1. Introduction

During the hydrogeological cycle, surface water and groundwater interact and eventually mix depending on several factors such as topography, geology, temperature, altitude, and other environmental conditions. This interaction can lead to either abrupt or gradual changes in the chemical and isotopic composition of the water (e.g., Clark and Fritz, 1997).

Stable isotopes are widely used in hydrogeology and hydrogeochemistry as tools to assess potential changes in chemical composition, determine the origin of water, flow direction patterns, mixing processes, and identify recharge and discharge zones in both surface and groundwater aquifers (e.g., Dansgaard, 1964; Hem, 1989; Fetter, 1994; Hitchon et al., 1999; Fitts, 2002; Dickin, 2018). Radiogenic isotopes, on the other hand, provide valuable insights into relative or absolute residence times and aquifer renewal rates (e.g., Burke et al., 1982; DePaolo and Ingram, 1985; Veizer, 1989; Farrell et al., 1995).

This study focuses on the central region of the Middle Magdalena Basin (MMB) in Colombia, a mature hydrocarbon province with a cumulative oil production of 2.75 billion barrels as of  December 2021 (e.g., Pastor-Chacón et al., 2023) and an estimated annual cumulative production of approximately 11.7 million barrels of oil as of December 2024 (Agencia Nacional de Hidrocarburos, 2024). While many oil fields in the MMB historically exhibit low water cuts (typically <10%), current production relies increasingly on enhanced oil recovery (EOR) techniques, such as water injections, to offset reservoir depletion. The basin is also characterized by intense tectonic deformation, with fault systems potentially acting as critical pathways for natural fluid migration (e.g., hydrocarbons, connate waters) and anthropogenic fluid movement (e.g., injected water, flowback from hydraulic fracturing) (Castelblanco et al., 2025). However, the origin of water in production systems, whether naturally infiltrated meteoric water, connate formation water, or EOR-derived fluid, remains poorly understood. This research addresses this gap by investigating groundwater’s isotopic and geochemical signatures in the Oligocene Mugrosa Formation, aimed at disentangling their possible sources and evaluating the role of fault-controlled hydrologic connectivity in fluid redistribution.

Waterflooding has been used as a secondary recovery method in five commercial oil fields within the MMB: La Cira, Galán, Casabe, Yariguí-Cantagallo, and Infantas (Castro et al., 2010). Recently, EOR techniques have been initiated as pilot projects in several of these fields (e.g., Yariguí-Cantagallo, Casabe, Chichimene, and La Cira-Infantas) to improve volumetric sweep efficiency and increase overall recovery factors (Gutiérrez et al., 2024).

Hydrogeological understanding of the MMB has evolved significantly over the past two decades through regional investigations. In fact, several government institutions, including IDEAM (Institute of Hydrology, Meteorology, and Environmental Studies of Colombia) (Vargas et al., 2013; García et al., 2014), the Colombian Geological Service (Hincapié et al., 2004), and the Autonomous Regional Corporation of Central Antioquia (Mejía, 2008), have conducted studies to assess groundwater aquifers in the MMB. A foundational study by Hincapié et al., (2004) classified hydrogeological units based on aquifer productivity, porosity, and permeability, employing the nomenclature established by the International Association of Hydrogeologists (IAH). However, it was not until a decade later that hydrogeological provinces across Colombia were systematically delineated and classified according to tectonostratigraphic frameworks and structural boundaries (Vargas et al., 2013).

Mejía (2008) evaluated groundwater quality and hydrogeochemical facies in the MMB, reporting a predominance of sodium-bicarbonate waters with localized occurrences of calcium-bicarbonate facies. García et al., (2014), expanded on this by characterizing the aquifer systems of the Neogene Real Group and Mesa Group. These units comprise unconfined and semi-confined multilayer aquifers within recent alluvial and fluvial terrace deposits adjacent to the Magdalena River, as well as poorly consolidated detrital sediments. Infiltration rates in these units range from 0 to 500 mm/year, with saturated thicknesses between 80 and 230 m and transmissivity values from 100 to 200 m²/day. More recently, Malagón (2017) and Malagón et al. (2021) applied principal component analysis and hierarchical clustering to 140 groundwater samples, offering a refined geochemical classification. Their findings confirmed calcium-bicarbonate waters as the dominant components throughout the basin, with sodium-bicarbonate and chloride types limited to specific areas.

