Carbon reduction techniques, such as carbon capture and storage or geothermal energy, involve injecting fluids into the subsurface. These injections can lead to changes in the stress field of the rocks, which might induce seismic activity. Moreover, the occurrence of significant seismic events can generate opposition to the implementation of these climate solutions among local communities.
Therefore, to assess the geohazard risks associated with this particular phenomenon, it is critical to analyze factors such as: 1) the properties of the rocks – either their geomechanical, hydraulic or thermal characteristics, and 2) the characteristics and orientations of geological features, such as faults and fractures. Our study focuses on demonstrating techniques that can help improve reservoir characterization and mitigate the potential for induced seismicity.
We demonstrate here the utilization of novel seismic attributes to enhance the imaging of faults that might cause seismicity when not seen in the exploration stage. These attributes can be applied to any well-imaged seismic dataset, regardless of whether it pertains to CCS or geothermal activities. In this study, we analyze data acquired from the Kevin Dome region of the Sweetgrass Arch in Toole County, Montana, which was collected as part of the Big Sky Carbon Sequestration Partnership project. The Kevin Dome area is characterized by a significant structural closure that naturally contains CO2 and has been identified as an ideal site for studying underground carbon storage. Although the Environmental Protection Agency regulations did not permit the storage of anthropogenic CO2 in the Devonian dolomite Duperow Formation, the BSCSP adjusted the project’s focus to gather data that aid in the reservoir characterization. This includes well logs, core samples and seismic data.
In the Kevin Dome dataset (figure 1a), a 3-D nine-component seismic survey was conducted. However, our analysis focused solely on the PP-PSTM seismic volume. Within this dataset, three prominent faults (F1, F2 and F3) cutting through the seismic reflectors below the targeted Duperow Formation are evident (figure 1b). However, although comprehensive reports that integrate all available information into a static model suggest that these faults act as sealing faults and intersect the injection interval, faults within the target Duperow Formation are not clearly visible in the seismic data.
A couple of novel attributes are presented here that are not as well known but have the same great potential as coherence and curvature to optimize the visualization of these faults and mitigate the risk of CO2 injection in the Kevin Dome CCS project.
Seismic Attributes
Seismic attributes have found widespread application in structural and stratigraphic interpretation. Common geometric attributes used for structural interpretation include coherence and curvature. In addition, recently developed attributes such as the multispectral coherence and aberrancy represent valuable tools to enhance the seismic interpretation.
Coherence is renowned for mapping faults and the edges of various architectural elements. However, coherence might not be effective when the offset of a fault falls below the resolution of the seismic data, resulting in the seismic reflector appearing to be continuous. In such cases, alternative approaches like multispectral coherence, which combines the coherence computed along different spectral components, has demonstrated its effectiveness in delineating tuning discontinuities that were not clearly imaged using the conventional broadband coherence, thereby enhancing fault visualization.
Curvature and aberrancy attributes have the capability to map faults at a sub-seismic scale when they manifest as folds adjacent to faults and no clear fault offset is visible in the seismic amplitudes. Curvature mathematically represents the first derivative of the reflector dip, allowing it to measure changes in dip and highlight concave features in upward or downward directions. Since geological features might exhibit curvature at different scales, it can be calculated as long-wavelength or short-wavelength curvature.
On the other hand, aberrancy represents the second derivative of the reflector dip, providing a measure of flexures where curvature undergoes the most significant changes. Consequently, aberrancy can map planes associated with potential small-offset faults, which are characterized by subtle flexures in the seismic reflector. Several studies have demonstrated the utility of aberrancy in mapping such faults by quantifying the strength of the deformation (aberrancy magnitude) and their azimuth (aberrancy azimuth).
Overall, incorporating multispectral coherence and aberrancy in addition to coherence and curvature attributes expands the toolkit for structural interpretation, enabling a more comprehensive analysis of seismic data and enhancing fault delineation at various scales.
