Hydrochemical-isotopic tendencies to define hydraulic mobility of formation water at the Samaria-Sitio Grande oil field, Mexico

Abstract

The chemical (major elements) and isotopic ( 2H, 3H, 13C, 18O) composition of formation water is presented for the Sitio Grande oilfield, SE-Mexico, extracted from 28 production wells from the carbonate reservoir at a depth between 3585 and 4545 m.b.s.l. The linear δ 18O/δD-trend explains the formation of reservoir water as part of three subsequent stages: a) the evaporation of marine water at the surface, causing enrichment of both, δ 18O and δD-values, b) a reversal trend with decreasing δ 18O-and δD-ratios by the extreme evaporation of brines and, c) the subsequent mixing with isotopically depleted meteoric water. A SW-NE directed flow direction of deep groundwater systems is indicated by parallel-oriented isoline trends of stable isotope ratios and conservative elements, supported by the dominance of parallel directed microfractures and extensional faults, and by tracer test results. The arrival of artificial tritium, three years after tracer injection in the well SG-85, reflects a) the importance of long-term monitoring of tracer tests, and b) an estimated flow velocity of 2.2×10 −5 m/s. As the arrival of the tracer was detected exclusively in three production wells towards the SW of the injection site, lateral migration of groundwater occurs mainly along defined, channel-like conducts with a narrow dispersion angle of 39 °. Surface water, injected as part of an enhanced oil recovery program, was not detected in the production zone by chemical and isotopic methods. This fact is not a proof for lacking hydraulic conductivity, as the large reservoir size and the termination of the injection program in 1996 could have caused a complete dilution of the injected fluids. Restricted vertical flow movement between reservoir layers and insignificant temperature-dependent reaction processes is indicated by the heterogeneous distribution of δ 18O-and Cl-values throughout the groundwater column of most reservoir zones. Little influence of water-rock interaction, such as Ca-Mg exchange processes between formation water and host rock is reflected by the heterogeneous abundance of calcium in the liquid phase in both, dolomite-free and dolomitized carbonate host rock layers.