Flow dynamics in shallow aquifers (250-365 m) of the Real Group and Mesa Group were investigated by Piña et al. (2022) using stable isotope analyses. Their results revealed seasonal isotopic variability indicative of varying groundwater residence times, surface-groundwater interactions, and regional flow regimes driven by hydraulic gradients. This study also provided evidence of hydraulic connectivity along fractured zones of the Eastern Cordillera, highlighting significant data gaps regarding geology and structures, as well as the lack of long-term groundwater monitoring.

This paper explores the interaction between meteoric and brine waters of the Mugrosa Formation reservoirs through percolation. We hypothesize that meteoric water infiltration primarily occurs through the flanks of the main structures in the area and along fault zones, reaching depths of approximately 1,950 m. These interactions result in an increase of δ¹⁸O and a decrease in the pH of reservoir samples. Other infiltration mechanisms may involve seal leakages and the tortuosity of the overlying formations. The findings obtained in this research are hoped to benefit water management strategies in oil-producing basins.

2. Geological setting

The Middle Magdalena Basin (MMB) is located along the Magdalena River valley, between the Central and Eastern Cordilleras in the Colombian Andes. It is currently classified as an intermontane basin (Sarmiento, 2011) (Fig. 1). The northern boundary is El Banco uplift, the northeastern boundary is the Bucaramanga-Santa Marta Fault, the southeastern boundary is the La Salina and Bituima thrust faults, the southern boundary is the Ibague Fault, and the western boundary is the Mesozoic igneous and metamorphic basement of the Central Cordillera. The study area is located close to the southeastern boundary, where the La Salina thrust fault system has generated several fault propagation folds, notably the Nuevo Mundo Syncline (NMS), the Lisama-Provincia anticlines, and the Peña de Oro syncline. Additionally, folds (both synclines and anticlines) related to the Arrugas and Casabe thrust faults can be found toward the western flank of the basin (Morales et al., 1958; Dengo and Covey, 1993; Cooper et al., 1995; Gómez et al., 2003, 2005a; Caballero, 2010; Sarmiento, 2011).

Fig. 1. Location of the study area, sampling sites, and regional schematic cross-section across the sampling areas. The schematic cross-section was adapted from Morales et al. (1958) and Gómez et al. (2005a, b). MMB: Middle Magdalena Basin.

Multiple petroleum systems have been identified in the MMB. The Rosa Blanca and Tablazo formations (Lower Cretaceous), and the La Luna Formation (Upper Cretaceous), are recognized as primary source rocks, while the Lisama, La Paz, Esmeraldas, Mugrosa, and Colorado formations (Paleocene to Oligocene) serve as the main reservoir rocks identified to date (Fig. 1). The claystone-rich intervals within the Esmeraldas and Colorado formations act as the primary regional seals for the Cenozoic reservoirs (Morales et al., 1958; Barrero et al., 2007; García et al., 2009; Sarmiento, 2011) (Fig. 1).

The Mugrosa Formation comprises thick, laterally continuous sandstone packages interbedded with shale layers, reflecting high-energy fluvial to deltaic depositional environments. Its total thickness ranges from approximately 500 to 800 m. This formation is unconformably overlain by the Colorado Formation and underlain by the Esmeraldas Formation (Caballero, 2010; Sarmiento, 2011). The Mugrosa and Colorado formations represent alluvial syntectonic deposits, ranging from alluvial fans (on the west) to meandering rivers (on the east), deposited during the Paleogene and Neogene uplift of the basin (Caballero, 2010).