Results
Broadband coherence reveals intriguing features within the northwest Near Upper Duperow horizon and southeast Near Mid Duperow horizon. However, it remains uncertain whether these features are stratigraphically or structurally related (figures 2a, 3a). In contrast, the application of multispectral coherence (figures 2b, 3b) provides better delineation of some features with a more linear pattern (northwest-southeast trend) in the northwest area of the dataset (cyan arrows, figure 2b). Nevertheless, faults (F1, F2 and F3) observed below the Duperow in the amplitude volume remain elusive within the target formation. Notice, however, that in the cross sections (figure 3b) those faults are more clearly highlighted than in the broadband coherence. Upon analyzing the seismic expression of these faults, we observe that despite their clear interruption of reflectors below the Duperow Formation, within the formation, they appear as continuous and subtly folded reflectors. Consequently, any attribute reliant on discontinuities might fail to capture them effectively.
On the other hand, curvature analysis successfully highlights one of the main faults (figures 2c, 3c) across the three horizons. In the Near Lower Duperow, a subtle positive curvature response is indicative of a probable second fault (yellow arrows). Moreover, curvature provides valuable insights into the features observed in the northwest zone of the Near Upper and Mid Duperow horizons, notice they have a bowl-shaped pattern (negative curvature surrounded by positive curvature) typical of stratigraphic features likely associated with karstic processes (cyan arrows). Additionally, curvature highlights other lineaments with an northeast-southwest trend (green arrows) in the southwestern portion of the three horizons, possibly indicating a previously undetected fault.
The most promising results for structural interpretation are obtained through the application of aberrancy analysis (figures 2d, 3d). By mapping the point of maximum curvature change (flexure), aberrancy attributes successfully identify all three main faults within the target zone, characterized by a near north-south trend. Furthermore, aberrancy highlights additional lineaments with a northeast-southwest trend (green arrows), consistent with the features observed within the curvature attribute. Notice that all these features have a clear geometric pattern, which could indicate that even if the northwest-southeast and northeast-southwest features could be stratigraphic, there must have been a structural control.
While coherence and curvature analysis remain essential tools in the seismic interpreter’s repertoire, the inclusion of aberrancy attributes enhances the ability to detect subtle faulting and potentially fracture zones that might cause inducted seismicity. It is important to emphasize that these subtle features are best imaged in seismic volumes where noise removal is a key component of the processing steps.
Conclusion
In our analysis of the Kevin Dome dataset, we observed that, when it comes to smaller offset faults and fractures, conventional discontinuity attributes such as broadband coherence might not provide optimal results. In addition, although multispectral coherence has shown promise in analyzing clearly faulted strata in other studies, it fails to reveal faults at sub-seismic resolutions, whereas curvature is only able to detect one of the main faults in the study area. This limitation highlights the importance of incorporating additional techniques to improve fault imaging, particularly in cases where sub-seismic faulting is prevalent.
Here, in the context of the Kevin Dome dataset, the clear necessity of integrating the aberrancy attribute into standard workflows for mapping small-offset faults are demonstrated. Aberrancy proves to be more sensitive to small flexures that could potentially be indicative of sub-seismic faults through which fluids could travel. By recognizing the importance of aberrancy, we enhance our ability to detect these subtle features, thereby increasing our understanding of the potential for induced seismicity associated with fluid migration.
In conclusion, our findings show that aberrancy attributes have the same potential as coherence and curvature attributes had when they were introduced for the first time, and we strongly advised for its inclusion as a crucial attribute when mapping potential faults at different scales. Its sensitivity to small flexures that might be associated with small-offset faults provide valuable insights that can contribute to assessing the risks of induced seismicity. As we continue to refine our seismic interpretation workflows, the incorporation of aberrancy offers a promising avenue for improved imaging and characterization of faults, ultimately aiding in the development of safer and more sustainable resource extraction and storage practices.
Acknowledgements
The authors would like to acknowledge the Attribute Assisted Seismic Processing and Imaging Consortium and sponsors for the seismic attribute software and funding, and Schlumberger for Petrel access for mapping and interpretation, as well as the Department of Energy for seismic data, and Zanskar Geothermal and Minerals Inc. for research funding and support.
(Editors Note: The Geophysical Corner is a regular column in the EXPLORER, edited by Satinder Chopra, founder and president of SamiGeo, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer.)