Overpressure And Thermal-Contraction Estimation From Fracture Minerology And Diagenetic Modeling

Renee J. Perez and James R. Boles
University of California, Santa Barbara, Geological Sciences Department Room 1006, Webb Hall, Santa Barbara, CA 93106-9630
Emails: [email protected]; [email protected]

    Intragranular microfractures and three types of microveins are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. Intragranular microfractures are associated with burial stresses and formed early during diagenesis, but they are filled with quartz cement precipitated at temperatures equal or higher than 100°C. The first type of microveins is cemented by bituminite-pyrite, they formed at temperatures between 60 and 100°C, and may be associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. 
    Fluid inclusion homogenization temperatures (175°C), higher than maximum burial temperatures (~160°C), suggest that upward flow, caused by a vertical pressure gradient, transported silica vertically and crystallized into the second vein type. Upward decompression may have also caused a Pco₂ drop, which, at constant temperature, allowed simultaneous carbonate precipitation into the second vein type. The third type of microveins is open (uncemented) or partially cemented by siderite-hematite and other iron oxides. The presence of hematite and iron oxides in microfractures are evidence for oxidizing conditions during meteoric fluid invasion that may be associated with the uplift of the Misoa formation. The coupling of published quartz cementation kinetic algorithms with unaxial strain equations suggests that thermoelastic contraction, caused by uplift, erosion, and cooling in the basin is a feasible mechanism for the last fracture generation.

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