Structural interpretations by Gómez et al. (2003, 2005a, 2005b), Caballero (2010), Sarmiento (2011), and Pastor-Chacón et al. (2023) suggest that the La Salina and Casabe thrust faults controlled the formation of the Lisama anticline as well as the NMS as fault propagation folds. The La Salina thrust fault system also controlled the formation of fault propagation folds and the syntectonic sedimentation of the Mugrosa and Colorado formations, which were deposited with sediments derived from the Serranía de los Cobardes and the Lisama anticlines at the foothills of the Eastern Cordillera (Gómez et al., 2003, 2005a, b; Caballero, 2010; Sarmiento, 2011) (Fig. 1). This fault system has been suggested as the main migration pathway of petroleum and connate water from Cretaceous source rocks to the Paleogene reservoir rocks, including the Mugrosa and Colorado formations (Dengo and Covey, 1993; Cooper et al., 1995; Rangel et al., 1996; Ramón et al., 1997, 2001; Gómez et al., 2003, 2005a; Caballero, 2010; Sarmiento, 2011; Pastor-Chacón et al., 2023; Rivera et al., 2025) (Fig. 1).

3. Materials and methods

Formation water samples were collected from twelve producing wells completed in the Mugrosa Formation, at operational depths ranging from 800 to 1,950 m (Fig. 1). These wells are unaffected by EOR operations, ensuring that the geochemical data represented the natural, unaltered conditions of the reservoir. Sampling was conducted in two study areas: 1) the eastern sector-NMS, and 2) the western sector-Casabe field. We collected each sample in pre-cleaned, sterilized amber glass bottles equipped with cap-seals and refrigerated immediately to minimize alterations in chemical composition. Samples containing both hydrocarbon and water phases were separated using a separation funnel.

3.1. Physicochemical analysis

Both in situ and laboratory measurements of physicochemical parameters were conducted. These parameters include total dissolved solids (TDS), conductivity, pH, and temperature. In situ measurements were obtained using a WTW 2E30-101B02 multiparameter instrument, whereas laboratory analyses employed a two-channel HANNA multiparameter instrument for measuring pH, oxidation-reduction potential, and specific ion measurements (pH-ORP-ISE), and total dissolved solids (TDS)-Salinity-resistivity (Tables 1 and 2).

3.2. Chemical analysis

Ion concentration, reported in tables 1 and 2, includes the following major cations: Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Sr²⁺, Al³⁺, and Mn²⁺, as well as major anions: Cl^-, HCO₃^-, CO₃^2-, SO₄^2-, and S^2-. Cation concentrations were determined using a flame atomic absorption spectrophotometer (Agilent A240S) equipped with an air-acetylene-nitrous oxide flame. Prior to every analysis, the samples underwent standard digestion with concentrated nitric acid, following method SM3030E.

Chloride concentrations were determined by potentiometric titration using silver nitrate (AgNO₃) as the titrant. Sulfide concentrations were determined using the iodometric-potentiometric method (SM 4500 S²⁻-F). A 25 mL sample aliquot was acidified with 2 mL of 6N HCl and subsequently titrated with 0.025N sodium thiosulfate (Na₂S₂O₃). Sulfate concentrations were established using the standard turbidimetric method (SM 4500 SO₄^2--E). A 25 mL sample aliquot was mixed with a pH buffer, followed by the addition of 0.1 g of BaCl₂. The mixture was stirred and left to rest for 5 minutes before measurement. Carbonate and bicarbonate concentrations were determined by titration. A 25 mL sample aliquot was titrated with 0.02N NaOH until the final endpoint, using phenolphthalein and methyl red as indicators.

Total acids (or total acid number, TAN) were determined by titration with NaOH until neutralization. The NaOH concentration used, along with the formation of salts and the dissociation of organic acids reacting with NaOH, was analyzed to quantify total acidity.

3.3. Stable isotope analysis (δ¹⁸O and δ²H)

Stable isotopes of oxygen (δ¹⁸O) and deuterium (δ²H) were analyzed in refrigerated samples at the Beta Analytic-Testing Laboratory in Miami, USA. The analysis was conducted using cavity ring-down spectroscopy (CRDS). A 1.6 mL aliquot of each liquid sample was placed in a vial with a screw cap and septum, then injected into the Picarro L2140-I vaporizer for measurement by CRDS. To ensure high precision, the CRDS system generated six measurement peaks per sample, each lasting nine minutes. Due to potential isotopic memory effects between consecutive samples, only the average of the last three injections was used for final calculations. We used a minimum of three sets of reference standards (e.g., EVIAN, LOUSE, and USGS 50) covering the δ¹⁸O and δ ²H spectrum to bracket the samples. These standards established a linear calibration curve to normalize the δ¹⁸O and δ²H values on the Vienna Standard Mean Ocean Water (VSMOW) scale. The standard deviation of all analyses was ±0.03. The results are reported in table 3.

