February 21. Ohio oil fields

You don’t expect much oil and gas in the Cambrian. Partly that’s because it’s so old and deep, the rocks that might hold oil or gas may have been buried so deeply that the hydrocarbons, the oil and gas, may have volatilized, turned to gas and seeped out. Or the pressure could have reduced the porosity to not much. And since oil and gas come mostly from decaying plants, you have to wonder if there was enough life around to accumulate to be cooked into oil. But we do find some oil in Cambrian rocks.

Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.

In Morrow County, Ohio, just north of Columbus, in the 1960s there was quite an oil boom for a while. It discovered a bunch of little fields in a special kind of oil trap. The upper part of the Cambrian in Ohio includes carbonates – limestones and dolomites, the kinds of rocks that flowing water can dissolve to make caves. At some point not too long after they were deposited, parts of those layers were eroded into hills, and the hills, standing above water line were dissolved – not really into caves, but little dissolution cavities developed in the rocks, excellent spaces to have an oil reservoir.

Ordovician rocks – we’ll talk about the Ordovician next month – were deposited over those rocky hills and served as a tight, impermeable seal to keep fluids from escaping. Oil migrated into those little hills, and stayed trapped until the 1960s, when thousands of wells tapped hundreds of fields. They have produced about 38 million barrels of oil and 35 billion cubic feet of natural gas over time – not that much in the grand scheme of things, but not too shabby, either. To put it in perspective, the United States today consumes almost 20 million barrels of oil every day, so all of the 38 million barrels produced by those wells in the 50 years since the 1960s amounts to about 2 days’ consumption.

Where did the oil come from? Good question. It’s in Cambrian rocks now, but did it start there? Oil reservoirs are not usually where the oil originates. It starts in a rock with lots of organic material, a source rock. Heat, from burial, cooks that solid organic matter over sometimes millions of years, and oil is generated. Then it migrates until it reaches a suitable place to accumulate, a reservoir. The oil in Cambrian reservoirs in Ohio is probably from organic-rich black shales of Ordovician age – younger than the reservoir. How do you push light oil DOWN into older formations? Well, you don’t, with some unusual exceptions. The deep Appalachian basin where the Ordovician shales were heated up – oil people call it the oil kitchen – was deeper than the reservoirs up in central Ohio. The oil did migrate up, as it pretty much must – but it got into older rocks that were physically above the younger source beds. Seems counterintuitive, but it happens more often than you might think.

—Richard I. Gibson

Photo: Oil wells in back yards in Cardington, Ohio, 1964. Photo from Ohio Geological Survey.
http://pubs.er.usgs.gov/publication/70020638