Formation Fluids in Faulted Aquifers: Examples from the Foothills of Western Canada and the North West Shelf of Australia
J.R. Underschultz, C.J. Otto, R. Bartlett, 2005. "Formation Fluids in Faulted Aquifers: Examples from the Foothills of Western Canada and the North West Shelf of Australia", Evaluating Fault and Cap Rock Seals, Peter Boult, John Kaldi
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Faults and fault zones commonly represent key geological factors in determining migration fairways and assessing the retention and leakage history for hydrocarbons in the subsurface. Although formation pressure data are sparsely acquired from within fault zones themselves, hydrodynamic analysis of faulted aquifers can be used as an indirect indicator of the fault zone hydraulic properties. Case studies from the foothills of Western Canada and the North West Shelf of Australia are used to define a workflow for hydrodynamic analysis in faulted strata and to identify the manifestation of fault zone hydraulic properties on adjacent aquifer pressure systems for various tectonic settings.
Faults with significant displacement can form hydraulic barriers. In this case, fluid flow in the aquifer next to the fault is predominantly parallel to the structural grain, and a discontinuity occurs in the potentiometric surface for the aquifer being crosscut. Localized hydraulic communication (leakage), either across a fault in an aquifer or vertically along a fault zone between aquifers, tends to occur (1) where the fault zone bends out of plane from the dominant stress field; (2) where the main structural grain is crosscut by steeply dipping high-angle faults; or (3) where deformation is transferred from one fault zone to another through a relay zone or transfer fault. These are manifest by chemical or thermal anomalies and potentiometric highs or lows closed against the fault trace. Although conditions of fault zone conductivity tend to be localized, they can limit the trapping potential of structural closures by allowing the leakage and further migration of hydrocarbons.
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This volume constitutes the proceedings of the AAPG Hedberg conference on seals held in Barossa Valley, South Australia, in 2002. The key driver for both the Hedberg conference and this publication was the recognition that knowledge of risk in the estimation of sealing capacity and fault-seal potential is important in making judgments at the exploration, appraisal, and development stages of the petroleum business. In addition, incorporating seal risk in the overall assessment of hydrocarbons in place can affect decisions to drill prospects and the location of appraisal and development wells, as well as reserve estimation. Improved methods to estimate seal capacity and fault integrity can lead to savings in well costs, improved recoveries through optimum placement of wells, and improved estimates of hydrocarbon in place. This volume contains 18 chapters that reflect the spectrum of presentations at the conference. The knowledge imparted by these chapters will be a window on the state of seal knowledge at this juncture of time and includes topics such as seal failure related to basin-scale processes, the role of geomechanics in seals, and the economic evaluation of prospects with a top seal risk.