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.