Small Clues to Huge Stories about Energy and Climate Change
As collections go, this one is truly minuscule, maybe a handful of samples in a drawer. And, notes the collector, the samples are falling apart.
But don’t let that fool you. Geophysicist William “Will” Gosnold’s collection of shale samples tells a huge story — actually, several stories — about oil reserves, geothermal potential, and long-term climate change, told in rock cored from several hundred meters down.
“Yes, it’s true, we have very few of these samples available,” said Gosnold, chair of the Department of Geology and Geological Engineering at the University of North Dakota and a Chester Fritz Distinguished Professor. “And they’re very fragile. To do the kinds of measurements we need on these samples, we handle them very carefully as soon as they’re recovered. That means wrapping them carefully in a kind of wax paper, and keeping them wet in a tube filled with wet sponges and under pressure if possible so they don’t delaminate or dehydrate.”
Gosnold demonstrates: contained in a small plastic bag are a couple of inch-thick disks of shale-like material. These were brought up in a core sample from holes drilled about half a mile into the Earth in the Eagle Ford Shale formation in Texas, one of several locations around North America where he’s obtained samples.
“Anytime you bring rocks up from depth, if they can expand, they will,” he said. “And now, as you can see, this is a piece of shale that is delaminating and falling apart. So when they come up, we take such samples immediately to our lab where we can do our measurements before anything happens to them. That’s critical. Shale won’t retain its integrity long. After that, the samples lose the physical properties we want to measure.”
Most core samples are much sturdier because they come from dolomite, limestone, sandstone, and other rock types that don’t fall apart as easily as shale does, Gosnold said.
Gosnold has collected samples over the years for various reasons. Originally, his research — funded in large part by the U.S. Department of Energy in its 1970s quest for energy alternatives — was driven by the need to identify good sources of geothermal energy. The big question Gosnold wanted answers to was how far down one would have to drill to get to a consistent source of geothermal heat, the kind that could power electric generation turbines on the surface. But as he worked on the samples he cored, Gosnold discovered something else: evidence of long-term climate change.
Today, Gosnold’s rock collections tell several key stories.
“It appears that the surface temperature change since the last glaciations (the past 10,000 years) is significantly greater than we thought,” he said. “We see this signal in a gentle curvature of the geothermal gradient from the surface to depths of 1.6 kilometers (one mile). To test this we need to obtain a fresh set of core samples from the surface to that depth. This climate signal has implications for our understanding of the thermal maturity of oil source rocks in the Williston Basin and it will help us develop an accurate picture of our geothermal resources.”