This month's column, the first of a two-part series, is titled "High-Resolution Aeromagnetic (HRAM) Surveys: Exploration Applications from the Western Canada Sedimentary Basin -- Exploration in Highly Deformed Terrains." [PFItemLinkShortcode|id:46336|type:standard|anchorText:See Part 2|cssClass:|title:|PFItemLinkShortcode]
The application of aeromagnetic data to hydrocarbon exploration has moved from primarily mapping basement structures and lithologies to imaging and mapping structures within the sedimentary section. High-resolution aeromagnetic (HRAM) surveys are now relatively inexpensive tools for 3-D mapping of faults and fracture systems propagating through hydrocarbon-bearing sedimentary levels.
Advances in data acquisition techniques (enhancements in magnetometers, compensation software/hardware for suppressing airplane noise, positioning utilizing GPS systems, pre-planned drape surveys, gradiometers measuring horizontal and vertical total magnetic intensity gradients, etc.), as well as data processing and displaying procedures (such as micro-levelling), have significantly improved data quality and resolution, providing levels of detail that are compatible to those derived from seismic, well and surface geological data.
Current industry standards view "high-resolution" aeromagnetic data as fixed-wing surveys with flight-line spacing of 400-800m and tie-lines every 1,200-2,400m, and with mean flight clearance of 100-125m.
The Canadian petroleum industry has been receptive to the introduction of HRAM data as a tool for exploration. Consequently, a large volume of HRAM data has been collected over both mature and frontier portions of the Western Canada Sedimentary Basin (WCSB), including part of the highly deformed Canadian Fold Belt.
HRAM Data In Western Canada
A portion of HRAM data collected in the WCSB is shown in the inset of figure 1. The imagery represents an amalgamation of several different speculative HRAM surveys that were collected over the past decade by industry vendors and the Geological Survey of Canada (GSC).
The area contains approximately one million line-kilometers of HRAM data (representing about a third of the total data collected in the entire basin), covering parts of the WCSB and Mackenzie Corridor.
We focus first on the Canadian Fold Belt region because:
- The geological structures that will be shown are clearly detached from the basement, and thus illustrate that HRAM data can image sedimentary structures.
- Fold belt regions are usually characterized by extremely complex structures that are difficult (and extremely expensive) to image with seismic data.
- The collection of HRAM data is done in a non-invasive manner, and therefore can be collected in environmentally sensitive areas.
- The rugged topographic relief of fold belts allows us to demonstrate the ability of the new surveys to overcome the effects of topography on the magnetic signature of structures.
- Fold belt regions appear to represent a significant portion of the remaining uncharted areas for frontier oil and gas exploration.
HRAM Mapping -- Fort Norman Area, Northwest Territories
In 1998 the GSC began to collect a series of non-exclusive HRAM surveys along the Mackenzie Corridor. HRAM data was collected at 800m flight-line spacing and mean elevation of 125m above ground.
One such survey was collected over the Fort Norman area, along a partially exposed portion of the Mackenzie Mountains thrust belt (figure 2). In this area, the structural style of the fold belt is further complicated by the presence of a major northeast-trending tectonic element known as the Gambill Shear Zone (GSZ).
Figure 2a illustrates that in the subsurface, the Gambill Diapir acted as a transfer zone linking the GSZ with the Norman Wells Thrust (MacLean and Cook, 1999). Geological mapping (2b) and the digital elevation model (2c) show that the structural features associated with these tectonic elements are only partially exposed at the surface.
However, the HRAM data (2d) clearly demonstrate that the transfer zone reflects the presence of a complex shear element that exhibits basement-involved, right-lateral faulting, which resulted in a complex pattern of surface and near-surface anticlines.
In addition, one can notice that the overall magnetic intensities do not reflect the rugged topography found in this region.
The block diagram shown in figure 3 illustrates that the GSZ consists of high-angled wrench faults and a series of tight salt core anticlines that developed and wrapped around a structural high to the north. Strike-slip reactivation of northeast-trending basement faults may have triggered the development of salt diapir structures along this fault zone during Laramide deformation.
Magneto-Stratigraphy In the Coleman Area
Our experience in fold belt regions shows that HRAM data collected at sufficient resolution and flown at low altitude can resolve magnetic signatures that are associated with structures found within the sedimentary section.
Studies have shown that HRAM data can image the internal geometry of folded strata, given that they detect minute variations in the magnetic response of near-surface deformed rock units.
For example, an HRAM TMI (total magnetic intensity) profile nearby the Coleman gas field in the southern Canadian Fold Belt reveals that lithostratigraphic units produce magnetic "highs" and "lows" (figure 4). In this case, the diagram illustrates that the Crowsnest Formation (volcanic sediments) is associated with a magnetic high, suggesting that it contains minerals with relatively higher magnetic susceptibilities than the surrounding rocks of the Blackstone Formation and Blairmore Group.
Consequently, this formation forms a "magnetic marker bed" that may reveal the size and shape of folds at a consistent stratigraphic level.
Conclusions
Since HRAM surveys are strongly influenced by near-surface structures, they are particularly useful in areas where the reservoir targets are located within the upper thrust sheets, where there is significant correlation between surface and subsurface structures.
We have concluded that HRAM data also can detect the presence of reactivated basement faults as well as "tear faults." The recognition of these faults is crucial in exploration, since they can either enhance the reservoir potential of rock units or raise concern about the presence of structural compartments within a targeted fold.
The quality of HRAM surveys in rugged areas, however, may deteriorate due to strong topographic effects, which are very difficult to remove with current processing techniques.
The best way to overcome this problem is to collect data with helicopter-mounted systems. Although significantly more expensive, these systems are flown while draping the landscape, and as such, minimize the effect of topography.
The benefits of this technology will be illustrated in a future article.
Editor's note: Berger, Fortin and Wang are with Image Interpretation Technologies (IITech), Calgary, Canada.