--> Abstract: Hydrothermal Volcanism-Related Dolomitization (Jurassic, Lebanon): Field Geometry, Petrography and Geochemistry, by Fadi H. Nader, Rudy Swennen, Rob Ellam, and Sarah Gleeson; #90078 (2008)

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Hydrothermal Volcanism-Related Dolomitization (Jurassic, Lebanon): Field Geometry, Petrography and Geochemistry

Fadi H. Nader1, Rudy Swennen2, Rob Ellam3, and Sarah Gleeson4
1Sedimentology & Stratigraphy, IFP, Rueil-Malmaison, France
2Geologie, Katholieke Universiteit Leuven, Heverlee-Leuven, Belgium
3SUERC, East Kilbride, United Kingdom
4Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

This contribution discusses distinct dolomite textures in the same HTD reservoir and the role of dolomite diagenesis in enhancing or destroying reservoir quality. The Kesrouane Formation, part of a broad Jurassic carbonate platform deposited in the Middle-East where equivalent rock units are known to include economic hydrocarbon reservoirs, includes two distinct dolomite types: (1)fine-to-medium crystalline seepage-reflux dolomites, and (2)coarse crystalline hydrothermal dolomites. Exposures of the latter dolostones form belts along regional faults, where Late Jurassic volcanics also occur. Dolomitization occurred between the Kimmeridgian and Barremian times. The resulting dolomites are almost stoichiometric, well-ordered and show a clean, sucrosic texture with abundant intercrystalline porosity (~10%). TH values of primary inclusions in dolomite cement are from 50 to 80°C (δ18O: -9.3 to -4.0‰ V-PDB; δ13C: -1.5 to +2.1‰ V-PDB; 87Sr/86Sr: 0.70742), supporting a hydrothermal origin taking into account the timing of their occurrence with respect to the burial curve. The dolomitizing fluids appear to be mesosaline (3.5 to 12.0 eq. wt% NaCl) consisting of a mixture of marine-related waters and fluids of higher salinities. Crush-leach analyses may indicate Potassium enrichment of these fluids due to interaction with Late Jurassic volcanics.

The dolomitizing fluids appear to have invaded the nearby limestone facies according to their original porosity and permeability, forming HTD fronts. Enhanced porosity due to the initial hydrothermal dolomitization of micritic limestones is observed at the fronts' margins. The center of the dolostone bodies appear to be affected by over-dolomitization, which have resulted in increasingly stoichiometric interlocking dolomite crystals (50-52 mole% CaCO3), light δ18O values, and lower matrix porosity and permeability (<6%, <0.5mD).

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas