The Appalachian Basin’s Marcellus shale is fast becoming one of the nation’s premier natural gas plays, thanks to its vast extent and choice rock properties. Its reservoir characteristics do vary, however. Speakers at Hart Energy Publishing’s Developing Unconventional Gas—East conference, held in Pittsburgh in October, offered insights into this complex unconventional reservoir.

Jack Ward, executive vice president, exploration and production, for Houston-based PetroEdge Energy LLC, discussed results derived from regional mapping. Ward looked at the distribution of porosity in the shale, which carries two main pore types: inorganic porosity between fractures and grains, and organic porosity within diagenetically altered kerogen.

An area where both types of porosity occur could be especially prospective.

While it seems intuitive to suspect that rocks with higher total organic carbon (TOC) content have higher porosities, the correlation is not straightforward. Ward stressed the need to carefully calibrate log data, to adjust for the presence of kerogen.

The Marcellus is quite thick in northeastern Pennsylvania, but thin in the southwestern region of the state. And yet, both areas are delivering excellent wells.

“The net differential between the plays is less than the gross differential, because northeast Pennsylvania has thick intervals of low organic content,” said Ward.

On a net/gross basis, the difference between the two major play areas is actually small. “We are seeing the impact of the coarse clastic influx on the eastern side of the basin,” he said. “As we add terrigenous material, our ratio of organic to inorganic material is changing.”

His conclusion? The southwestern play has both high porosity feet and high TOC, and is dominated by organic porosity. The northeast play is quite different, displaying a mix of organic and inorganic porosity.

James Coleman, a Reston, Virginia-based geologist with the Eastern Energy Resources Team, U.S. Geological Survey, cited four key factors affecting successful exploration in the Marcellus shale.

The starting point for any resource play is to understand the areas of gas occurrence and the potential volumes. Coleman noted that numerous sources validated the immense size of the Marcellus resource, with estimates ranging up to 220- to 867 trillion cubic feet of gas, on a P-90 to P-10 spread.

Water is another concern across the Marcellus play. An average 17 trillion gallons of precipitation fall in the Marcellus fairway each year, and 7 trillion gallons recharge the groundwater.

“On an annual basis, adequate supplies are probably available,” he said. “However, seasonally low periods may restrict on-demand calls.” Operators need to plan ahead and secure adequate storage capacity for periods of low water availability.

Another issue is adequate disposal and/or recycle capacity for drilling and frac fluids. Up to 300,000 gallons of water per day per well are needed for drilling operations, and that water is typically recycled at the well site. Hydraulic fracturing has major water requirements: 2 million to 8 million gallons of water are required for each frac job.

Treatment options for water are surface storage and evaporation, injection, recycling and wastewater treatment. One persistent issue is that permitted water-disposal locations are scarce to nonexistent, and the permitted sites that do exist are unable to properly treat used frac water.

At least one operator is building its own water-treatment facilities to reuse frac water.

The fourth factor Coleman cited involves possible consequences of habitat fragmentation from drilling and production activities. Drillsite access roads, drill-pad footprints and pipeline corridors each impact the land.

A particular concern in the Marcellus area is the effect of development on threatened and endangered bird species, including bald eagles, osprey, blackpoll warblers and yellow-bellied flycatchers. Horizontal pad drilling is one mitigating solution, since development with horizontal wells would require five wells per square mile, versus 16 wells for vertical-only development.

Coleman stressed that the U.S.G.S. is currently addressing all these concerns through improved data collection and new studies.

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