--> Abstract: Petrographic and Geochemical Study of Hydrothermal Dolomitisation Fronts (Cantabrian Mountains, Spain): Implications from Reservoir Characterization and Diagenetic Modeling, by Mumtaz Shah, Fadi H. Nader, Rudy Swennen, Benoit Vincent, and Remi Eschard; #90078 (2008)

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Petrographic and Geochemical Study of Hydrothermal Dolomitisation Fronts (Cantabrian Mountains, Spain): Implications from Reservoir Characterization and Diagenetic Modeling

Mumtaz Shah1, Fadi H. Nader1, Rudy Swennen2, Benoit Vincent1, and Remi Eschard1
1Sedimentology & Stratigraphy, IFP, Rueil-Malmaison, France
2Geologie, Katholieke Universiteit Leuven, Heverlee-Leuven, Belgium

This contribution discusses typical HTD fronts (100s of meters long and 10s of meters wide) that are restricted to faults along which hot and over-pressured dolomitizing fluids episodically circulated. The rates of periodic influx of such fluids are evident from the cataclastic nature of the dolostones and the floating clasts cemented by coarse crystalline dolomite. Three types of dolomite textures are observed: sucrosic (crystal size > 400μm), interlocking saddle (crystal size 200-500μm) and zebra dolomite (alternation of replacive and cement phases). These dolomite textures were later altered by either cataclastic crystal breakage or dissolution/precipitation (recrystallization).

The sucrosic dolomites have δ18O values ranging from -16.8 to -10.8‰ VPDB and δ13C from -3.2 to +1.7 ‰ VPDB. The interlocking dolomites have similar δ18O values, but more positive δ13C (+0.4 to +1.8‰ VPDB). Replacive phases of zebra dolomites show δ18O values around -15.0‰, while the cement phases exhibit lighter values (ca. -18.0 ‰). These cement phases have fluid inclusions with TH from 130 to 190°C and Tm around -14°C, indicating very hot and saline dolomitizing fluids (~18 eq. weight% NaCl). The broad spread of δ18O values invokes multi-phase dolomitization. Porosity and permeability analyses demonstrated that dolomite alteration (e.g. brecciation, dissolution/precipitation) played a major role in the enhancement/destruction of porosity. The sucrosic dolomites that undergone recrystallization (overdolomitization) show the lowest porosity values (<5%), although these textures include coarse-sized crystals. The interlocking dolomites show higher porosity and permeability (10%, 0.5mD) when brecciated and lower values when overdolomitized. Zebra dolomites show porosity exceeding 15% and permeability above 1mD.

 

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