Early detection: A Canadian geophysicist is finding success by incorporating existing 3-D data to determine fracture networks in the Bakken Shale.
A Canadian geophysicist is finding success by incorporating existing 3-D data to determine fracture networks in the Bakken – and that could lead to identifying future prospects earlier in the exploration process.
Elaine Honsberger, chief geophysicist at Enerplus in Calgary, Alberta, is talking about her past experience and current activity in the Elm Coulee field, a giant Williston Basin oil discovery producing from the middle Bakken Formation.
“All of the mapping work to locate and drill Enerplus wells in Elm Coulee has been done with detailed geologic mapping based on knowledge gained from wells as the field has been developed,” Honsberger said. “To date, we have not used seismic to locate or drill any wells in Elm Coulee.
“That said,” she continued, “having now had a good long look at 3-D seismic over Elm Coulee, I am optimistic the additional knowledge we gain by integrating our geological, engineering and geophysical interpretations will be helpful to the future development of the Bakken.
“Primarily, I see 3-D seismic as a very useful tool that can be used to better understand the tectonic history of the Bakken,” she said, “which I believe is the key driver of the natural fracture network.
“If we can understand the tectonic history of the Bakken, we may be able to differentiate between areas where we expect to drill higher EUR (estimated ultimate recovery) wells versus lower EUR wells,” she added.
“The concept is particularly intriguing for Bakken fields early in their development life cycle.”
Honsberger spoke at the annual 3-D Seismic Symposium this spring in Denver. The program, which attracted a record 700 attendees this year, is sponsored by the Rocky Mountain Association of Geologists and the Denver Geophysical Society.
Honsberger, in speaking specifically about the use of seismic in the Bakken play, said “there’s a wealth of seismic that has been shot in North Dakota, where the Bakken is undergoing an extreme pace of development.
“I’m a curious person, so I wanted to understand the role seismic could play by using the Elm Coulee Bakken field as an analog,” she said. “I wanted to have a look at it and see if 3-D could help determine the fracture networks I believe are in place.
“The bottom line is I wanted to learn about the Bakken and determine if there was enough evidence to support using seismic for the Bakken play in Montana and North Dakota,” she said.
Critical Picks
The Elm Coulee field is located along the southwest rim of the Williston Basin in northeast Montana. The field, also known as Sleeping Giant, covers more than 500 square miles and has about 700 horizontal wells producing from the Bakken Formation.
Honsberger, in seeking to determine how 3-D seismic might relate to resource and performance indicators, wanted first to understand the basin’s structural framework and tectonic history as it impacted the generation and locale of natural fracture networks.
According to some estimates, the Elm Coulee Field – first commercialized in 2000 – holds over one billion barrels of oil in place, she said, “and that means less than 15 percent has been recovered.”
In 2007, 3-D was shot over a 48-square-mile area for a target below the Bakken.
“What I wanted to do was reprocess this data to understand the structural framework,” she said. “In Elm Coulee, optimizing basic processes such as statics and velocity derivation is imperative to achieve the optimal input for migration, which can then be used to predict Bakken depth and the structural network.”
With about 100 horizontal Bakken wells drilled and producing over the 3-D area, that background material offered a useful database to determine what information can be derived from seismic for the Bakken play in Elm Coulee, she said.
“In Elm Coulee, the thickness of the middle Bakken is typically less than 25 feet,” she said. “That’s very thin, and means seismic methods will have difficulty estimating geological facies variations.
“This put the focus of seismic mapping on really understanding the structural history,” she added.
The two most critical seismic picks used to characterize the structural history and tectonics were the Bakken and the Winnipeg just below the Red River. Honsberger began by looking at the Bakken depth surface, computed from integrating the 3-D seismic data and the existing well information.
“Disappointingly, it was an uneventful surface,” she noted. “However, as I pulled back layers of earth and looked at the Winnipeg I was able to quickly determine major and minor lineaments form the seismic structure surface at the Winnipeg interval.”
But she still needed another approach to map the structural history at the Bakken, so she used the Winnipeg to calibrate, which attributes were most useful at showing lineaments, concluding that dip/azimuth was the best attribute.
In doing so she discovered that the structural lineaments at the Bakken were apparent through attribute mapping – and the fault system interpreted at both the Bakken and Winnipeg provided strong evidence for a large-scale, basement-driven regional strike/slip system that extends across the Williston Basin with primary fracture directions of 45NE and secondary fracture directions of N60W.
The 3-D seismic also provided evidence of geologic features called “flower” structures commonly associated with strike/slip faulting.
“Based on the seismic evidence, the Elm Coulee Field is located on a major strike/slip system in the 3-D area,” she concluded, “with potential for other strike/slip faults elsewhere in the field.”
Making Connections
Honsberger noted there appear to be compartments within the Elm Coulee field that produce similarly – and prediction of these compartments with better reservoir performance may be possible by combining lineament mapping from seismic with some well production for calibration, she said.
“EUR is basically what we expect wells to ultimately produce,” she said.
When EURs for wells within and surrounding the 3-D area were normalized for horizontal well length and then gridded, Honsberger observed that regions within Elm Coulee produce similarly where distinct NE/SW and NW/SE faults may act as fracture propagation barriers between compartments and as fracture fluid thief zones during completion – and possibly as baffles during production.
“These regions with similar EURs may be related to basement-driven ‘faulting,” she said.
Seismically mapped lineaments could help to define these EUR regions, she said.
“We may be able to high grade these regions,” she said. “It could tell us where to drill high EUR wells early.”
When the structural lineaments taken from seismic were overlaid onto the EUR map, it was apparent that the possible boundaries mapped strictly from EUR data are very similar to the lineaments mapped at the Bakken level from seismic.
Bottom line: She determined that analysis of the Sleeping Giant 3-D indicates that 3-D seismic is a useful tool for interpreting structural lineaments at the Bakken and Winnipeg zones.
She also concluded these structural lineaments provide strong evidence for tectonic activity that could have influenced Bakken reservoir quality in multiple ways by impacting the depositional environment and the diagenetic alteration processes, and also through the creation of a natural fracture network.
“The Bakken structural lineaments derived from seismic may be useful for defining regions of similar reservoir performance when some well control is available to calibrate the regions,” Honsberger said.
“The concept is particularly intriguing,” she concluded, “for Bakken fields early in their development life cycle.”