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Structurally-Controlled Carbonate Diagenesis: Creation of Hydrothermal Dolomite and Leached Limestone Reservoirs

Graham R. Davies, GDGC Ltd., Calgary, Alberta [email protected] and Langhorne Smith, New York State Museum

Structurally-controlled carbonate diagenesis, covering the spectrum of hydrothermal dolomite (HTD) to leached limestones, is a field of rapidly growing significance in petroleum exploration. HTD reservoirs, some with associated productive leached limestones, are major producers in the Ordovician, Devonian and Mississippian of North America. Structurally-complex rifted North and South Atlantic margins record similar diagenetic processes and reservoir creation in Jurassic to Cretaceous sections. In Europe and through into the Middle East, Jurassic and Cretaceous hosts, including the world’s largest oil and gas fields, show strong structurally-related diagenetic controls on reservoir development and enhancement.

HTD is formed under burial conditions from Mg-charged brines emplaced via structural conduits into a carbonate host, typically limestone, at temperature and pressure greater than the ambient T and P of the host. Original limestone facies and permeability play a major role in lateral extent of dolomitization, replacement textures, pore type, and pore volume. Transient, short term but high temperature (tTI) hydrothermal events may result in ‘forced maturation’ of kerogens in this setting.

Extensional and strike-slip (wrench) faults are the preferred structural locations for hydrothermal dolomitization and/or limestone leaching, with a bias toward the upper hanging wall site. The seismic signature for simple dilational or pull-apart sites is a structural ‘sag’, often with high positive correlation to HTD distribution. Underlying sandstone aquifers, basement highs, and shale top seals and internal aquitards are other variables in localization of HTD facies and solutional porosity.

Rock fabrics in an HTD system record short-term (‘instantaneous’) shear stress and pore fluid pressure transients. They include dilational ‘floating clasts’ breccias, rimmed microfractures in shear sets, boxwork vugs and zebra fabrics compartmentalized by shear microfractures, and hydrofracturing of low-permeability hosts. Younger tectonic fracturing may be a critical factor in economic production and high flow rates.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York