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Davies, Graham R.1
(1) Graham Davies Geological Consultansts Ltd, Calgary, AB

ABSTRACT: Hydrothermal (thermobaric) Dolomite and Leached Limestone Reservoirs: General Principles Genetic Connections, and Economic Significance in Canada

Dolomite emplaced under burial conditions by CaCl2 brines at temperature and pressure greater than the ambient T and Pf of the host limestone through the influence of extensional fault and fracture systems, typically with a wrench component, may be designated as hydrothermal (HTD) or thermobaric (TBD) in origin. Critical proof is provided by fluid inclusion temperature data tied to burial-thermal history plots. Original facies determines flow pathways, type and volume of porosity. Associated matrix-replacive dolomite commonly is of same hot brine source, and may be the principal reservoir. Limestone peripheral to HTD/TBD may be leached to form genetically related reservoirs. Cooling and pressure decline of thermobaric fluid may drive leaching.
Dolomite rock fabrics preserve evidence for formation under shear stress or dilational stress release–these include zebra fabrics compartmentalized by bilateral microfractures (new term), rimmed microfractures (new term) in shear sets, ‘boxwork’ vugs, ‘floating clast’ breccias, others. Episodic, high rate fluid flow within opened faults and fractures is documented by crossbedded detrital dolomite internal sediments. ‘Collapse’ above breccia/dolomite zones may contribute to seismically-recognizable sags. Most HTD/TBD probably was emplaced relatively early at depths less than 500m. Boiling or effervescence may have been active processes.
SEDEX and MVT mineral deposits and HTD/TBD reservoirs are tectonically and structurally linked, with extensional and strike-slip elements common to most. Elevated heat flow is favored by extensional tectonic settings such as volcanogenic backarc basins.
In western Canada, HTD/TBD reservoirs and associated leached limestones contain at least 13 TCF (IER) of gas, and possibly as much as 30 TCF. Reservoirs are of Devonian and Mississippian age. In eastern Canada, HTD/TBD reservoirs are mainly of Ordovician age.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.