3.4. Radiogenic isotope analysis (⁸⁷Sr/⁸⁶Sr)

Radiogenic isotope analysis (⁸⁷Sr/⁸⁶Sr) was conducted at the Center for Geochronology and Isotope Geochemistry at the Complutense University of Madrid, Spain. The analysis required a specific pretreatment process depending on the Sr ion concentration determined by conventional analysis. For each sample, 10 to 20 mL was evaporated in a clean Teflon crucible on a heating plate at temperatures below 80 °C. The residue was then re-dissolved in 2 mL of HNO₃ Suprapur ®, followed by a second evaporation at 120 °C. After drying, 3 mL of 3M HNO₃ was added to facilitate chromatographic separation.

Chromatographic separation of Sr was performed using Sr-Resin™ (Triskem International), which contains crown-ether (4,4’(5’)-di-t-butyl cyclohexane-18-crown-6). Sr was then eluted using 0.05M HNO₃, and the Sr-concentrated fraction was identified and evaporated to dryness. The final analysis was conducted using thermal ionization mass spectrometry (TIMS).

The ⁸⁷Sr/⁸⁶Sr ratios were normalized to a measured value of 0.1194 to correct for potential mass fractionation that may have occurred during filament loading and instrumental analysis. Additionally, the Sr isotope standard NBS 987, which has a certified ⁸⁷Sr/⁸⁶Sr ratio of 0.710248±0.000003 (per the NBS 1982 Certificate of Analysis, Standard Reference Material 987), was analyzed repeatedly. From nine replicates, a standard deviation (STDEV) of 0.000021 was obtained. The results are reported in table 3.

3.5. Meteoric water line (global and local)

This study used the equation proposed by Harmon Craig in 1961 for the Global Meteoric Water Line (GMWL) (Craig, 1961). The Local Meteoric Water Line (LMWL) was constructed using data collected near the study area, obtained from the International Atomic Energy Agency (IAEA) through its WISER program (Water Isotope System for data analysis, visualization, and Electronic Retrieval) from the 2020-2021 dataset (https://nucleus.iaea.org/wiser).

For the construction of the Local Meteoric Water Line (LMWL), δ¹⁸O and δ²H data from the past 30 years were analyzed, specifically selecting values from the MMB. This study therefore presents, for the first time, a meteoric water line for the central region of the MMB (e.g., the La Fortuna and Lisama areas), providing a valuable framework for interpreting the isotopic fractionation processes affecting meteoric water and groundwater in the basin.

4. Results

4.1. Physicochemical analysis of groundwater

The samples analyzed exhibit different physicochemical parameters depending on their sampling location within the basin (Fig. 2; Tables 1 and 2):

Fig. 2. Physicochemical characteristics of groundwater in the MMB area. A. Depth versus pH. B. Depth versus electrical conductivity. The samples have been grouped based on whether they belong to the western or eastern sectors of the study area.

Eastern area-NMS: The groundwater samples here show a higher pH, ranging from 7.65 to 8.10. Electrical conductivity values are lower than in the western area, with 9.01 mS/cm for sample P3-1 and 34.50 mS/cm for sample P4-1. Similarly, TDS values are also lower, ranging between 4,640 ppm (P3-1) and 9,495 ppm (P4-1). Temperature differences are minimal, ranging between 25.8 °C and 26.0 °C.

Western area-Casabe field: Groundwater in this region has a pH ranging from 6.00 to 6.89, an electrical conductivity between 55.10 and 78.40 mS/cm, TDS values between 20,740 ppm and 39,290 ppm, and temperatures between 25.7 °C and 26.5 °C.

4.2. Chemical analysis

Piper diagrams serve as a graphical tool for classifying groundwater based on its chemical composition by organizing ion concentrations (cations and anions) (Piper, 1944). This analysis allows for the identification of two key aspects: 1) The origin of dissolved ions in the water; and 2) The evolution of the water’s chemical composition due to water-rock interactions in the Mugrosa Formation.

