Characterization of a Seismic-Scale Fault Zone by Means of Borehole Image Analysis

The complexity of large-scale fault zones, such as might be observable on seismic, often makes it difficult to determine the orientation and sense of movement of the fault on an image log. This paper presents a case, in which an image log was acquired in a single well drilled through a seismic scale fault. Detailed interpretation of the image log helped resolve an ambiguity in the interpretation of the seismic data.

The image reveals that the fault zone is characterised by alternating intervals of featureless sandstone, breccia, steeply dipping bedding and fractures. The orientation of the main fault is ambiguous and both a “normal” and a “reverse” drag scenario can be envisaged. However, the deviation of the wellbore allows a 3-dimensional reconstruction of the fault zone geometry which, when compared to a classic fault core and damage zone model, renders the reverse fault scenario unlikely. The wellbore penetrates the fault zone at a low angle which results in an exaggerated length of the zone as seen on the image. This allows for an unusually detailed characterization of the seismic scale fault, which may increase our understanding of the role of faults as seals or conduits for flow in the reservoir.