Friday Rock Blogging: Oil Shale

i-a89e00c4d30015bb780677e2708e9718-oil-shale-varves.jpg This is a thin section from some Colorado shale. It's part of the Green River Formation, which is a series of rocks laid down about fifty million years ago when the West was wet. The shales come from a set of lakes that occupied part of what is now Colorado, Wyoming, and Utah.

If you look carefully - behind the white blotches, where the contrast is too blown out to say much but they might be grains of sand or bits of shell that fell into the lake where this was forming - you'll see that the shale was deposited in alternating layers of dark stuff and light stuff. The dark stuff is organic matter and the light stuff is inorganic, and the layering is probably an annual cycle of algal bloom and death.

There's a trillion barrels of oil inside this rock.

The problem, for those who would like to release all those beautiful carbon atoms into the atmosphere, is that the oil is only half-cooked, so the organic matter is still solid. For oil to be oil - and more to the point, for it to be a commercially viable form of energy mining with current technology, and at current market prices - it really can't be solid. You can't pump a solid, you have to strip-mine it and haul it away for processing (which is expensive) and then you've got a bunch of useless gravel left over and dump trucks are expensive and all those pesky environmentalists are whining about how you've denuded the landscape.

If you cook the shale, though, the organic gunk will break down into lighter liquid components. In more petrochemically convenient circumstances, the earth does this for us - if you bury the rocks deeply enough, geothermal heat will cook the oil down for you, and then it can slowly percolate back up through cracks in the rock until it hits one of our wells. But that never happened to the Green River Formation.

With oil prices on the rise, several companies have started pilot projects in the Green River Formation to test methods of extracting the oil, either by installing large underground ovens, or by pumping in other chemical reagents designed to convert the oil to a handy liquid form. See this 2006 New York Times story for more information, or check out the ginormous bibliography of relevant technical articles maintained by the USGS.

i-a82e3ba702e8abae9fde32860043238f-green-river-formation-bluffs.jpg For the last bit of context (well, okay, it's a gratutious landscape photo), here's an outcrop of the Green River Formation near the town of Green River, Wyoming.


More like this

This was an interesting article recently published on shales:

Schieber, J. et al (2007) Accretion of Mudstone Beds from Migrating Floccule Ripples. Science, 318, 1760 - 1763.

Mudstones make up the majority of the geological record. However, it is difficult to reconstruct the complex processes of mud deposition in the laboratory, such as the clumping of particles into floccules. Using flume experiments, we have investigated the bedload transport and deposition of clay floccules and find that this occurs at flow velocities that transport and deposit sand. Deposition-prone floccules form over a wide range of experimental conditions, which suggests an underlying universal process. Floccule ripples develop into low-angle foresets and mud beds that appear laminated after postdepositional compaction, but the layers retain signs of floccule ripple bedding that would be detectable in the rock record. Because mudstones were long thought to record low-energy conditions of offshore and deeper water environments, our results call for reevaluation of published interpretations of ancient mudstone successions and derived paleoceanographic conditions.

They conclude:

Elucidating the mechanisms of mudstone deposition not only helps to better understand the rock record but also benefits hydrocarbon exploration, hydrogeology, and coastal and shelf engineering. Managing mud is important for the maintenance of harbors, shipping lanes, and water reservoirs, especially given the impact of climate change. How mudstones act as barriers to fluid migration (oil and water) is probably linked to depositional processes that affect mud microfabrics. For example, if a mud accumulated from current-transported floccules, one might expect a network of larger pores, poorer sealing capacity, and easier release of liquid and gaseous hydrocarbons. Conversely, accumulation in still water from dispersed clays and low-density floccules should lower permeability and may produce an oil shale that clings tightly to its generated hydrocarbons. These qualities are also critical for the ability of a mudstone unit to protect aquifers from contamination and to compartmentalize groundwater reservoirs.

They go on to mention that they found some of these low amplitude bedforms in the Green River Formation (though they aren't any more specific and don't provide photos). It would be interesting to know more about these and how they might relate to the varved part of the sequence.

