Reading the actual report, the oil is buried very deep, not at the surface.
The formation is discontinuous, lacustrine in origin, and outcrops extensively in Wyoming, Colorado, and Utah. "Mining" that is commonly proposed is open pit mining. Fish fossils from the Green RIver Formation are well known as well.
Granted, with the formation outcropping (at the surface), the base of a full section could be 1000 feet down, but thermal maturity in basins with ordinary heat flow is attained at roughly 9,000 ft. (or more) of burial. without magmatic involvement.
(In much of the oil industry "very deep" is in excess of 15,000 ft.)
The Green River oil is trapped in the non-interconnected pore space in the shale, and is not subjected to sufficient lithostatic pressure from overlying formations (which are relatively thin or absent for most of the areal extent of the formation) to migrate out of the shale.
That's good news and bad news.
The good part is that it is a near-surface resource if and when economical extraction methods are devised. The bad news is that even the horizontal drilling and fracking methods used in the Bakken Formation or the Marcellus Shale (for example) won't get the job done.
The Parachute, CO plant relied on cooking the oil out of mined shale. Others (Shell, iirc) have tried cooking the oil in situ and forcing it into collector wells from a central heated wellbore. Neither method has proven to be economical, both have their problems, and the EPA would be all over either process like white on rice, especially on Federal Land.
The oil is there, but we need a better way to extract it.