Shale vs tight
A couple of weeks ago, we looked at definitions of unconventional resources. Two of the most important play types are shale gas and tight gas. They are volumetrically important, technologically important, and therefore economically important. Just last week, for example, Chevron bought an unconventional gas company for over $4B.
The best-known examples of shale gas plays might be the Barnett in Texas, the Marcellus in eastern US, and the Duvernay in Alberta. Tight gas plays arguably had their hyper-popular exploration boom five or so years ago, but are still experiencing huge investment in areas where they are well-understood (and have nice reservoir properties!). Prolific examples include the Bakken of northern US and the Montney of Alberta.
So if we were to generalize, perhaps over-generalize: what's the difference between shale gas plays and tight gas plays?
| Shale gas | Tight gas | |
|---|---|---|
| Grain-size | Mostly mud | Substantially silt or fine sand |
| Porosity | up to 6% | up to 8% |
| TOC | up to 10% | up to 7% |
| Permeability | up to 0.001 mD | up to 1 mD |
| Source | Mostly self-sourced | Mostly extra-formation |
| Trap | None | Facies and hydrodynamic |
| Gas | Substantially adsorbed | Almost all in pore space |
| Silica | Biogenic, crypto-crystalline | Detrital quartz |
| Brittleness | From silica | From carbonate cement |
Over-generalization is a problem. Have I gone too far? I have tried to indicate where the average is, but there is a space in the middle which is distinctly grey: a muddy siltstone with high TOC might have many of the characteristics in both columns; the most distal facies in the Montney are like this.
Why does this matter? Broadly speaking, the plays are developed in the same way: horizontal wells and fracture stimulation. The difference is really in how you explore for them.