According to the Piper classification diagram, the groundwater in the study area is predominantly of the sodium-chloride type (Fig. 3). Two distinct subgroups are identified, corresponding to samples from the Eastern area-NMS and the Western area-Casabe field. Within each subgroup, samples can be further distinguished according to their ionic concentrations, indicating consistent geochemical signatures for each area. The Eastern area-NMS samples are relatively enriched in HCO₃ and CO₃ relative to Ca and Mg, whereas the Western area-Casabe samples show the opposite trend, with a marked increase in Cl concentrations toward the west (Fig. 3).

Fig. 3. Piper diagram showing the spatial distribution of groundwater types in the Mugrosa Formation. Eastern area-NMS samples (P3-1 and P4-1) have relatively higher concentrations of bicarbonate (HCO3) and carbonate (CO3) compared to calcium (Ca) and magnesium (Mg), whereas Western area-Casabe field samples show higher chloride (Cl) concentrations. The diagram highlights the contrasting geochemical signatures and spatial zonation of groundwater across the study area.

Gibbs diagrams illustrate the relationship between TDS concentration and the proportions of cations (Na/(Na+Ca)) and anions (Cl/(Cl+HCO₃)) in groundwater (Gibbs, 1970). These diagrams help identify the dominant processes influencing the chemical composition of groundwater, which include: 1) Meteoric water (influence of rainwater infiltration); 2) Water-rock interaction (e.g., dissolution, ion exchange, and mineral alteration); and 3) Mineral precipitation (increased salinity due to temperature-driven evaporation) (Fig. 4).

Fig. 4. Gibbs diagrams illustrating geochemical processes controlling groundwater composition in the Mugrosa Formation. A. TDS versus (Na/(Na+Ca)); this diagram helps identify whether groundwater chemistry is dominated by atmospheric precipitation, water-rock interaction, or mineral precipitation. B. TDS versus Cl/(Cl+HCO3); this diagram highlights the influence of ion exchange mechanisms or mineral precipitation. Both diagrams suggest that meteoric water infiltration plays a significant role in the water-rock interaction, as observed in the groundwater chemistry of the analyzed samples, particularly in the eastern part of the basin, where lower TDS values and distinct ionic compositions indicate a stronger influence of meteoric water recharge.

The physicochemical characteristics of groundwater enable us to predict its evolution and flow within the system (Chebotarev, 1955; Mifflin, 1988; Bustamante, 2017); however, the groundwater flow trajectories are primarily dominated by the hydrostatic gradient and geological factors (e.g., faulting, structures) of the studied area. Several studies (Mifflin, 1988; Clark and Fritz, 1997; Cook and Herczeg, 2000; Bustamante, 2017; Malagón et al., 2021; Piña et al., 2022) have shown that the calcium-bicarbonate-type waters transition to sulfate and chloride waters as depth increases in the infiltration process in the subsurface (Fig. 5). Based on the ionic content present in these waters, previous authors classified the basinal flow patterns as local, intermediate, and regional.

Fig. 5. Mifflin diagram showing the classification of groundwater flow systems (local, intermediate, and regional) based on hydrochemical composition. Groundwater samples from the Mugrosa Formation are plotted to evaluate their possible association with regional flow.

Figure 5 shows that the water samples have compositions possibly related to deep regional flow due to the interaction between groundwater and the reservoir rock (i.e., Mugrosa Formation), as indicated by the analytical results obtained in this investigation.

4.3. Isotope analysis (δ¹⁸O, δ²H,  ⁸⁷Sr/⁸⁶Sr)

Isotopes are widely used in hydrogeochemical studies, offering insights into physicochemical processes such as evaporation, condensation, and water-rock interactions. Additionally, stable isotopes can contribute to the definition of groundwater flow patterns when combined with conventional ion analyses, while radiogenic isotopes can provide clues about the relative age of basinal waters (Dansgaard, 1964; Hem, 1989; Fetter, 1994; Hitchon et al., 1999; Fitts, 2002; Davis and Elderfield, 2004; Dickin, 2018).

