How do you spot a coal layer from your well logs?
Just remember this: CBM consisits of coal layers.
So technically, you are looking for coal layers/beds.
First, you can see coal cuttings from the shale shakers or from the mudlogs. You will notice an increase in gas readings over a CBM interval.
Second, you can easily detect coal layers from well logs.
In general, you are looking for these wireline trends:
1. Bulk-density measurements less than 2 g/cc.
2. Gamma ray measurements less than 60 API.
3. Neutron porosity measurements greater than 50%.
4. Sonic transit time greater than 80 µs/ft.
5. Shear transit time greater than 180 µs/ft.
6. Resistivity greater than 50 Ohm/m. Sometimes, the coal layers with extensive natural fractures and cleats could retain conductive water that lowers the resistivity reading. The bottom line - resistivity could be low, but may well be a good CBM layer.
7. VpVs ratio of more than 2.
8. GR and RHOB deflect to the left.
So there you go, the eight cool ways of detecting coal bed methane layers. These are super general trends for coal. You just have to compare with what you have as the log signatures could be different.
Also, when you see coal layers, it doesn't necessarily mean that you've got a CBM reservoir. Check with geology.