A mid-Devonian ‘perfect storm’

Marcellus Owes Its ‘Beauty’ to Algal Blooms

Looking for algae? Field trip participants study the Marcellus Shale Photo courtesy: Greg Wrightsone
Looking for algae? Field trip participants study the Marcellus Shale Photo courtesy: Greg Wrightsone

Today, the Marcellus shale gas play in the Appalachian Basin of the northeastern United States appears to hold the lead position on the hype scale for the many domestic shale gas plays.

It’s a big ‘un, for sure.

The Marcellus trends northeastward from West Virginia into Pennsylvania and New York and has a potentially prospective area of 44,000 square miles, according to AAPG member Bill Zagorski, vice president of geology for the Marcellus shale division at Range Resources, which pioneered this relatively new play.

Zagorski, who in 2009 was officially given the title “Father of the Marcellus” by the Pittsburgh Association of Petroleum Geologists, talked about the play in the Discovery Thinking forum at the recent AAPG Annual Convention and Exhibition in New Orleans.

According to him, reserve potential over this vast acreage is humongous. Current-day estimates range from 50 Tcf to more than 500 Tcf.

The large natural gas reserve projections for individual wells and the play as a whole are directly tied to some darn good geological rock properties. These include high total organic carbon, high porosity and high permeability.

Credit this in large part to blooming algae.

Really.

Gregory Wrightstone
Gregory Wrightstone

“The superior rock properties may be partly explained by the paleogeography and the depositional framework of the Marcellus, and the significant role that algal blooms may have had in the development of this resource,” said AAPG member Gregory Wrightstone, director of geology at Texas Keystone Inc. in Pittsburgh.

He noted that algal blooms in the Middle Devonian Marcellus depositional basin are proposed to have played a key role in the creation, accumulation and preservation of the Marcellus.

“Algal blooms are suggested to have greatly increased the production of organics,” Wrightstone said, “and also enhanced preservation by creation of both local and basin-wide anoxia.”

Bloom Town

During mid-Devonian time, organic-rich deposition took place in a large, nearly enclosed sea – the Marcellus Basin. Marine phytoplankton was the main contributor to organic material in the organic-rich facies.

The phytoplankton growth no doubt was enhanced by the subtropical warmth and solar radiation indigenous to the basin’s paleographic location in the sub-tropics.

The Marcellus Basin was bounded on the east by the Acadian Highlands. The likely arid conditions on the Highlands’ west flank contributed significantly to the quality of organics deposited in this environment owing to probable non-aeolian sediment starvation.

Fold is in Union Springs member of Marcellus Shale at Oriskany Falls quarry.
Fold is in Union Springs member of Marcellus Shale at Oriskany Falls quarry.

“Several recent investigators have identified aeolian silts as the major contributor of quartz within the organic-rich black facies of the Marcellus,” Wrightstone said. “It’s probable that large dust storms may have been a recurring phenomenon during the deposition of the Marcellus.”

The dust blown into the nearly landlocked basin most likely came from Middle Devonian soils containing naturally occurring nitrates, sulfates and iron. Upon entering the sub-tropical basin, these nutrients would have triggered formation of algal blooms.

This sudden increase in growth, or blooms, of the basin’s omnipresent phytoplankton population would have significantly increased the amount of organic material available for deposition and accumulation.

“Far reaching anoxic events such as those associated with modern blooms may have occurred as basin-wide events during the Marcellus deposition, leading to enhanced preservation of the organics,” Wrightstone said.

“Just as with modern algal blooms, large anoxic ‘dead zones’ are created with explosive algae population,” he said. “The algae consume the available nutrients and die off, stimulating a bacterial process that breaks down the dead algae.

“The process uses a huge amount of oxygen, creating a ‘dead zone’ that then kills off the area surrounding the ‘bloom,’ making it uninhabitable,” Wrightstone said. “Since the Marcellus sea was nearly enclosed, repopulation of the basin would have occurred exceedingly slowly.

“Phytoplankton growth within the basin would have continued, with enhanced preservation of the organics owing to lack of a developed marine ecosystem,” he added. “Additionally, the nearly enclosed basin would have likely led to a much shallower wave base than in an open marine setting, allowing for reduced oxygenation and preservation on the basin floor.”

A Lesson For All?

Organic rock quality varies across relatively small areas of the basin, and some larger regions have superior capabilities for production. These local variations may be partly explained by localized algal blooms, according to Wrightstone.

He succinctly summarized some of the salient points relative to deposition and preservation of the Marcellus:

  • The excellent rock properties of high TOC, porosity and permeability found in the Marcellus Shale likely are related in large part to the depositional processes in place during the Middle Devonian.
  • Controls on the depositional processes of the Marcellus Shale may include paleogeography, nutrient sourcing of algal blooms by frequent dust storms and preservation of the organics by basin-wide bloom-related anoxic events.
  • The Marcellus organic-rich units were deposited and accumulated in a “perfect storm” scenario of maximum organic production, maximum preservation and minimum non-aeolian sediment influx.

Even if you’re not playing the high-profile Marcellus, you may want to file this info away in your shale folder.

“The important role of algal blooms,” Wrightstone said, “is likely to be applicable to many of the other shale plays around the world.”

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