Petroleum wells of the Activo de Produccion Luna oil field at the Mexican Gulf Coast are partially invaded by formation water at a production depth between 5000 and 6000 m. Measured 14C activities between less than 0.9 and 13.7% modern carbon reflect a homogenous, late Pleistocene-early Holocene age (40-10 ka) for the regional infiltration of meteoric and marine water into the reservoir. Before infiltration, both components were partially affected by atmospheric evaporation, which explains the hypersaline composition of some formation waters. Very positive δ18O values (up to +12.5‰) of the formation waters are caused by strong secondary water-rock interaction processes and reflect close to equilibrium conditions between the carbonate host rock and the fluids. The formation of biogenic and/or thermocatalytic methane in some parts of the petroleum reservoir is indicated by δ13C values up to +20.4‰. Southwest-northeast-directed hydraulic migration of the deep aquifer between camps Sen and Escuintle-Pijije-Caparroso is indicated by interference tests and pressure drawdown characteristics, whereas northwest-southeast-trending thrust faults restrict communication toward the Luna and Tizón camps in the most northeastern part of the oil field. On a local scale, vertical zonation trends of the fluids with decreasing salinity toward upper parts of the common aquifer are related to separation processes by gravity and/or by the rising of condensed vapor. The migration of the fluids is mainly related to southwest-northeast-trending fractures and microfractures, whereas northwest-southeast- and northeast-southwest-trending reverse and normal faults, respectively, behave irregularly as barrier or as flow conduits. Recently, the extraction of petroleum caused an increased mobilization of the hydrodynamic aquifer system.

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