The following blogs on velocity modeling are my notes and thoughts on velocity modeling. I am currently taking a deep dive into this topic. I will try to break the concepts down into lay terms, if you want to comment, please feel free to do so! I would love to learn from you as well.
The first thing needed to do when velocity modeling is to create your workflow. In order to create a workflow, you need to know why depth conversion is so important.
- Now I just used the term "depth conversion", we create a velocity model to convert our time volume into a depth volume.
Simply put, depth conversion is important because you need to know how large a prospect is to determine whether its worth drilling! There are some questions that need to be answered to come to this conclusion:
1) What/where is your source rock?
2) Is/was a migration pathway?
3) What/where is the seal?
4) What is the reservoir rock?
5) How/what is the trap?
Accurately knowing what depths (ft) the oil is located helps you determine the best place to drill, what size rig is needed, how long it will take to drill, and be aware that the highest part of the structure in time may not be the same when converted to depth!
A typical workflow goes something like this:
Load data --> Build model --> Interpret, QC data while building layer by layer --> Test model --> Create the technical cases --> Create velocity volumes for each case.
Now there are a lot of terms that go into each level of this model workflow: Checkshots, Stochastic, Interval Velocities, RMS Velocities, Lateral variability, Faust, Anisotrophy, VSP, Vertical variability, Synthetics, and the list goes on...as the terms come up I will define.
Different rocks have different densities, and as a result sound waves move through rocks at different speeds. A general guide to the different rocks and speed (m/s) are:
Shale ≈ 1500-300
Sandstone ≈ 1600-4000
Limestone ≈ 4000-5500
Dolomite ≈ 5000-6000
The reason for such variability for one type of rock is due to compaction and diagenesis.
Compaction - a process where the sediment/material loses its space between the material due to loading, and is part of the process of lithification.
Diagenesis - the re-crystallization of one material into a new material. The temperature and pressure of the material at depth transforms one rock into another type of rock.