Some of the largest recent finds in the Gulf of Mexico (GoM) are in places where nobody thought they could be—ultradeep water.

How did the Wilcox Formation, a prolific play onshore the U.S. and Mexico, end up there? How could it have traveled so far from home?

A recent article in Interpretation offers up compelling data that imply that drastically changing sea levels in the GoM (“drastic” by geologic time, at least) might have led to erosion that allowed these sediments to “roll down the shelf,” as it were, and be deposited into the deeper basin. And this evidence, in large part, comes from an outcrop that has attracted attention for years but only recently has been studied in detail.

Steve Cossey, an independent geologist, told me that the outcrop is “very insignificant” but contains an unusual black bed in the middle that for years was assumed to be coal. But after taking samples and sending them to laboratories, a very different story unfolded.

“I talked to a coal expert who told me how to take the samples,” Cossey said. “I very carefully took the samples and sent them to Calgary. He said, ‘Well, it’s not coal.’ He sent them off to a colleague who was an expert in bitumens and oil shale, and she pronounced it a fossilized bitumen.”

As in a petrified surface oil seep. Where none should have been. With turbidites, suggesting deepwater deposition, both below and above the bitumen bed, this implied a rather swift emptying and refilling of the GoM.

Don Van Nieuwenhuise, director of professional geoscience programs at the University of Houston, said that a drastic lowering of sea level in the GoM, most likely due to it being landlocked during part of its evolution, would explain the Paleocene-Eocene thermal maximum, a period during which temperatures increased dramatically. But why?

“If you lower sea level, you’re going to be reducing the pressure on the gas hydrates,” Van Nieuwenhuise said. “They’ll start to come out of crystal form into gas form, and they’ll bubble up.”

Not only that, but reduced pressure could reduce overburden pressure on hydrocarbon reservoirs as well. He said that a drop in sea level of 183 m (600 ft, and maybe more) would drop the overburden pressure by about 400 psi. “Some reservoirs whose seals are right at balance will leak out,” he said.

Cossey plans to go back with a bobcat to dig out more of the outcrop. However, some of the area of interest is under a dwelling, so he’s hoping to incorporate electromagnetic techniques to look for resistivity underground. And he’s hoping to find some fossilized biostratigraphy that will help date the deposit more accurately.

“We’ve got the smoking gun in the outcrop and the bullet (the seep) that came with it,” Van Nieuwenhuise said. “The more we look at it, the more amazed we get.”

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