Circulación de fluidos en fracturas en el miembro superior de la Fm. Armàncies (Eoceno, Pirineo oriental)

Abstract

The presence of oil shows associated with fractures provides a significant opportunity to unravel the type, origin and evolution of fluids involved in fractures cementation and their relationship to oil migration. Two stages of calcite cement were distinguished in the fractures of the Eocene Armàncies platform carbonates: i) C1 calcite cement is characterised by fence-like crystals with abundant twin planes and inclusions, dull red luminescence, relative high Fe and Mg content and d18O values ranging from -6.2‰ to -4.8‰ VPDB. C1 precipitated from meteoric fluids during burial in reducing conditions. ii) C2 calcite cement is characterised by blocky clean crystals showing dark brown to red luminescence alternating with bright yellow bands, relatively low Fe and Mg content, and d18O values from -9.6‰ to -8.9‰ VPDB. C2 contains aqueous and hydrocarbon fluid inclusions with mode homogenisation temperatures of 120ºC. This cement precipitated from a fluid at higher temperature and relatively less reducing conditions than C1. Porosity associated with C2 is filled with liquid oil that reached 115ºC. Therefore, free oil and fluid inclusions oil come from the same migration pulse. Oil migration timing was synchronous with C2 and continued when calcite cementation completed