Fluid migration history from analysis of filling fractures in a carbonate formation (lower cretaceous, middle Magdalena valley basin, Colombia)

Abstract

The integration of Conventional Petrography, SEM, Rare Earth Element geochemistry (REE) and Fluid Inclusions analysis (FI), in the fracture fillings at the Rosablanca Formation (Middle Magdalena Valley basin), make it possible to relate opening and filling events in the veins with hydrocarbon migration processes.Petrographic and SEM data indicate that the veins are fracture filling structures, with three types of textures:1) Granular aggregates of calcite (GA); 2) Elongated granular aggregates of calcite (EGA); and 3) Fibrous aggregates of calcite and dolomite (FA). The textural relationship suggests that GA must have been formed in an environment of widespread extension of the basin, while EGA and FA must have been formed in a compressive environment.
 The geochemical analyses of REE carried out in the dominant fill of the veins (GA) indicate that these fillings must have been formed in a closed system (intraformational fluid movement) for the drilling well Alfa-1, while in the drilling wells Alfa-2 and Alfa 3, these fills (GA) must have been formed in a characteristic environment of open system (transformational fluid movement). Two pulses of hydrocarbon migration were identified through the study of fluid inclusions: In the first event, light hydrocarbons and aqueous fluids (H2O-NaCl-CaCl2) migrated trough the primary porosity and fractures at temperatures between 60ºC- 90ºC. In the second event, light hydrocarbons associated with aqueous fluids (H2O-NaCl-CaCl2) migrated through fractures at temperatures between 70ºC - 120ºC. Data obtained in this investigation will strengthen the knowledge about the hydrocarbon migration history and entrapment in the Middle Magdalena Valley basin (VMM) particularly in the lower Cretaceous age reservoirs.