April 23. Pinnacle Reefs

Coral reefs developed around the margins of the Michigan Basin during the Silurian, but even in deeper water some corals thrived. They grew upward in cylindrical, column-like structures, keeping pace with the rate of rising sea level, like modern atolls. Some reached heights of a few hundred feet, and because of their shape they’re called pinnacle reefs. 

Pinnacle reefs are especially common in a northeast-southwest trending zone across northern Lower Michigan. A belt that’s about 150 miles long and 40 miles wide. The reefs probably grew on a linear break in the sea floor – there’s some evidence in geophysical data for such a break, which would have been the easiest place for corals to get started because it was somewhat shallower than the deepest part of the basin, to the south.

When the tall cylindrical reefs were eventually buried beneath later sediments including salt as the water evaporated, the porous limestone structures built by corals became excellent traps for oil and gas. The surrounding sediment is the seal, the stuff that keeps the oil and gas from escaping from the reservoir, which is that tall little reef. They are usually no more than a half-mile across, but there are more than 700 of them across northern Lower Michigan that have been found to contain oil and gas. A typical oil field there contains around a half-million barrels of oil, which is maybe 45 minutes worth of oil consumption in the whole United States today. To have a good oil reservoir you need good porosity – that’s the holes in the rock where the oil is found – and good permeability, which is how interconnected the pore spaces are. These little reefs have both – they average about 6% pore space, but some are as much as 37% - that’s really huge, more than a third of the rock. The permeability can be as much as 500 times that of a “good” oil reservoir.

Most of those little fields were discovered after 1970, which was the peak of oil production in the United States. In 1970, the U.S. produced around 10 million barrels of oil per day. Today, even with the dramatic additions in production from North Dakota, the U.S. only produces about 7.9 million barrels per day – but that’s significantly up from the 5 million that was usual through most of the 1990s and 2000s.

There are Silurian pinnacle reefs in Illinois, Indiana, Ohio, and Ontario as well, and some contain oil, but the reefs in Michigan are the richest.

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Today’s birthday is William Dean Thornbury, born April 23, 1900, in English, Indiana. He became a professor of Geomorphology at Indiana University and wrote the textbook that was used in geomorphology classes across the United States for decades. Geomorphology is the study of landforms and the processes that create them.

—Richard I. Gibson

Links:
Pinnacle reefs 
Porosity and permeability 
Map from Michigan DEQ