The physicochemical processes affecting groundwater can be analyzed using the δ (²H/¹H) and δ (¹⁸O/¹⁶O) ratios, which describe variations in the relative abundance of heavier versus lighter stable isotopes. These values are expressed in per mil (‰) relative to Standard Mean Ocean Water (SMOW) (Dansgaard, 1964; Hem, 1989; Fetter, 1994; Hitchon et al., 1999; Fitts, 2002; Dickin, 2018). These isotope ratios help determine whether a sample exhibits isotopic depletion or enrichment in δ¹⁸O and δ²H.

The results, shown in figure 6 and table 3, indicate for the samples from the east area, δ¹⁸O values between -5.14 ‰ and -1.54 ‰ and δ²H values ranging from -26.28 ‰ to -17.56 ‰. In contrast, the samples from the west area exhibit δ¹⁸O values ranging from -1.87 ‰ to 0.65 ‰ and δ²H values between -25.41 ‰ and -18.63 ‰.

Fig. 6. Stable isotope behavior and isotopic fractionation processes in groundwater from the MMB area. A. Variation in δ2H versus δ18O values in water samples, showing greater δ18O enrichment in the western samples compared to those from the eastern area. Tendency lines from Clark and Fritz (1997). B. Relationship between chloride (Cl-) and δ18O. This relationship proved useful for evaluating isotopic fractionation, water-rock interaction processes, and relative groundwater residence time, allowing the distinction between meteoric waters and mixtures of meteoric and connate waters. GMWL and LMWL lines constructed as shown in section 3.5.

The ⁸⁷Sr/⁸⁶Sr ratio can be compared with the ⁸⁷Sr/⁸⁶Sr composition of seawater over geological time (Burke et al., 1982; DePaolo and Ingram, 1985; Veizer, 1989; Farrell et al., 1995). In the present study, the ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7081 and 0.7121 in the east area (well depths between 793 and 1,463 m), and between 0.7080 and 0.7097 in the west area (well depths between 1,798 and 1,951 m) (Table 3). These values, together with the stable isotope data illustrated in figure 6, indicate that the western samples are relatively enriched in δ¹⁸O compared to those from the east, suggesting longer residence times and enhanced water-rock interaction in the deeper reservoir intervals. The positive relationship observed between δ¹⁸O and chloride (Cl^-), a chemically conservative tracer, further supports mixing between meteoric waters and connate fluids, consistent with regional flow pathways in the Mugrosa Formation.

5. Discussion

5.1. Hydrogeochemical characteristics of groundwater

5.1.1. Ionic and isotopic correlations

The increasing trend of electrical conductivity with depth (Fig. 2) aligns with the expected geochemical evolution of groundwater in sedimentary basins, where prolonged water-rock interactions at depth enrich the water in dissolved ions, producing brackish compositions. However, the anomalously relatively low pH values in deeper zones challenge conventional models that predict higher pH due to carbonate buffering (Drever, 1982). This discrepancy can be resolved by incorporating the organic acids identified in our analyses (Tables 1 and 2), which are linked to redox reactions during water-hydrocarbon-rock interactions (Surdam et al., 1993; Lundegard and Kharaka, 1994; Palmera, 2023). These acids lower the pH and drive the dissolution of carbonate and feldspar minerals, further modifying ionic balances and enhancing conductivity in a feedback mechanism.

The spatial distribution of water types in the Mugrosa Formation (sodium-chloride type groundwater, with higher Cl^- concentrations in the west and relatively higher HCO₃^- and CO₃^2- concentrations in the east) reflects contrasting fluid origins and migration pathways (Fig. 3). In fact, elevated Cl⁻ concentrations in the western part of the study area correlate with prolonged residence times and mixing with Cretaceous connate waters that ascend via faults, such as La Salina and Casabe (Fig. 1). In contrast, the eastern area’s higher pH (7.65-8.10) and carbonate (CO₃^2-) and bicarbonate (HCO₃^-) content indicate dominant meteoric recharge near the NMS (Fig. 3). The cation evolution pattern, observed by analyzing the Na/(Na+Ca) and Cl/(Cl+HCO₃) ratios, further supports this model: meteoric waters in the east evolve from sodium-rich compositions to sulfate-dominated and eventually chloride-rich fluids in the west, consistent with progressive water-rock interaction, mineral precipitation, and connate mixing (Chebotarev, 1955; Mifflin, 1988; Clark and Fritz, 1997; Cook and Herczeg, 2000; Schiavo et al., 2009) (Fig. 4).

