Hydrogeochemical characterization of oilfield waters from southeast Maracaibo Basin (Venezuela): Diagenetic effects on chemical and isotopic composition

Free water and water in emulsion with oil from hydrocarbon wells onshore-offshore Lake Maracaibo were extracted and analyzed. The use of classical and new chemical diagrams evidenced that Na-HCO3 and Na-Cl are the main chemical facies, whereas Na-SO4 facies was rare and revealed only in faulted areas in which shallow groundwater was involved in deep circulation. Cretaceous Ca-Cl waters were not revealed, but their imprint on some samples was traced and evidenced by Langelier-Ludwig, Brine Differentiation and Ca-excess vs. Na-deficit diagrams. These square plots, allowing a discrimination between the water contributions from the different source rocks, should be more exploited as hydro-geochemical tools in hydrocarbon exploration. Original data on monocarboxylic anions showed acetate concentration higher than ten times that of formate at sampling conditions, but speciation under deep conditions indicated a reversed activity dominance. Calculation of saturation indexes and kinetic modeling showed an active precipitation of chalcedony, kaolinite and clays, especially where the temperature was lower (T < 100 degrees C). Formation waters and lake water showed quite similar chloride concentration and hydrogen isotope composition. However, thermodynamic calculations at the oilfield P-T conditions revealed chlorite and illite, produced during previous diagenetic stages of Misoa sandstones, recrystallization to smectite and vermiculite in present-day stage. The diagenetic water produced in these reactions could have enriched the delta O-18 isotopic composition of approximately +2 parts per thousand. This isotope diagenetic effect was more evident in Na-HCO3 waters, which also showed a decrease in chloride at constant boron concentration as a consequence of clay dewatering. In this regard, a new boron geo-thermometer useful for formation waters is also proposed: T degrees C = [5.1364xln(1/B)(mg/1)]+5.728(+/- 0.180)/-0.164(+/- 0.012) (C) 2016 Elsevier Ltd. All rights reserved.