Conceptual hydrochemical model of late Pleistocene aquifers at the Samario–Sitio Grande petroleum reservoir, Gulf of Mexico, Mexico

Carbon-14 concentrations between 0.83 and 11.79 pmC of formation water from the Activo Samaria–Sitio Grande petroleum reservoir in SE-Mexico, extracted from 3500 to 4500 m.b.s.l., indicate a common infiltration event of surface water during the late Pleistocene period. Mixing of two components – meteoric water and seawater, previously evaporated at the surface – explain the widespread mineralization (TDI = 15–257 g/L) of Na–Cl and Na–Ca–Cl type reservoir water. Statistical discrimination by clustering and a heterogeneous chemical–isotopic fluid composition indicate the existence of 4 different water types as part of local aquifer systems, which are separated by normal and thrust faults. Tectonic horst and graben structures show an ambiguous, individual hydraulic behaviour – as permeable conduits and/or as impermeable barriers, causing the local limitation of aquifer extent. The recent increase of water production in petroleum wells is not related to the injection of surface water, but the long-term extraction of oil reserves is modifying the original position and flow direction of the reservoir aquifers. The rise of the initial groundwater level reflects the final stage of an exhausted petroleum reservoir with coning effects of underlying aquifer systems. The flexible change towards superior production intervals could represent a feasible technique to avoid the abrupt closure of invaded production wells.