Migration creates ‘gas halo’

With Marcellus, It’s All About the Fractures

American Association of Petroleum Geologists (AAPG)
Terry Engelder
Terry Engelder

When veteran Marcellus shale player Range Resources issued a December 2007 press release revealing results of its pilot horizontal well program in the shale, it served to verify the rock’s significant productive potential.

Indeed, it jump-started the now-famous Marcellus shale play, which has been a headline maker ever since.

The Marcellus shale member of the Devonian black shales spans a distance of approximately 400 miles, trending northeastward from West Virginia and into New York. Its proximity to the gas-hungry eastern markets is fortuitous.

“This is an unconventional play with a huge area to it,” said AAPG member Terry Engelder, a Pennsylvania State University geosciences professor who has studied the Devonian black shales in the Appalachian Basin for 30 years.

“The size of this continuous shale,” he added, “makes this a very unique resource in terms of potential.”

The Marcellus Shale: The play is all about the fractures in the rock and how you tap into them. Photo courtesy of Terry Engelder
The Marcellus Shale: The play is all about the fractures in the rock and how you tap into them. Photo courtesy of Terry Engelder
All About the Fractures

An acknowledged Marcellus shale expert, Engelder authored a paper in 1985 predicting the Appalachian Basin could contain a well developed set of natural hydraulic fractures. Later, natural hydraulic fracturing in the basin was confirmed in a series of papers by Engelder and various co-authors.

“The play is all about the fractures in the rock and how you tap into them,” he said. “The Marcellus has two sets of vertical fractures, or joints – the J1 and the J2.

“The east-northeast trending J1s are more dense, more closely spaced and are cross-cut by the less well-developed, northwest-trending J2 joints,” he noted.

Once the play kicked off, Engelder quickly became an in-demand interviewee for the mainstream media as well as the go-to guy for numerous players and wannabes who virtually lined up to pick his brain, so to speak.

He recently gave the EXPLORER a quick look at what’s going on in the subsurface of this exciting play.

In His Own Words

ENGELDER: Petroleum and natural gas are both generated in the organic rich source rock, and they have to break out of, or somehow move out of, the source rock to the reservoir rock in the process called migration. There has been long-term questioning about the exact details of this migration – and especially the role that larger scale fracturing plays in all of this.

It was more than 20 years ago when my students and I started looking directly at the source rocks with the idea in mind they might be reservoir rocks also; this, in effect, is what a gas shale is. Gary Lash (see September Explorer) joined in the hunt with me about 10 years ago.

The generating capacity of these source rocks can be large, which means a lot of hydrocarbons could have moved out of the source rock and yet the gas left behind is economic gas.

The question is what is the evidence for early migration out of these source rocks. Some of them, especially the Marcellus in the northeast part of the play, contain more than one joint set that can be related to the generation by natural hydraulic fracturing (NHF) under high gas pressure.

The question remains why some gas shales are more heavily fractured than others. The answer is, we don’t exactly know. One thing that’s clear, though, is the presence of fracturing in gas shale allows for much higher productivity of gas.

In the Marcellus, the most productive wells in terms of initial production are in the northeast part of the Marcellus fairway, e.g. Susquehanna, Bradford and Tioga counties. The fractures developed as a consequence of high pressure methane found not only in the gas shale itself, but fracturing has occurred in a plume above the gas shale.

The reason we believe this is there are two major joint sets in the Marcellus.

The J1 joint set appears to be unique to gas shales. The J2 set appears to break out of the gas shales and populate the rock above those gas shales. This second joint set may appear about 1,000 feet or even as much as 4,000 feet above the gas shale.

We interpret this to mean that a large enough volume of gas was generated so the section above the gas shale became over-pressured to the extent it also was hydraulically fractured. So the section above the gas shale became charged with high-pressure gas as well.

There appears to be a strong correlation between fracturing above the gas shales by NHF and the productivity of the source rock. The correlation indicates a gas column above the gas shale that could have extended maybe 3,000 to 4,000 feet above the Marcellus – although it’s usually not that much. This is what we call the gas halo.

Of course, much of the gas in the halo has bled back to hydrostatic during exhumation, leaving only the Marcellus over-pressured.

One measure of productivity, then, of a gas shale may well be the extent to which fracturing occurs – not only in the gas shale itself, but in the halo or gas plume that occurs over the top of the gas shale.

Regional development is limited ... The most productive wells in the area are found where the largest and most well-developed gas plumes are, as indicated by natural fractures of the rock seen in outcrop.

In Other Words …

ENGELDER: Here’s the bottom line: The early joint set, the J1, was generated as a result of high-pressure gas in the black shale. The volume of high-pressure gas was relatively modest and insufficient for cracking outside the gas shale itself. As the gas shale continued to mature, enough gas became present to crack outside the gas shale into the overlying rocks (the J2 set).

In some instances two of my graduate students, AAPG Student member Yunhui Tan and Tom Johnson, have mapped the J2 set, cutting up through the section as much as maybe 7,000 feet. This is unusual and confined today to the Marcellus area, where gas production is the most dramatic in the northeast Marcellus fairway.

It is no coincidence that the operators are having so much trouble with stray methane in northeast Pennsylvania as well.

During initial maturation, petroleum is the primary product, with natural gas being secondary, so there’s not a large volume of natural gas under these circumstances. If hydraulic fracturing occurs, it’s limited to the gas shales themselves, as in the case of J1 joints.

As maturation continues, more and more gas evolves, and the volume during the latter phase is sufficient to charge the gas shale and the rocks immediately above – and will cause fractured rocks right above the gas shale.

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Engelder's Paper

AAPG member Terry Engelder will present “The Correlation Between Natural Fracturing and Gas Shales” at the upcoming AAPG International Conference and Exhibition in Milan.

Engelder’s paper will be given at 2:05 p.m. Monday, Oct. 24, as part of the AAPG/EMD technical session on “Shale Dynamics: Rock Properties to Hydrocarbon Generation.”

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