Technology sweeps data from basement

HRAM ‘Flies Low’ Over Subtleties

Looking for “sweet spots,” “preferred trends” and other features that help define resource play exploitation targets?

For many of you, the answer is yes – the industry’s ongoing shift from conventional exploration to resource play exploitation demands the ability to indentify these extremely subtle geological features.

Question two: If so, what tools are you using?

Of all the potential technologies to help seek such targets, at least one AAPG member believes that there’s a technique that is emerging as an important alternative to conventional exploration.

He believes high resolution aeromagnetic (HRAM) surveys – paradoxically, the art of “flying low” over plays to assess their potential – is “gradually gaining recognition” as a valuable tool.

“New advances in processing filtering of HRAM data have significantly increased the value of the data for structural mapping and integration,” said Zeev Berger, president of Image Interpretation Technologies in Calgary, Canada.

Berger, who has written on the subject in the EXPLORER’s Geophysical Corner, is now teaming with employees Michelle Boast and Martin Mushayandebvu to spread the word that HRAM has been successfully used recently in several areas.

“The emergence of several resource plays in North America has allowed us to critically review the contribution of HRAM to exploration and exploitation of several active resources plays,” Berger said.

Those plays include:

  • Texas’ Barnett Shale.
  • The Williston Basin’s Bakken formation (both United States and Canada).
  • The Woodford and Fayetteville shales in the Arkoma and Ardmore Basins (Oklahoma/Arkansas).
  • The Doig and Montney formations.
  • Canada’s Devonian shale play in the Horn River Basin, northeast British Columbia.
  • Several resource plays in the Piceance Basin of Utah.

Results, Berger said, show that many of these resource plays contain “sweet spots” and “preferred trends” that are largely controlled by basement structures and/or shallow fracture systems that are difficult to detect with seismic and well data – but can be clearly seen with HRAM data.

“This experience has lead us to initiate several large spec HRAM surveys,” he said.

Fault Finding

High resolution aeromagnetic surveys are usually collected using airplanes (loosely draping over the terrain) or helicopters flying at a constant elevation above the ground.

Fixed-wing airplane surveys are usually flown at nominal heights above the ground – from 125 to 150 meters – while helicopters, which are used to collect data over extremely rugged terrains, can lower their sensors to less than 50 meters above the ground.

Most HRAM surveys are collected with flight line spacings ranging from 200 to 800 meters and tie line spacings ranging from 600 to 2400 meters.

“The increased resolution of HRAM data improves the imaging of the basement structures, and also provides information on shallow sedimentary structures that can be buried or obscured by thick cover of soil and vegetation,” Berger said.

He said the capacity of using HRAM data to detect geological features in the sedimentary section was “nicely illustrated” with data collected by the Department of Energy in Utah’s Piceance Basin.

“This basin is characterized by extremely rugged topography,” he said, “which has made the collection and processing of seismic data extremely expensive.”

The HRAM data – flown at a line spacing of 400 meters – covers the Divide Creek Anticline located at the basin’s southeast corner.

“The complex structural core of the Divide Creek Anticline is well imaged on HRAM data,” he said. “It shows the typical structural pattern of a wrench fault-related positive flower structure that is bounded on both sides by near surface thrust faults.

“The HRAM data also shows the presence of other faults and fracture systems that have not been previously recognized in this area,” he added. “The high frequency characteristic of the magnetic features indicates that they reflect the expressions of near surface sedimentary structures.”

More Examples

Berger and his team point to other Canadian and American examples that they say show “the advantage of using HRAM data for imaging deep seated basement structure.”

One example, taken from an emerging new resource play in British Columbia, shows how basement structures and topography seem to exert “significant control on the development of sweet spots in the clastic section of the Triassic Doig formation,” Berger said.

“We believe that, in this region, the Doig reservoir sand was preferentially deposited in topographic low areas formed around uplifted basement blocks,” he said.

A different relationship between basement faults and reservoir distribution can be observed in the Williston Basin area.

“Here, the emerging carbonate Bakken play appears to be focused along the uplifted edges of basement blocks, which are cut and offset by major northeast trending strike slip faults.”

Extending 3-D Data’s Value

Regarding HRAM’s increased value for structural mapping and integration, Berger says that the traditional “band pass” filtering, derivatives and a range of residuals of the magnetic data “are being augmented with more quantitative treatment of the data, which relays mostly on depth to source analysis.”

His team says the use of different filtering technique to enhance geological structures is “nicely demonstrated” with HRAM data that was collected over the Green River Basin’s Jonah Field – a field formed by a major northeast trending wrench fault system.

“We used a special band pass filter design to enhance high frequency magnetic features,” Berger said. “This filter shows the near surface expression of the fault trace as well as many other known structural features in the area.”

A “more sophisticated” treatment of the data was demonstrated when the team used the extended Euler deconvolution technique to map and identify faults and fracture system in the area.

“The fault and fracture system correlate well with faults that were mapped with 3-D seismic data,” Berger said, “illustrating how HRAM data can be used to extend the interpretation of existing 3-D seismic.

“The control of basement structures on the development of hydrocarbon plays is a long-standing concept in exploration geology, he continued. “The mechanisms that lead to the propagation of basement structures and topography to the sedimentary section have been well documented and are usually attributed to structural reactivation and differential compaction processes. Yet, these concepts always remain vague and intuitive because basement features have been very hard to map at the same structural level as the sedimentary features above.”

The availability of HRAM data, however, “helped us breach the gap.”

“HRAM images can often provide structural information that can be correlated with seismic and well data,” he said. “Therefore, integrating HRAM survey into exploration programs of mature and frontier basins is gradually gaining recognition.

“We think that these examples demonstrate the important role that HRAM surveys can play in exploration and exploitation of unconventional plays.”  

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Emphasis: Geophysical Review