When it comes to the ubiquitous shale plays, it’s no surprise to find that a raft of observers tend to view the Marcellus play as just more of the same.
The Marcellus has its own special character.
“When I first started looking at it, I didn’t see the kind of relationships I normally see,” said AAPG member J.P. Fagan, president of Centennial Geosciences in Littleton, Colo. “It was kind of turned on its ear a bit, so to speak.”
Fagan uses aeromagnetic data to help operators steer their Marcellus programs.
He noted that aeromagnetic surveys typically are used to find deep-seated faults that can give rise to shallower structures. These generally are identified with a focus on drilling areas closely associated with them.
“This is the exact opposite of what should be done in the continuous (unconventional) shale,” Fagan emphasized. “In Marcellus shale exploration, one of the geologic hazards to be avoided is structurally complex areas with deep-seated faulting.”
Building on past aeromagnetic data experience gained elsewhere, Fagan decided to look back on an aeromagnetic survey he had flown in 2006 in the northern core area of the Marcellus play along the New York-Pennsylvania border.
He went through reported production numbers, took the magnetic data and spotted the wells.
“I went in and took production data in six months bins and normalized the amount of gas produced by the number of days reported to get the average daily production rate,” Fagan said.
And then he started looking at how this related to the magnetic data.
“I found that where you get near the edge of a basement fault contact, the magnetic field changes quickly,” he said. “If you think of the basement like a series of blocks – sort of like tiles – we found the wells drilled near the tile edges had lower daily production rates in general compared to those kind of interior to a basement block.
“In taking higher resolution data, one of the processing tools we get is the horizontal derivative that shows how fast the magnetic field is changing with respect to the plane of the ground,” Fagan noted. “One of the theorems of gravity magnetic interpretation is if you have faults that are near vertical, you will have maximum horizontal gradient over the fault.
“This is a basement fault, and you don’t want to intersect this with horizontal legs on a well; it would eat up all your hydraulic fracture energy,” he emphasized. “You want magnetically quiet areas, which occur where the magnetic field changes at a slow rate.
“Even small faults can divert a large amount of the stimulation energy.”
Finding the Right Direction
Given the rapid magnetic changes observable near the basement faults, this yields a couple of choices if you want a good well – and who doesn’t?
- Drill the well in the in the middle of the block.
- Site the well on the edge of the block and drill toward the center to encounter more enhanced natural fracturing without encountering the main fault.
“Parallel to the big faults, there will be secondary fractures associated with the main basement fault, and these are the ones you want,” Fagan said. “You want to intersect these smaller systems and not lose your hydraulic fracturing energy to the big open fault systems.
“Magnetics is not a be-all/end-all,” Fagan emphasized. “But it can help you go in the right direction.
“By incorporating aeromagnetic data into a shale exploration program,” he said, “the greater the chance of avoiding faults and large scale fractures that lower peak well deliverability.”