Vintage 2-D seismic data is helping to map major faults in the world-class Marcellus Shale play, while newer 2-D proprietary or spec seismic is identifying even smaller structural features.
That was the insight provided by AAPG member James Morris, chief geophysicist for Range Resources Corp.’s Marcellus Shale division in Pittsburgh since 2004, who discussed seismic applications in the Marcellus Shale at the 16th annual 3-D Seismic Symposium earlier this year held in Denver.
“Vintage 2-D will allow you to map a regional structure and identify major faults,” Morris told the 700 attendees. “Newer 2-D can help identify even smaller structural features and faults.”
This newer 2-D seismic data can provide up to five times the special sampling and three times the frequency content, he said.
Also, high fold, high frequency, wide azimuth 3-D seismic has provided detailed isochrones of shale units, local lineaments, open fracture directions and possibly other rock properties, he said.
The north central and southwestern part of Marcellus are the main drilling and seismic activity right now,” Morris said. In the northeastern Marcellus Shale play area, there are more proprietary and spec/multi-client 2-D and 3-D seismic surveys being acquired than in the southwest, he said.
This area is within 60 miles of the Allegheny Structural Front and within the thickest part of the Salina Salt basin.
The Marcellus Shale lies under the Allegheny Plateau, west and northwest of the Allegheny Structural Front. The two main areas of drilling and seismic activity currently are located in northeastern Pennsylvania and in southwestern Pennsylvania to northern West Virginia.
“With the Devonian Marcellus Shale getting increased focus from independents, and more recently major oil and gas companies, Marcellus reserve estimates also have been rising,” Morris said.
From 1985 to 2009, Marcellus reserve estimates rose seven fold from 67 TCF to 489 TCF – “a world class gas field by any yardstick,” he said.
With about 45,000 square miles of prospective area – mainly in Pennsylvania but also in lower New York and north central West Virginia – the Marcellus is probably one of the largest in size, too.
Morris presented a case study and showed numerous slides of Marcellus charts and data.
“Almost all Marcellus operators acquire some kind of seismic data, from vintage 2-D to modern 3-D before drilling, especially in the fold belt region,” Morris said. “Acquiring 3-D seismic can be quite a challenge in the rugged terrain of northeastern Pennsylvania.”
“It’s pretty rough terrain,” he added. “We had a hard time accessing here.”
Steep-sided canyons dropping 1,000 feet with boulder-capped ridges make shot hole drilling difficult and often require heli-drills, he said.
“Roads within the canyons are sometimes vibrated to add fold for imaging problems when shot hole access is limited,” he said.
The velocity of the Marcellus Shale in the southwest is generally slower than the northeast. The Marcellus can be divided into two stratigraphic units in the far western Allegheny Plateau, he said.
The Upper Marcellus has an average velocity of about 11,500 feet/second, while the Lower Marcellus has an average velocity of 10,000 feet/second or less.
“The detection and resolution out of the Upper and Lower Marcellus and these small horst blocks can only be imaged with high resolution seismic, either 2-D or preferably 3-D,” he said.
“The western Allegheny Plateau sits on the northwestern boundary of the Rome Trough and is dissected by numerous northwest to southeast cross strike discontinuities,” Morris said. “These deep seated features appear to have influenced shallower structures right up through coal deposition.
He said that 3-D structure, 3-D coherency and 3-D curvature maps on the Marcellus and encasing formations all show the footprint of these deep features and can divide the shale into structural provinces, providing important insights to well performance.
“Since the western Allegheny Plateau seismic data is so good, we’re in the process of trying to extract rock properties via seismic inversion. The first step involves modeling the Marcellus for anomalies that might be detected with various seismic inversion volumes,” he said.
Some preliminary results are the stratigraphic interpretation of the high-resolution acoustic impedance cube.
“Interpretation shows identification of potential maximum flooding surfaces relating to different depositional cycles within the Upper Marcellus. These distinct depositional cycles may exhibit differing acoustic and stratigraphic reservoir properties carrying operational significance all the way through gas field life cycle from intelligent geosteering to multi-stage frac design,” Morris said.