June 17. Shales

You’ll probably be happy to have a break from all the Mississippian limestones. Today’s topic is shales. Mississippian shales in the United States have become pretty important as oil and natural gas source rocks and as tight reservoirs in various places.  

The Heath Formation in central and eastern Montana is a world-class source rock, containing as much as 26% total organic carbon. You may recall me raving about the Chainman Shale in Nevada, which has as much as 8% total organic carbon. The Heath formation has some interbedded carbonate rocks, similar to the interbedding of shale and dolomite in the Bakken Formation, and using horizontal drilling techniques, some wells in the Heath produce close to 500 barrels of oil per day. But it’s not nearly as uniform and prolific as the Bakken in North Dakota, leading to a 2013 headline in the Billings Gazette about the Heath, indicating that the “Shale formation in Montana frustrates oil drillers.” In mid-2013 there were only a dozen producing wells, and the most recent discoveries only yielded around 20 barrels a day. It’s not worth spending $5 million on a well for that kind of return. Explorers haven’t given up, but for now the Heath is on the back burner.

The Heath Formation is late Mississippian in age, about 320 to 340 million years old. It’s about the same age as the Barnett Shale in Texas, the Fayetteville Shale in Arkansas, the Chainman Shale in Nevada, and other shales. They all formed in basins along tectonically active margins of North America, although some were more active than others. The standard view of these organic-rich shales is that they formed in deep water, poorly oxygenated, and stagnant. Recent studies seem to suggest that it was more complicated than that, and that there might even have been some periods of wave erosion in the deposits at times, so the water must have been relatively shallow for that to happen.

In 2013, the Barnett Shale in the Fort Worth Basin of Central Texas produced about 4.5 billion cubic feet of natural gas per day, almost 7% of all the natural gas produced in the United States. The Marcellus Shale, which we talked about last month, was the nation’s leader in natural gas production.

—Richard I. Gibson

References:
Multiscale Erosion Surfaces of the Organic-Rich Barnett Shale, Fort Worth Basin, USA
Mohamed O. Abouelresh, 2013, Journal of Geological Research


Map from USGS