Los paleofluídos en el grupo Oncala, Cuenca de Cameros (La Rioja, España): datos de inclusiones fluídas, isótopos de oxígeno y SEM

Abstract

Combined microthermometric and Raman study of fluid inclusions from quartz-calcite veins from the Oncala Group (Cameros Basin), shows that earlier fluids must be described by reference to the complex H20+NaCI+C02+H2S+(N2+CH J+Se system. Moreover at the time of trapping fluids were heterogeneous (effervescence) consisting at this stage of an aqueous low salinity fluid and a C02+H2S-rich gas phase. Younger fluids were low-salinity aqueous solutions. Oxygen-isotope thermometry on quartz-calcite pairs suggests that temperature of vein-forming fluids was close to 2 7 0°C, implying that regional temperatures in the Oncala Croup probably were lower than in the overlaying Urbion Croup. Also composition of flu ids was significatively different in both realms. A case of thermal inversion and hydrological incomunication betweeen the two groups is thus envisaged, uppermost black shales acting thus as a first order geological barrier between the two groups. The composition of vein-forming fluids, and the presence of pyrobitumen, in the Oncala Croup rocks suggests that conditions of deep burial diagenesis in the gas windows were attained. Thermochemical sulfate reduction involving hydrocarbons (oil), probably resulted in the complete destruction of the latter releasing pyrobitumen as a solid residue.