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1 Graham Davies Geological Consultants (GDGC) Ltd., Alastair Ross Technology Center, 3553-31 Street NW, Calgary, Alberta, Canada, T2L 2K7; gdgc{at}telus.net
2 Reservoir Characterization Group, New York State Museum, Room 3140 CEC, Albany, New York 12230; lsmith{at}mail.nysed.gov
Graham Davies received his B.Sc. (honors) degree and his Ph.D. from the University of Western Australia. His doctoral thesis was on modern carbonates in Shark Bay, Western Australia (published in AAPG Memoir 13). After a postdoctoral fellowship with James Lee Wilson at Rice University, he joined the Geological Survey of Canada (GSC) in Calgary. After 7 years with the GSC, Graham cofounded and became principal owner of AGAT (Applied Geoscience and Technology) Consultants/Laboratories. Since 1983, he has operated through Graham Davies Geological Consultants Ltd. He has published about 70 articles on the geology of Australia and Canada and has authored or coauthored more than 600 consulting reports. Graham received the Douglas Medal of the Canadian Society of Petroleum Geologists in 2002 for his work on Arctic Paleozoic carbonates and evaporites and on the Triassic and other aspects of western Canadian geology. His principal current interest is in hydrothermal dolomites.
Langhorne Smith currently heads the Reservoir Characterization Group at the New York State Museum. He holds a B.S. degree from Temple University and a Ph.D. from Virginia Polytechnic Institute and State Technology. He worked for Chevron as a development geologist for two years and then did two years of postdoctoral work at the University of Miami. His current research interests are in carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs.
Structurally controlled hydrothermal dolomite (HTD) reservoir facies and associated productive leached limestones are major hydrocarbon producers in North America and are receiving increased exploration attention globally. They include multiple trends in the Ordovician (locally, Silurian and Devonian) of the Michigan, Appalachian, and other basins of eastern Canada and the United States, and in the Devonian and Mississippian of the Western Canada sedimentary basin. They also occur in Jurassic hosts along rifted Atlantic margins, in the Jurassic–Cretaceous of the Arabian Gulf region and elsewhere.
Hydrothermal dolomitization is defined as dolomitization occurring under burial conditions, commonly at shallow depths, by fluids (typically very saline) with temperature and pressure (T and P) higher than the ambient T and P of the host formation. The latter commonly is limestone. Proof of a hydrothermal origin for HTD reservoir facies requires integration of burial-thermal history plots, fluid-inclusion temperature data, and constraints on timing of emplacement. Hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary-exhalative lead-zinc ore bodies and HTD-hosted Mississippi Valley–type sulfide deposits. All three hydrothermal deposits show a strong structural control by extensional and/or strike-slip (wrench) faults, with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall. Transtensional sags above negative flower structures on wrench faults are favored drilling sites for HTD reservoir facies.
Saddle dolomite in both replacive and void-filling modes is characteristic of HTD facies. For many reservoirs, matrix-replacive dolomite and saddle dolomite appear to have formed near-contemporaneously and from the same fluid and temperature conditions. The original host facies exerts a major influence on the lateral extent of dolomitization, resultant textures, pore type, and pore volume. Breccias, zebra fabrics, shear microfractures, and other rock characteristics record short-term shear stress and pore-fluid-pressure transients, particularly proximal to active faults. High-temperature hydrothermal pulses may alter kerogen in host limestones, a process designated "forced maturation." Basement highs, underlying sandstone (and/or carbonate?) aquifers (probably overpressured), and overlying and internal shale seals and aquitards also may constrain or influence HTD emplacement.
Although many questions and uncertainties remain, particularly in terms of Mg and brine source and mass balance, recognition and active exploration of the HTD play continues to expand. Increasing use of three-dimensional seismic imagery and seismic anomaly mapping, combined with horizontal drilling oblique to linear trends defined by structural sags, helps to reduce risk.
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