ORIGIN AND MIGRATION OF FLUIDS IN THE SANT CORNELI ANTICLINE

Fluid inclusion analyses of fracture-filling cements can provide important information about temperature and pressure conditions at the time of cement formation. Stable isotope analyses of the same fracture-filling cements can also constrain the origin of the parent fluids the cements precipitated from. This information yields direct evidence for fluid migration and evolution at the time of formation, and is crucial to hydrocarbon modelling and exploration. Fluid distribution and migration in the subsurface is largely impacted by the evolution of folds and their associated fractures. Fracture networks created during folding affect the connectivity of fluid reservoirs, thus controlling the fluid distribution within a fold.

Five systematic joint sets (J1 to J5) were observed in the Sant Corneli anticline in the south-central Spanish Pyrenees. These joints are filled with calcite cements that range from 1mm to several cm in thickness. The syn-tectonic strata and cross-cutting relationships of these sets are ambiguous in the field, leaving the timing of these fracture sets weakly constrained at best. Each joint set was sampled at multiple locations and from multiple stratigraphic levels. Detailed petrography of the joints was conducted to determine the type and degree of mineralization and composition of the cements. The J1 and J2 sets exhibited drusy, elongate blocky and small blocky calcite. The J3, J4 and J5 veins were characterized by drusy, small blocky and large blocky calcite. The J4 veins contained goethite and hematite cements. Late quartz cement was observed in J1, J3 and J4 sets, which were sampled from the same stratigraphic level. Fluid inclusion petrography of each joint set indicated multiple generations of fluids had passed through the fractures; primary, secondary and pseudosecondary inclusions were observed. Fluid inclusion microthermometry of fluid inclusions trapped within the calcite cements yielded very little data due to the small size of the inclusions (generally <20µm). Carbon and oxygen stable isotope analyses were also conducted for each generation of cement for each joint set. Carbon isotope data can indicate the source of the carbon (host rock or not) and therefore whether the subsurface reservoirs were open or closed systems. Oxygen isotope data could suggest the origin of the fluids.