Shale Oil Deposits Caused By Volcanic Ash From The Time Of The Dinosaurs

Much of the world's oil and gas is trapped in sedimentary rocks known as shale. Rather than being pooled in large basins like other deposits, shale fluids are held in tiny pores, micrometers to millimeters across within the rock. To extract economic amounts of fuel from these shales requires the hydraulic fracturing, or fracking, that technological advances only recently made economically viable. Consequently, across much of the world, long-ignored shale oil and gas have suddenly become both an economic boom and an environmental curse.

While some have raced to exploit these deposits, others have sought to understand them. Professor Cin-Ty Lee of Rice University had seen hints of a connection to volcanic eruptions, but found nothing conclusive until he came across hundreds of layers of ash in West Texas' Cretaceous rocks. This area, along with territory as far north as Montana, was once the floor of a shallow ocean. "We had seen ash layers before, but at this site we could see there were a lot of them, and that got us thinking,” Lee said. “It's almost continuous. There's an ash layer at least every 10,000 years.” Over a period of 10 million years, that adds up.

The ash was traced back to volcanoes in what's now California, part of a huge Pacific rim arc that erupted 90-100 million years ago. However, Lee noticed something strange about the ash layers. "Their chemical composition didn't look anything like it would have when they left the volcano," he said. In fact, three-quarters of the silicon, iron, and phosphorus the volcanoes would have emitted in the ash had been leached out.

What was left reminded Lee of what happens when farms apply too much phosphorus as fertilizer, with the excess running into lakes or oceans. Phytoplankton gobble the precious nutrient up in huge blooms, taking oxygen with it, leaving the waters uninhabitable to anything else. "We found the amount of phosphorus entering the ocean from this volcanic ash was about 10 times more than all the phosphorus entering all the world's oceans today,” Lee said.

The lack of oxygen in these dead zones kills all the organisms in deeper waters that might otherwise recycle dead phytoplankton. Something similar on a larger scale would leave vast amounts of dead organic material, which could turn into hydrocarbons if concentrated by the heat and pressure of subsequent layers of ash, Lee argues in Scientific Reports. The theory explains why carbon in Cretaceous seawater has more heavy isotopes than previous models predicted.

This explains not only the West Texas shale deposits, but the Bakken shales laid down at the same time. These have set off a fracking boom in North Dakota, and a looming battle in Montana. The Marcellus shale, where fracking first became widespread, is 300 million years older but may have been formed through a similar process.

In the deposits Lee investigated, the oil and gas are interleaved between the ash layers, and Lee thinks multiple layers of thinly spaced ash could indicate the presence of undiscovered deposits elsewhere. Unfortunately, there's no way we can mine and burn them without returning the world to the ice cap-free climate of the Cretaceous, and the disasters that would accompany it.

Meanwhile, Lee said the findings could improve our understanding of modern dead zones and the consequences for ocean ecology of sudden inputs of phosphorus or iron causing dramatic flowerings of microscopic life.