Interaction between groundwater and meteoric water determined by stable isotopes (δ18O, δ2H), radiogenic isotopes (⁸⁷Sr/⁸⁶Sr), and water chemistry: a case study from the Middle Magdalena Basin, Colombia

Abstract

In mature oil-producing basins, managing water resources presents challenges due to the potential of enhanced oil recovery (EOR) techniques to counteract reservoir depletion. The water sources in production systems, whether from naturally infiltrated meteoric water, connate formation water, or fluids derived from EOR, add complexities to the isotopic and geochemical signatures of groundwater in reservoir fluids. The present study reports new data on the isotopic and chemical composition of groundwater associated with Oligocene Mugrosa Formation reservoirs in the Middle Magdalena Basin (MMB), Colombia. This research integrates conventional hydrochemical parameters (pH, electrical conductivity, and major ion composition), stable isotope data (δ18O and δ2H), and radiogenic isotope ratios (87Sr/86Sr). The water samples were collected from production wells with selective completions in operational levels (depths ranging from ~800 to 1,950 m) of the Mugrosa Formation across fields P1-P6, ensuring samples representative of the targeted reservoir intervals. These water samples are unaffected by EOR operations, thereby providing data that reflect the natural, undisturbed conditions of the reservoirs. Ionic and isotopic analyses indicate interaction between meteoric and connate waters within the MMB. Meteoric waters, which are chemically less evolved and isotopically depleted in δ18O and δ2H, exhibit short relative residence times in the system and are primarily present along the eastern part of the basin, particularly in the eastern flank of the Nuevo Mundo Syncline. In contrast, connate waters are characterized by longer relative residence times, more evolved chemical compositions, and isotopic enrichment in δ18O and δ2H. These latter samples are located on the western flank of the Peña de Oro Syncline and in proximity to the Arrugas and Casabe thrust faults. This study suggests that groundwater in the MMB originates from two primary sources: 1) infiltration of meteoric water, and 2) upward migration of deep saline waters derived from Cretaceous formations. Meteoric water infiltrates along the eastern basin margin and migrates through the Mugrosa Formation, where water-rock interactions progressively alter their chemical composition, most notably increasing sodium, calcium, and potassium concentrations. Connate waters from underlying Cretaceous formations ascend along reverse fault planes, mixing with meteoric waters and modifying their chemical and isotopic signatures. This mixing results in elevated chloride concentrations and an enrichment in δ18O relative to local meteoric water values.