Cationic ratios provide further evidence of hydrochemical zonation. For example, Na and K dominate over Ca and Mg, with Ca+Mg ratios increasing westward (Fig. 7A), while Na appears to prevail over K during ion-exchange processes associated with water-rock interactions (Fig. 7B). This trend suggests shorter residence times for the eastern samples, as indicated by their lower Cl⁻ concentrations and lower Cl-(Na+K)/Cl ratios, compared to the western samples (Fig. 7C). The latter may reflect a longer interaction with Cretaceous-derived brines and enhanced mineral dissolution (e.g., carbonates and feldspars) (Hem, 1989; Drever, 1997; Tostado, 2010). Such patterns may reflect the Mugrosa Formation’s fluvial architecture (Caballero, 2010; Sarmiento, 2011), that is, coarse-grained channel sands in the east facilitate rapid meteoric infiltration, while finer-grained floodplain deposits in the west prolong fluid-rock contact, enhancing ion exchange and organic acid generation.

Fig. 7. Ionic relationships indicating dominance patterns and ion exchange processes in groundwater from the Mugrosa Formation: A. (Na+K) versus (Ca+Mg) plot, highlighting the dominance of Na and K ions over Ca and Mg. B. K versus Na concentrations, illustrating the role of water–rock interactions, with Na typically present in higher concentrations than K. C. Exchange bases index plot suggesting an enrichment in Na and K resulting from mineral dissolution and cation exchange with clay minerals.

5.1.2. Water-rock interaction and relative residence time in the hydrogeological system

The isotopic and geochemical data reveal that the hydrogeological system in the study area is affected by two processes: meteoric water percolation and connate water migration. These processes occur in two different structures (compartments) that are isolated by the Umir Formation, which is the regional seal, and connected through the thrust fault systems (e.g., La Salina, Arrugas, Infantas, Casabe) (Figs. 6 and 8). The interaction between meteoric and connate waters resulting in depleted δ¹⁸O values (-5.14 ‰ in P3-1) and high CO₃^2-/HCO₃^- ratios in the eastern area confirms significant meteoric recharge along the eastern flank of the NMS, where short residence times maintain a near-neutral pH (7.65-8.10). Furthermore, the ionic and isotopic composition of groundwater in the eastern area aligns closely with the local and global meteoric water lines established in this investigation, indicating a predominance of meteoric recharge (Fig. 6A).

Fig. 8. Ionic and isotopic correlations used to infer groundwater mixing processes and relative residence times. A. 87Sr/86Sr versus δ18O plot. Three distinct water groups are identified based on δ18O values and their spatial association. Group 1 corresponds to meteoric waters from the eastern area, characterized by depleted δ18O values (sample P3-1), indicative of recent recharge. Group 2 comprises mixed meteoric and connate waters, with intermediate δ18O values (samples P1-4 and P4-1). Group 3 consists of connate waters from the western area, exhibiting enriched δ18O values, suggesting prolonged residence times and significant interaction with the host rock. B. Mg versus Ca plot for groundwater residence time differentiation. Two distinct groups are recognized: one characterized by elevated Ca concentrations, associated with samples from the western area (wells P1, P2, P5, and P6), and another with lower Ca concentrations, corresponding to the eastern area (wells P3 and P4). Magnesium concentrations remain relatively stable along the y-axis. C. 87Sr/86Sr versus Sr plot. The observed trend mimics that of panel 8A, delineating three distinct groups. However, in this case, Sr concentrations provide further resolution: eastern samples exhibit lower Sr contents compared to western samples.