Paging sedimentologists to the thread... paging sedimentologists... Eric, are you out there?

I don't know where the rippled parts of the Green River Formation are in relation to the varves (NB non-geologists: "varve" is the technical term for these annual deposition cycles) so it's hard to speculate, but IIRC we have other evidence pointing to a traditional low-energy deposition environment for the varved shale (paleoshorelines, and I think you need an anoxic environment to preserve the organics?).

I was looking at some oil shales in a core library a while back, and the oil had capillaries out of the rock, into the wooden spacers used to divide the core sections. There it happily dissolved the "permanent" marker ink used to label the intervals. Luckily the ink labeling had been preceded by pencil...

I don't know where the Green River Formation is but in the early 80's I worked on an oil shale recovery project outside Parachute, CO on the western slope. (about 17 miles west of Rifle, some 11 miles north of Interstate 70.) I was working for Daniels out of SC but I think the ultimate employer was Standard Oil. Exxon had a similar plant under construction in the next valley over. AFAIK, Exxon's was shut down and ours was started up and then mothballed. It would seem like a good place to start testing the viability of the recovery process in terms of today's prices and costs, since the mine, crusher and retort are all complete.

From what I heard working out there, the area was full of oil shale. You could actually light the rocks with a torch. There's oil out there, or down there, or in there, whatever. It's just how much it costs to bring it out.

Tom H

It is not uncommon to find current generated structures in connection with varves or pseudovarves (not true annual varves), so I don't see a problem with the floccule ripples appearing in the Green River Formation. If they occur in the actual varved succession, however, this would mean that the lamination might not be annual varves but some other, more frequent rhythmicity.

...then you've got a bunch of useless gravel left over...

Oh, it's even worse than that. The most obvious method of processing the kerogen in the rock is to heat it up, but this causes the clays in the sediment to expand. So once you've mined and extracted the oil from the oil shale, you have a larger volume of material to dispose of than you originally mined.

It's referred to colloquially as 'the popcorn effect', and this is one of the major roadblocks to tapping this as a hydrocarbon resource.

Chris - the stuff being mined in Alberta are called tar sands. It's basically just the long-chain fractions of petroleum in a poorly consolidated sediment. I'm not 100% sure what happened to the short-chain fractions, but I think it was just volatilization due to surface proximity.

So is the popcorn effect also a problem for heating in situ? How do they compensate for that? It seems like you'd be at risk of healing all your fractures and making it impossible to extract anything.

IIRC we have other evidence pointing to a traditional low-energy deposition environment for the varved shale (paleoshorelines, and I think you need an anoxic environment to preserve the organics?).

Yeah, you're going to need anoxia for the organic preservation. From memory there is quite a bit of evidence that Green River is lacustrine (or at least a substantial chunk of it). I'm not overly familiar with it though.

On the subject of varves, there is decent astronomical evidence.

Fischer, A.,G. and Roberts, L.T. (1991) Cyclicity in the Green River Formation (Lacustrine Eocene) of Wyoming. Journal of Sedimentary Petrology, 61, 1146-1154.

There is also some geochronology that supports the varve counts IIRC. This kind of stuff is pretty strongly suggestive of annual deposition; I'm not sure how Schiebers' observation fits in with this. Thats why I find his offhand remark in the paper is rather frustrating. If you're going to say something, back it up! Still interesting work though.

If you're going to say something, back it up!

It was a Science paper, though. The polite observation is that the page limit is too short to allow for proper support of one's points...

I'm not sure about the effect of the popcorn effect on in situ processing; it may involve driving drifts beneath the areas slated for processing and/or explosive fracturing.

There's a trillion barrels of oil inside this rock.

That's a lot of oil! It will be interesting to see as time goes by if it becomes more economical to retrieve it. Maybe if we can give renewable energy a kick in the pants to upscale production that oil will get to sit there for millions of more years?
Dave Briggs :~)