Hydrothermal Dolomite and Leached Limestone Reservoirs: Tectonic, Structural, Mining and Petroleum Geology Linkage

Graham R. Davies¹ and Langhorne Smith^2
1 Graham Davies Geological Consultansts Ltd, Calgary, AB
² New York State Museum, Albany, NY

Dolomite and leached limestone reservoirs formed by episodic upflow and throughflow of hot, high pressure (hydrothermal/thermobaric) brines into host limestone via fault/fracture systems are major oil and gas reservoirs in North America, and are being recognized increasingly on a global scale. They include many Devonian and Mississippian fields in western Canada, the Ordovician of eastern Canada and NE USA, probably the Ordovician of southern USA, along Mesozoic shelf edges of the Atlantic margin, apparently many reservoirs in Saudi Arabia, UAE, Oman and environs, probably in Kazakhstan, and elsewhere. ‘Hydrothermal' means emplacement at temperature higher than ambient temperature of host (>10oC difference?). Thermobaric (used synonymously with hydrothermal) stresses role of pressure (pore pressure and normal shear stress) as well as temperature, as revealed by rock fabric evidence for concomitant shear microfracturing, hydrofracturing (zebra), dilatational brecciation. Hydrothermal dolomite (HTD) reservoirs are one end of a sedimentary hydrothermal spectrum, connecting outboard shale-hosted SEDEX Pb-Zn and inboard HTD-hosted MVT deposits. Extensional tectonic settings, including backarcs, with associated volcanism, and perhaps grading into early oblique contraction, appear to be a common link at time of emplacement. Optimum upflow sites are on transtensional locations on wrench faults. Extensive evidence supports early and therefore shallow burial (+ 500m?) emplacement for HTD. Combining fluid inclusion temperature (Th) data with burial-thermal history plots is critical step in defining hydrothermal condition. New evidence suggests episodic high-temperature pulses may ‘cook' kerogen in host limestone to form bitumen, methane (‘forced maturation'). Leaching of limestone peripheral to HTD may create microporous limestone reservoirs.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005