Sealing by Shale Gouge and Subsequent Seal Breach by Reactivation: A Case Study of the Zema Prospect, Otway Basin
Paul J. Lyon, Peter J. Boult, Richard R. Hillis, Scott D. Mildren, 2005. "Sealing by Shale Gouge and Subsequent Seal Breach by Reactivation: A Case Study of the Zema Prospect, Otway Basin", Evaluating Fault and Cap Rock Seals, Peter Boult, John Kaldi
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The Zema prospect, located in the Otway Basin of South Australia, hosts an interpreted 69-m (226-ft) paleohydrocarbon column. Two faults are significant to prospect integrity. The main prospect-bounding fault (Zema fault) shows a significant change in orientation along strike, with some parts of the fault trending northwest-southeast and other parts trending east-west, all at a consistent dip of about 70°. The fault shows a complex splay and associated relay zone at its western tip. An overlying fault shows a similar northwest-southeast trend.
Shale volume (Vshale) derived from the gamma-ray log was tied to seismic horizon data in order to model across-fault juxtaposition and shale gouge ratio on the Zema fault. Shale volumes of greater than 40% correspond with paleosol shale lithotypes identified in the core that are characterized by high mercury injection capillary entry pressures of 55 MPa (8000 psi), capable of supporting gas columns far beyond the structural spillpoint of the trap. Vshale values of 20–40% correspond to silty shale lithotypes characterized by mercury capillary entry pressures equivalent to gas column heights of less than 30 m (100 ft). Sands correspond with Vshale values of less than 20%.
Juxtaposition modeling of the Pretty Hill reservoir interval that is displaced across the Zema fault against the Laira Formation seal demonstrates the existence of both sand-on-sand juxtaposition and sand-on-silty shale juxtaposition above the paleofree-water level. Hence, juxtaposition alone cannot explain the observed paleocolumn. It is therefore likely that fault damage processes on the fault plane were responsible for holding back the original 69-m (226-ft) column. Shale gouge ratio values show a gradual decrease from 32% at the top of the fault trap to less than 14% at the structural spillpoint. The fault damage zone is likely to consist of phyllosilicate framework rock types. Because the Zema trap was not filled to structural spillpoint, it is likely that the percentage of shale gouge in the fault zone not only provided the original sealing mechanism but also limited the original column height. This is supported by fault zone capillary entry pressures calculated from shale gouge ratio values, which indicate that the fault zone is only capable of supporting a maximum column height of 72 m (236 ft), just 3 m (10 ft) more than the interpreted column height of 69 m (226 ft).
Geomechanical analysis shows that the northwest-southeast-trending parts of the faults are optimally orientated in the in-situ stress field for reactivation. A spontaneous potential (SP) anomaly in the Zema-1 well, which was recorded in a northwest-southeast-striking fault damage zone through the seal, confirms the existence of open, permeable fracture networks. These are likely to have been generated by recent reactivation that caused the breach and subsequent leakage of the entire original hydrocarbon column.