The debut of high-resolution aeromagnetic surveys in the early 1990s was received with great interest by the oil industry for exploration of both mature and frontier basins. In less than a decade, many of the active sedimentary basins in North America were covered by spec HRAM surveys that were flown by a fixed-wing aircraft at a line spacing of 400-800 meters and elevation clearance of 125 meters. In some rugged and structurally-complex areas, individual explorers have also collected super-HRAM surveys that were flown by a helicopter at 50 meters line spacing with elevation clearance of 25 meters. The data collected by these surveys provided critical information about the effect of basement and sedimentary structures on the development of both conventional and unconventional hydrocarbon plays, particularly when these surveys were integrated with other seismic and non-seismic exploration tools.
The interest in the application of these HRAM surveys slowly subsided as the basins became more mature and were covered by high-quality 3-D seismic surveys. However, after a nearly 30-year hiatus, there has been a renewal of interest due to pressing demand from the oil industry, government agencies and concerned citizens to find effective methods for mitigation of the potential hazards associated with induced seismic events triggered by the implementation of horizontal drilling, fluid injections and intense frac’ing of unconventional reservoir rocks.
The basic principles of HRAM and super-HRAM data interpretation and integration are no longer widely known among petroleum geologists, and so will be reviewed here along with illustrations of how these surveys can be used to identify tectonically sensitive areas that are likely to become the focal point of induced seismic events.
Geological Mapping Capabilities of HRAM Data
The basic geological mapping capabilities of HRAM surveys are briefly illustrated in figure 1. The first example shows a comparison between regional magnetic and HRAM surveys of the Fort Worth Basin in Texas, which hosts the prolific resource play of the Barnett Shale (figures 1A and 1B). The HRAM imagery shows the magnetic expression of the faulted boundary of the Muenster Arch that forms the eastern boundary of the production zone of the Barnett shale. The HRAM data also reveals the presence of several strike-slip faults that cut and offset the Muenster Arch. Recognition of these faults is critical to the development of this resource play because they are connected to the active aquifer of the deeper Ordovician section (the Ellenberger Formation). There is also strong evidence to suggest that these faults are likely to become the focal point of induced seismic events.
Figures 1C and 1D show a comparison between regional magnetic and HRAM surveys of the southern portion of the Powder River Basin in Colorado. The HRAM image shows the unique structural position of two major Niobrara discovery wells that were recently developed in this area by horizontal drilling. The two wells appear to be formed along the downthrown side of several basement faults that have been charged by the migration of oil from the south.
Integration of 2-D and 3-D Seismic Data
The basic principle of integrating 2-D and 3-D seismic data with HRAM imagery data is illustrated in figure 2. The first example shows a block diagram that was constructed by using HRAM imagery and regional seismic data collected in the Bakken Resource Play of Southern Alberta in Canada (figure 2A). The diagram highlights the structural pattern of a major detachment fault which delineates the eastern boundary of a foredeep basin that was formed in this area by the loading of the thrust sheets of the Canadian Rockies from the west. This major fault, along with unique graben features that developed within the major fault zone, form the focal point for the development of both conventional and unconventional hydrocarbon plays in this area. The detachment surface also acts as an area of tectonic instability that recently recorded some significant induced seismic events related to horizontal drilling activities.
Figure 2B shows a block diagram of HRAM data and a 2-D seismic line of Divine Creek anticline in the Piceance Basin in Colorado. The HRAM image shows the magnetic expression of a major wrench fault system that forms this anticline and structurally deforms a large portion of the basin. This basin hosts several prolific resource plays that have been fully developed and appear to be partly influenced by the presence of regional scale fault and fracture systems associated with the late reactivation of basement structures.
The last two slides (figures 2C and 2D) illustrate how the integration of magnetic and 3-D seismic images can be used to establish the possible interaction between the basement and sedimentary structures. This example focuses on the analysis of the effect of basement uplifted horst blocks on the structural and topographic setting of the Ellenberger Formation in the deeper parts of the Fort Worth Basin. The recognition of the location of faults and sinkholes in the Ellenberger Formation is critical to the development of the Barnett Shale Play. It is important to note that the analysis of the topographic features that developed along the edges of the uplifts is extended beyond the coverage of the 3-D seismic survey.
Building Regional Tectonic, Seismic Hazard Maps with Super-HRAM Data
The process of building a tectonic hazard map of a sedimentary basin and the type of products that accompany this map is illustrated with an example from the Deep Basin and the Peace River Arch areas of the Western Canadian Sedimentary Basin in Canada (figure 3). The example is based on the integration of HRAM, remote-sensing data, production information, detailed seismicity records and other pertinent information. The regional map in figure 3A shows the major fault systems identified in this area, their structural style, timing of deformation, as well as the location of key hydrocarbon pools used to calibrate the tectonic model of the area. Also, indicated in black open circles, are areas that are recognized as Tectonically Active Fault Zones (TAFZ).
The contribution of super-HRAM data to the recognition of tectonically active fault zones in the study area is illustrated in figures 3B and C. Figure 3B shows a close-up look at an active strike-slip fault near the Parkland Gas Field in the Peace River Arch area. This fault cut and offset near-surface glacial stream channels that were deposited in this area during the last ice age. These channels manifest strong expressions on magnetic images because they contain basement rocks eroded from the nearby exposed Canadian shield. Naturally occurring earthquakes and some induced seismic events have been recorded along this fault in recent years.
Figure 3C shows a super HRAM survey that was collected along the Canadian Thrust Front that forms the western boundary of the Deep Basin area. The magnetic image shows the presence of two active strike-slip faults that appear to control the development of this tectonically active thrust belt. The intersection of the thrust front with these active strike-slip faults are regarded as major areas of tectonic instability which recorded high level of naturally occurring and induce seismic events in recent years.
Successful Integration of HRAM
High-resolution magnetic surveys collected over the active mature and frontier basins of the world have been successfully used to establish the structural framework of these basins and for early identification of “structurally controlled” sweet spots in both conventional and unconventional hydrocarbon plays. We hope that the examples shown here will encourage different stakeholders to use this technology to improve the safety of developing different resource plays that employ horizontal drilling, frac’ing and fluid injection techniques.
The keys to the successful integration of HRAM data into future development of resources plays is dependent on two main factors: First, the magnetic surveys should have the quality and spatial resolution that is needed to map both basement and sedimentary structures. Second, the users must acquire a thorough understanding of the basic principles associated with the collection, processing, interpretation and integration of these data sets.