Complex Low Angle Faulting in Petrel
Complex low angle faulting. The complexity of faulting in Petrel is not limitless, and as a general rule, a fault cannot be truncated by a fault which is itself truncated. However, there are ways of breaking down complex problems to simplify matters. In the case below, a single reservoir unit is split by a complex thrust structure. Because there is no connection between the two units, it is necessary to model them together in order to depth convert the footwall structure. The solution is to model the two reservoirs separately, but use the depth converted bottom horizon of the hanging wall structure to define the first zone of the depth conversion of the foot wall.
The figure below shows a section through the interpreted faults. The thick black line is the main thrust used to divide the model in two.
The areas above and below the main thrust were modeled separately. In addition, faults with minimal displacement were ignored in order to simplify the gridding process as much as possible.
The final models include most of the complexity of the original interpretation and can easily be depth converted using the same velocity model.
How to
- Build two separate fault models, one for the hanging wall and one for the footwall. Avoid complex truncations wherever possible. Remember, faults need not be defined above or below the input data for the model you are working on.
- Build the pillar grid for each model. Make sure the Hanging wall model extends beyond the footwall model (otherwise, you might have problemswhen it comes to depth conversion).
- Copy the input data for the horizons, so you have two sets of data, one for the hanging wall and one for the footwall.
- Build a surface of the main thrust fault you want to use to split the model.
- Use Operations -> Eliminate where to remove areas of the input data on the wrong side of the thrust for each model. (It can be useful to makecopies of the thrust surface slightly above and below the original to ensure that all extra data is removed)
- Build the horizons using the input data.
- Depth convert the hanging wall model.
- Export the bottom horizon of the hanging wall model in time (from the original grid) and in depth (from the converted grid), and use these tocreate a surface of the average velocity through the hanging wall model. (be aware of the two-way time option in the depth conversion settings).
- Use the exported time horizon from the hanging wall model to define the first zone for the depth conversion of the footwall model. Use V=Vo and drop in the velocity surface you made into the field for Vo. (Make sure the units agree with those stated in the Settings and again, be awareof the two-way time option).
- Use any additional surfaces to model the velocity between the last horizon in the hanging wall model, the thrust surface, and the first horizon inthe foot wall model