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7th Middle East Geosciences Conference and Exhibition
Manama, Bahrain
March 27-29, 2006
Hydrothermal
Dolomitization and Leaching of Carbonate Reservoirs
1 Reservoir Characterization Group, New York State Museum, Room 3140, CEC, Albany, NY 12230,
phone: 518 473 6262, fax: 518 486 2034, [email protected]
2 GDGC Ltd, 3553 - 31 St. NW, Suite 477, Calgary, AB T2L 2K7, Canada
Hydrothermal
alteration of carbonate reservoirs occurs when relatively high-pressure, high-temperature fluids flow at high
rates up active faults and into permeable formations that underlie sealing shales, evaporites or other low permeability strata.
Most reservoir-enhancing diagenesis is associated with strike-slip and especially transtensional faulting. Because of the
spatial link to faults,
hydrothermal
alteration commonly produces heterogeneity in reservoir quality and distribution.
Hydrothermal
alteration products include, but are not restricted to, saddle and matrix dolomite, recrystallized limestone
(including development of microporosity), pore- and fracture filling calcite, anhydrite, quartz, fluorite, barite, bitumen,
authigenic clay minerals, sulfides, and more. Significant leaching of limestone, dolomite, and other minerals by
hydrothermal
fluids is a common occurrence and can be a primary control on reservoir quality.
Hydrothermal
leaching likely
occurs from cooling, low pH fault-derived
hydrothermal
fluids. Cooler fluids can hold more carbonate and CO2 in solution
than relatively warmer fluids so cooling fluids should become progressively undersaturated and more acidic. Some
brecciation, leaching and microporosity development previously attributed to meteoric diagenesis may be
hydrothermal
in
origin.
Hydrothermally altered carbonate reservoirs appear to be very common in the Middle East. The Permo-Triassic Khuff
Formation has common saddle dolomite cemented breccias, zebra fabrics and sulfide deposits. Fault-related dolomitization
also occurs in some of the overlying Jurassic and Cretaceous reservoirs, but hydrothermal
alteration in these units more
commonly consists of leaching and microporosity development. A better understanding of the processes, products and
indicators of
hydrothermal
alteration will help improve the bottom line.