Thursday, April 19, 2012

A Buried Devonian Manhattan Made Of Calcium Carbonate

I'm continuing with the theme of carbonate reservoir rocks - and for good reason. I found this gem of a story about coral reef oil reservoirs from the Devonian carbonate depositional basins of Alberta, Canada in the comments thread of a post on the Oil Drum -

RockyMtnGuy writes:

Well, yes, carbonate reservoirs do test your wits. The ones in Alberta are particularly difficult to deal with. They are just like the girl with the curl right, in the middle of her forehead: when they are good, they are very good, but when they are bad, they are horrid.

A classic was the Rumsey Reef, which they found not too far from where I grew up.

In 1982, Gulf Canada Resources discovered a small pinnacle, called the Rumsey Reef, just to leeward of the Stettler-Fenn-Big Valley reefs. It produced over 3.7 million barrels of high gravity oil from one well; 90% was recovered during the first three years. During that period it flowed 3,000-4,000 bbls/day. A decade later, Gulf explorers, Lemon and Taylor (1993) presented a paper with the wistful title, “The Rumsey Leduc Pinnacle Reef: Where are the Rest?”

The Rumsey Reef is an oil field about the size and shape of a New York skyscraper, just full of oil waiting to be sucked out. Based on the geological history of the area, there must be thousands of similar reefs out there, but they just don't show up on seismic.

Probably most of the remaining Leduc pinnacle reefs in central Alberta, and we have measured many of them, are physically too small to be adequately resolved by reconnaissance seismic exploration, whether 2D or 3D. Our measurements suggest pinnacle reefs, similar to Rumsey in size, are pillars of coral growing between 550' to 700' high from the Cooking Lake carbonate platform. They appear to range from 70' to 225' in diameter.

Wow! .. where does one start?

The Rumsey Reef is an oil field about the size and shape of a New York skyscraper, just full of oil waiting to be sucked out. 

Today most of the Devonian strata of Alberta is in the subsurface, buried underneath younger sediment.

But imagine a sea floor 400 million years ago, where thousands of self assembled towers were being built by organisms scavenging calcium and carbonate ions from sea water to form skeletons made up of the mineral aragonite or calcite. A  living breathing city of underwater skyscrapers hundreds of feet tall, extending in narrow zones tens of kilometers in length, along the edges of depositional basins, where the shallow sea bed suddenly slope into an abyss. The Devonian Alberta basin was a flat shallow water area that gave way along steeper slopes possibly due to faulting activity to deeper waters. These are classic settings for a pinnacle reef to form.

The schematic figure below shows the different settings where reefs form. The patch reefs shaped like towers are the pinnacle reefs. They are called patch reefs because they occur as isolated communities.


A Pinnacle reef is a type of reef wherein the structure is a shape of a pinnacle or tower.  Reefs are biologically constructed structures that rise from the sea floor to form mounds. They are made up of the skeletons of organisms that prefer sunlight depths. Ordinarily, in shallow seas, the upward growth of reefs stop when the reef organisms start getting exposed to the atmosphere at low tides. The reef then grows laterally forming a broad structure.

In certain settings though, a rapidly subsiding sea floor creates space, maintaining a certain water depth, and the reef grows upwards maintaining its colonies in the optimal sunlight zone. Such conditions are present around volcanic islands. As volcanoes become dormant and erode and subside, reefs nucleate along their flanks and grow upwards as pinnacles. This situation is observed today along many tropical volcanic chains in the Pacific ocean.

Another environment for the formation of pinnacle reefs occurs when rapid sea level rise creates sufficient water depth. Pioneer colonies of reef building organisms tolerant of greater water depths may initiate the building of a bio-structure. As the reef grows into shallower water, a different species assemblage may become more common. The communities that build pinnacle reefs may change as the towers grow through different water depths.

The image below is of the coast of Belize. You can see a long north south trending white barrier reef in the center of the image. To the right is the open Caribbean sea. To the left are relatively deeper and quieter lagoons, environments where pinnacle reefs would likely grow. Some of the white isolated patches you see may be pinnacle reefs. This is just to give you an idea of the setting for pinnacle reefs, but I can't say for sure whether those patches are pinnacle reefs from this particular image.

Many of these Holocene reefs in Belize and the West Indies islands have grown on a Pleistocene limestone substrate (figure to the left, SEPM, which during the last sea level fall was weathered into an undulating topography known as karst. In early Holocene, sea level rose and flooded this limestone substrate. Reefs that took root in the depressions i.e. greater water depths, grew upwards to form pinnacles. In Alberta, the situation was different. The basin was an extensive shallow sea and the sea bed was broken into deeper and shallower areas by faults. My guess is that the Alberta basin experienced many episodes of sea level rise in the Devonian, and reef communities growing along the edges and flanks of fault blocks repeatedly grew upwards to form pinnacles.

Today, the dominant reef building organism are species of scleractinian corals, a group of coral organisms which have a symbiotic live in relationship with algae. In the Devonian, the community structure of reefs was different. There were corals present, but they predominantly belonged to two now extinct groups known as tabulate corals and rugose corals. Along with corals were stromatoporoids, an important coral like colonial organism, also now extinct. And there was a supporting cast of algae and molluscs.

The reef building activity in the Alberta basin lasted hundreds of thousands of years. It was not a continuous process. Sea level falls would have interrupted reef formation. A subsequent sea level rise would have allowed reef builders like stromatoporoids to colonize older reef substrates and resume the construction of these enormous towers. Each reef building episode may have lasted a few tens of thousands of years.

Eventually, water depths become considerably deeper, stopping reef growth. Sedimentary conditions changed and buried the reef under layers of mud or fine grained sediment. The reef itself, because it is built by organisms having branching structures is quite porous. Besides this primary porosity, the Alberta basin reefs underwent extensive alteration during burial. The minerals aragonite and calcite were replaced in patches by dolomite. This created more porosity as the replacement dissolved the original minerals and a denser dolomite occupied the space. Oil then migrated into the open pores and got trapped because the reef is capped by fine impervious material. Many of these living towers got transformed over time into a reservoir rock,  a buried skyscraper full of oil.

Regarding the Alberta basin, all these reefs are in the subsurface, and although they are invisible to seismic surveys, new exploration methods using  telluric currents have successfully identified more pinnacle reefs. Many of them will turn out to be prolific oil reservoirs.

But I can't get that 400 million year old underwater Manhattan of calcium carbonate out of my mind!

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