by Richard I. Gibson
Jargon confuses, and confusing jargon confuses more of us. Oil shale is a rock, a solid rock, that contains some solid organic material called bitumen. It is not liquid. One does not simply drill a well into oil shale and produce oil. It’s more like a mining proposition, like the tar sands in Alberta, only more so—although there are ways of heating it up below the ground, converting that solid bitumen into something that can come up a drill pipe.
There are some oil shales that are oil source rocks, and then there are some other shales that may be even better source rocks, while most people would not call them oil shales. Source rock has enough organic material in it, and has had the proper geologic history, that nature has done the converting, maturing the organic material into actual liquid oil which may (if you are lucky) then migrate into some kind of trap, where explorers may find and produce it.
Some of the best oil shale in the U.S. is in the Piceance Creek Basin of northwestern Colorado. Much of the oil shale there formed in a large, long-lived lake that occupied the area especially during Eocene time, about 50 million years ago. Some estimates suggest more than a trillion barrels of oil in the solid bitumen of these rocks. The problem is you can’t turn it into liquid oil without a lot of heat—a lot of energy, and a lot of dollars. Oil shale in the U.S. is not an economic proposition at current prices. I’ve seen some things that say sustained prices around $85-$100 per barrel could make oil shale economic, but the other stumbling block is huge up-front costs to build the processing plants. Don’t hold your breath for oil shale as some panacea any time soon.
Shale as source rock is another matter. Here, nature does the work. A good example is the marine Chainman Shale of Nevada, laid down during Mississippian time around 330,000,000 years ago. There are places where this rock is up to 8% total organic carbon—really remarkable, given that good source rocks may be 1% or 2%, and some of the best source rocks in Saudi Arabia are typically 5% total organic carbon. The Chainman Shale is not generally a target for exploration, but as a source rock it is critical to the complex oil systems in Nevada.
Nevada? Yep. One two-well field, Grant Canyon, produced around 6,000 barrels a day for quite a few years in the 1980s, the best onshore wells in the conterminous United States. That’s an astounding number when you realize that the U.S. average is 10 barrels per day per well. (Note that there are at least some possible reservoir targets in the Chainman, thin sand beds, but none have been proven and the Chainman is probably a source rock, not a reservoir.)
“Shale oil” is the phrase often applied to unconventional oil production, and the Bakken formation of North Dakota is the best example. It’s much less “unconventional” than real oil shale, because in shale oil, the oil is liquid. It’s just trapped in rocks that are very tight—meaning they have possibly abundant but often tiny pores to contain the oil, and their permeability, the ability of the oil to flow through the rock, is limited. So the way we try to exploit such deposits is not a simple vertical punch into the oil-bearing zone; you’d get precious little that way. Horizontal drilling is the common approach: drill down some 9,000 feet, make a right turn, then navigate your drill string through a 140-foot-thick zone that has oil—for another 9,000 feet. Fracture it to make the porosity better. That’s all expensive, but very doable, and that has turned North Dakota into one of the leading oil producing states in the U.S. in only a few years. Even so, the average North Dakota well only produces around 90 or so barrels per day.
So “oil shale,” or shale source rock, or “shale oil” – they are all really different!
