[ConservativeMind]

Is that what’s in North Dakota?

[dirtboy]

Yes, the Williston Basin.

[thackney]

Yes. It is called the Williston Basin.

It has multiple layers from different geological time periods. Cap rocks at different layer prevent oil migration to push upwards resulting in different layers of oil. Often oil has different qualities in different layers under the same surface area.

Within the simplified cross section above are more layers.