Testing 3D Seismic Attribute Strategies for Subtle Fault Mapping

John McBride, R. William Keach II, and Clayton K. Chandler
Brigham Young University

Discovering and mapping fine-scale faults is a critical activity for carbon sequestration planning. Particularly important is assessing the risk of subtle faults or fractures that may continue from a potential sink upward through a potential seal. Further, if such faults or fractures can be shown to have propagated from seismogenic depths (e.g., from Precambrian 'basement'), then a risk of reactivation may exist. In order to assess the potential strengths and weaknesses of using geophysical data to study the problem of subtle faults, we use a publicly available 3D seismic reflection data set from the Illinois Basin where a deep Paleozoic interval for carbon sequestration has been identified. The Illinois Basin provides special challenges for carbon sequestration planning because geologic anomalies (e.g., faults) can be faint and easily missed using 2D seismic data or even 3D data if only conventional display strategies are used. Previous studies have suggested that many large-scale structures in the basin are facilitated by deep-seated faults, which have propagated up continuously from Precambrian rocks into the shallow Paleozoic section. However, the same cannot necessarily be said for small-scale faults in the basin. Using the 3D seismic data from the Illinois Basin, we employ a suite of seismic attributes to demonstrate how deep faults at the target sequestration interval may initially appear to propagate vertically through the Paleozoic (and uppermost Precambrian) section in conventional displays, but when mapped in detail are seen to be much more complex. We have computed attribute displays based on discontinuity, positive and negative curvature, amplitude change in X and Y directions, and seismic shaded relief. Our results show how sets of small-scale faults can apparently grow and die out in a vertical section, while skipping laterally up or down section, thus lacking significant vertical continuity. However, such faults can appear continuous when viewed without detailed analysis and without optimal display parameters and orientation. Our study provides a general strategy for assessing subtle fault continuity and a cautionary tale for concluding significant fault continuity where none may actually exist.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013