Hydrothermal Dolomite and Leached Limestone Reservoirs: Representative Core Fabrics and Facies
Graham R. Davies1, Tom Boreen2, and Langhorne
Smith3
1 Graham Davies Geological Consultansts Ltd, Calgary, AB
2 Suncor Energy, Calgary, AB
3 New York State Museum, Albany, NY
In the last 10 years or so, there has been a strong upsurge in the number
of dolomite reservoirs interpreted to have formed or to have been influenced by
alteration of host limestones by saline fluids of hydrothermal origin.
Extensional but particularly transtensional locations on strike slip faults
appear to be the favored sites for focused hydrothermal fluid flow and more
massive dolomitization. However, there are many examples of dolomite reservoirs
in which characteristics of hydrothermal dolomite (HTD) fabrics and facies
extend for 45 to more than 200 kms – that is, they appear to be regional
hydrothermal dolomite trends. Although high resolution aeromagmatic mapping and
3D seismic interpretation across parts of these trends typically show structural
complexity and arrays of faults, questions remain concerning emplacement
mechanisms
and fluid source/volumetrics for these more extensive dolomite
bodies.
Texture, fabric, pore type and pore volume in HTD reservoirs are
controlled mainly by the facies of the original limestone host. Porosity and
permeability of the host at time of dolomite emplacement strongly influence the
size of the dolomite reservoir
and its lateral distribution away from fault
conduits. In relatively tight or low permeability limestones, such as the
Ordovician Trenton-Black River formations of the Michigan and Appalachian
Basins, structurally-controlled HTD
reservoir
trends may be sublinear and
narrow. In contrast, originally more permeable grainstones and
amphiporid-stromatoporoid floatstones-rudstones in the Devonian of the Western
Canada Sedimentary Basin (WCSB) may lead to more ‘regional'
reservoir
development. Dilational breccias, zebra fabric, arrays of shear microfactures
(rimmed microfractures) and related fabrics may be formed preferentially
proximal to periodically-active faults (fault proximity indicators). Leached
limestones with high volumes of microporosity may form peripheral to HTD and
contribute to hydrocarbon storage and production.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005