by the given map. Variograms parameters can also be populated from the current Data Analysis variogram.
Using multiple truncation diagrams per regions
It is possible to use one global truncation diagram (TD) for the simulation or assign different TDs for specific regions of the grid. Unlike the standard Facies conditioning, natively available in the Facies Modeling tool of Petrel, this feature allows to define only one variogram shared across the different regions, while respecting the various lithotype rules from one region to another.