OPG s DGR, Bruce Site, Tiverton,ON Geosynthesis Phase II Geology Update CNSC/NRCan Briefing December 8, 2009 Ottawa, Canada
Objectives Update on Phase II Site-specific geology work program Neotectonic landforms analysis (S. Slattery) Site-scale structural mapping (Dr. Cruden) Glacial erosion (Dr. Hallet) Karst (Dr. Worthington) Cap-rock natural analogues (Dr. Engelder) Lithofacies variations at the site-scale 2
Neotectonic study - S. Slattery, AGS Investigate evidence of seismically induced features such as liquefaction features and offset landforms (e.g. beach ridges and stream drainage patterns) using: interpretation of air photographs, analysis of a 10 m grid spaced DEM, LiDAR imagery, and field-based analysis of sediment-outcrop sections over 50 km area. The investigation did not identify neotectonic features at the DGR site or adjacent lands. 3
Site-scale structural mapping - Dr. Cruden, U of T Mapping of Devonian outcrop identified 2 main joint sets (ENE and NNW), which are broadly consistent with the regional data set. No significant faults or evidence of faulting was observed. 4
Karst Investigation - Dr. S. Worthington Report concludes that Middle Silurian carbonates are unlikely to have been affected and that the deeply buried Ordovician carbonates are unaffected by modern karstification processes. Evidence of modern karst is not observed in core or permeability measurements below the Upper Silurian or approximately 178 mbgs. Conditions necessary to generate karst (high flux, undersaturated groundwater) do not exist within the intermediate or deep groundwater systems. 5
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Erosion - Dr. B. Hallet, University of Washington Glacial Erosion Estimates from Studies of the Laurentide Ice Sheet (LIS) area Deep erosion such as that found within the Great Lakes is highly unlikely at the Bruce site. Tendency for ice lobes and interlobate areas to occur at generally the same locations in successive glaciations established in pre-pleistocene times. Key LIS erosion studies suggest that in most areas, significant (10 40 m) erosion did not occur during the Quaternary. 7
Erosion - Direct Geological Observations Multiple generations of cross cutting striations on the bedrock surface There are multiple lines of evidence to suggest that the contact forces between the glacier and the bed were modest (type and length of striae, and rare observances of friction cracks and lunate fractures). Overburden cover suggests that the bedrock at or near the site was shielded from active glacial erosion during the last stage of the last glaciations. Glacially Scoured Bedrock (Bruce) 8
Numerical Estimates of Glacial Erosion at the Bruce Site Site-specific erosion rates were estimated using: UofT Glacial Systems Model realizations empirical results from studies of glacial erosion rates at basin scale, maximum known amounts of erosion in the Great Lakes Basin, and glacial geologic observations in the region of the Bruce site. Although uncertainties remain in ice sheet reconstructions and model estimates of erosion, the numerical estimates predict erosion rates on the order of 0.4 m to 7.7 m per 100,000 years. The report concludes that many lines of evidence point to a remarkably coherent conclusion that bedrock erosion on this time scale is likely to range between a few meters and a few tens of meters. Modeled LGM hydraulic potential surface (Hallet, 2009) 9
Cap Rock Analogue Study Dr. T. Engelder, Penn State University Trenton-Black River fields of both the Michigan Basin and in New York area of the Appalachian Basin suggests that Ordovician shales were of sufficient seal quality to trap gas over geological time. (Lehmann et.al., 1995) 10
Presence of abnormal pressures supports the interpretation of effective seal rocks. (from Engelder, 2009) The likelihood of commercial shale gas at the DGR is low due to: TOC content < 2% Low thermal maturation - DGR shales barely reached the oil window. 11
The integrity of the shale-cap seal is suggested by: Lack of maturation Natural hydraulic fractures in shale caps are driven by overpressured natural gas. This mechanism requires a degree of thermal maturation that is not found at the DGR. Lack of interpreted basement faults that penetrate through the shales. This may be due to the high clay content of the seal rock (self-sealing) or age of the faults. 12
Site-scale lithofacies analysis and marker bed study 13
The consistency in lithology and corresponding hydrogeological and petrophysical properties of the Ordovician rocks implies transferability and predictability across DGR site. Lithofacies changes occur as small-scale conformable changes in quantities of shale, siltstone or limestone at the dm- to cm-scale. In situ hydraulic testing (30 m intervals) integrates smallscale lithofacies variations and demonstrates these variations do not control rockmass permeabilities. 14
Conclusions Preliminary Phase II Geosynthesis Geology results suggest: Field mapping within the DGR region (50 km) did not identify neotectonic features. Joint orientation data from site mapping and regional observation are consistent with two major joint sets at ENE and NNW. Site-specific evidence and the regional study suggest that the sedimentary sequence below approximately 180 m is not influenced by karst processes. Multiple lines of evidence suggest glacial erosion rates at Bruce site are in the range of meters to a few tens of meters per 100,000 years. Basin wide and site analogue studies suggest that the Upper Ordovician shales can provide an effective seal at geological timeframes. Borehole and geophysical correlations demonstrate that lithofacies at the DGR scale are laterally continuous and predictable. The lithofacies continuity at site-scale is favourable for site characterization. 15