The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, May 17, 2014

May 17. Bakken formation

Today’s topic, the Bakken formation of North Dakota and Montana, is probably familiar to most listeners, and there’s a vast amount of easy-to-find information available, so this will just be a summary. I actually started the Wikipedia page for the Bakken, back in 2007 when it was just taking off – if you go into the history of the article, and go back to the start, you’ll see it was started by Geologyguy – that’s me. The announcement a couple weeks ago that the Bakken had produced its one billionth barrel of oil also indicated that most of that production had come since 2008.

First, let’s talk a little about terminology. The Bakken is not an oil shale. I talked about oil shale in Estonia on March 27. Oil shale is a solid rock, no liquid. It contains a lot of organic material in it, which can be cooked to covert the organic stuff to liquid oil. The confusing, but somewhat better term for the Bakken is shale oil – liquid oil that is tightly trapped within very fine grained shale. The Bakken is the oil version of the Marcellus Shale that we talked about a few days ago – the Marcellus has natural gas, and the Bakken has oil. In both cases, the hydrocarbons are trapped in tiny, tiny pore spaces that are poorly interconnected, if at all. Consequently, techniques like horizontal drilling and hydraulic fracturing, which I described for the Marcellus on May 11, are used to extract the oil or gas.

In the Bakken formation, the oil bearing horizons are about 9,000 feet down, a little short of two miles. So the well is drilled that far, pretty much straight down, then the drill bit is turned to nearly horizontal and navigated through the oil-rich part of the Bakken, only about 140 feet thick. The horizontal portion of a typical Bakken well may extend for two or three miles, 10,000 to 15,000 feet. The kinds of production that such wells yield range from around 100 barrels a day to more than 1000 barrels a day – obviously 1000 is better than 100, but 100 is pretty good considering the average production of all oil wells in the United States is around 10 barrels per day per well. These wells in the Bakken are also a lot more expensive than conventional oil wells, costing $2 million to $5 million or more, when a standard conventional well might cost $1 million or even less.

During much of the early Paleozoic Era, the Williston Basin in western North Dakota and eastern Montana was a deep depression, much like the Michigan Basin that we’ve talked about several times. The water was deeper, and because it was kind of like a deep bowl in the sea floor, the water was also restricted in terms of circulation, so it became anoxic, at least at times. That happened on multiple occasions during the Devonian.

The Bakken lies above, and is therefore younger than the Devonian Jefferson Formation that we talked about the other day, and it’s also younger than another formation called the Three Forks. Both the Three Forks and the Bakken contain black shales, similar to those of the Marcellus, that accumulated in the deep, quiet, anoxic waters of the Williston Basin. But the Bakken is multiple layers, including typically a lower black shale, a middle dolomite, and an upper black shale. The dolomite, calcium magnesium carbonate, probably represents a change to a shallow, more well-oxygenated environment.

A few minutes ago I talked about the Bakken shale being tight – low porosity and low permeability, or interconnectedness of the pores, and that’s true, but it’s not 100% tight. Some of the oil has been squeezed from the two black shales into the dolomite, and the dolomite is in fact the main oil reservoir for the Bakken, so even calling it shale oil is misleading. The shale has some oil in it, and it most definitely served as the source for the oil that’s in the dolomite. Even in the dolomite, the porosity is quite low – maybe 5% of the rock – and the permeability is also very low. And it’s not all uniform – there are definitely “sweet spots,” places where there’s significantly more oil in the reservoir than elsewhere. It’s not a case of drill anywhere.

How much oil is there in the Bakken? Lots of guesses, and some of them are actually intelligent guesses. But you should ignore the hype that says there’s a trillion barrels of oil there. Or, more accurately, you should dig a little deeper. There might be a trillion barrels of oil in place – but by no means can all that oil be produced, and certainly all of it cannot be produced economically. The U.S. Geological Survey and the North Dakota Department of Mineral Resources give pretty reasonable estimates of 150 to 400 billion barrels of oil in place.

How much is producible? That’s the more useful question, and the answer is a moving target based on changes in technology and increasing understanding of both the volumes present, how they are distributed geologically, and how the production declines over time. Again, take various estimates with a few grains of salt. If a company producing oil estimates that there might be 20 billion barrels to produce, maybe that’s correct, but remember that it would certainly be to the company’s advantage to have such a high value for its reserves.

The U.S. Geological Survey, a reliable if somewhat conservative organization, estimated about 3.6 billion barrels producible in 2008, and they’ve more than doubled that estimate now, to 7.5 billion. Proved reserves – that’s a more reliable number, based on actual drilling rather than projections – amount to about two billion barrels or so.

All this is very good. The Bakken has propelled North Dakota from about the #10 state in terms of oil production to #2, after Texas. And its production continues to grow. In November 2013, according to the journal World Oil, North Dakota was producing an average of 972,000 barrels per day. That’s out of about 8 million barrels a day for the entire US, so North Dakota is producing more than 12% of all U.S. oil. North Dakota, combined with significant production increases in the Gulf of Mexico, has dramatically increased U.S. total oil production, from less than 5 million barrels a day 5 or 6 years ago to 8 million today. That’s still well short of the U.S. peak of oil production, at more than 10 million barrels a day back in 1970. Will the surge continue, and bring the U.S. to a new peak? Time will tell. You can find many headlines that say U.S. oil production will exceed that of Saudi Arabia within a year, or a few years. That’s not impossible, of course – Saudi Arabia produces about 9½ million barrels per day, only 1½ million more than the U.S. today. Personally, though, I wouldn’t bet the farm on it. Your mileage may vary, of course.

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On May 17, 1776, Amos Eaton was born at Chatham, New York. He was a geologist and botanist, and he significantly influenced education in the United States through a philosophy of applying science to daily life. In 1824 he co-founded the school that became the Rensselaer Polytechnic Institute in Troy, New York. Among his students was Mary Mason Lyon, founder of Mount Holyoke College, and James Hall, first state geologist of New York.

—Richard I. Gibson

Cross section from USGS

Map from Energy Information Administration

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