The anomalous ⁸⁷Sr/⁸⁶Sr ratios (e.g., 0.71213 in sample P4-1; Fig. 8A,C), may indicate two key processes (Mearns and McBride, 1999): 1) mixing with Cretaceous connate fluids, where overpressured brines possibly migrate through reverse faults (i.e., La Salina, Arrugas, Casabe) from the La Luna Formation, introducing radiogenic ⁸⁷Sr (from K-feldspars/micas) and Cl⁻ (Fig. 6B); and 2) extended water-rock interaction, where organic acid dissolve felspars and carbonates increasing the Na⁺/Cl⁻ and lowering the pH (6.00-6.89), which is consistent with prolonged fluid retention in low-permeability sediments. However, sample P1-4 from the western area stands out as an exception, exhibiting isotopic characteristics comparable to those of sample P4-1 from the eastern sector, which suggests a pronounced meteoric water influence (Figs. 6 and 8).

The east-west hydrochemical zonation in the MMB is supported by ionic and isotopic trends, including higher exchange-base index values with Cl⁻, increasing (Na+K) relative to (Ca+Mg), and a westward increase in Cl⁻ accompanied by δ¹⁸O enrichment (Figs. 6B, 7A, and 7C). These patterns indicate deeper saline inputs and enhanced silicate alteration, consistent with fluvial depositional controls. In the eastern sector, coarse channel sands facilitate rapid meteoric infiltration, whereas in the west, fine-grained overbank deposits extend residence times and promote ion-exchange processes (Fig. 8A, C). This study demonstrates that the covariations between ⁸⁷Sr/⁸⁶Sr and δ¹⁸O, between ⁸⁷Sr/⁸⁶Sr and Sr, and between Cl and δ¹⁸O serve as effective geochemical tracers for constraining groundwater origins and residence times.

In the MMB, fault-induced hydrocarbon migration from the La Luna Formation to the Mugrosa reservoirs (Ramón et al., 1997, 2001; Rangel et al., 2000; Thompson-Butler et al., 2019; Pacheco-Mendoza et al., 2024) underpins δ¹⁸O-Cl^- covariation, as oil-water-gas multiphase migration (Tissot and Welte, 1984) transports connate fluids, which mix with meteoric waters to reshape regional chemistry. Sample P4-1 shows an anomalous ⁸⁷Sr/⁸⁶Sr ratio likely due to an enrichment in radiogenic ⁸⁷Sr. Mearns and McBride (1999) reported that ⁸⁷Sr/⁸⁶Sr ratios depend on the composition of dissolved minerals; higher ratios are associated with K-feldspars and micas, while lower values correspond to marine carbonate rocks. The anomalous high ratio in sample P4-1 suggests a mixture of meteoric water with oil-related water, which typically has elevated ⁸⁷Sr/⁸⁶Sr ratios (Veizer, 1989; Mearns and McBride, 1999).

5.2. Groundwater flow paths/regional flow and hydrogeological model

Figure 9 illustrates the proposed flow paths of meteoric water within the Mugrosa Formation in the central MMB. The primary recharge areas are located on the eastern flank of the NMS at elevations between 150 and 200 m above sea level (m a.s.l.), where meteoric water infiltrates westward through the formation. Although the Cenozoic strata dip toward the east, the regional hydraulic gradient, driven by the topographic relief from the Eastern Cordillera foothills (ranging from 700 to 1,700 m a.s.l.) to the Magdalena River (at an elevation of between 70 and 75 m a.s.l.), governs the direction of groundwater flow. This flow could be further enhanced by permeable fault zones, such as the Arrugas Fault, along with geological outcrops that may serve as secondary infiltration points. Piña et al. (2022) described similar processes, albeit in shallower geological formations (depths ranging from 250 to 395 m), such as the Real Group and the Mesa Group (Miocene-Pleistocene), where meteoric water infiltrates through major geological structures. These authors also noted that limited knowledge of these structures and a lack of monitoring hindered the ability to track the infiltration process in the deeper and older formations in the MMB properly.

Fig. 9. Conceptual schematic hydrodynamic diagram illustrating recharge areas and potential groundwater flow paths from high to low elevations, together with the upward migration of deep Cretaceous connate waters along reverse fault planes. Diagram adapted and modified from Morales et al. (1958) and Gómez et al. (2005a, b).

As the meteoric water moves westward, it descends from the western flank of the NMS and from the Peña de Oro syncline (140 to 150 m a.s.l.) to lower elevations (about 80 m a.s.l.). In this down-path groundwater flow, there is an interaction with connate waters derived from Cretaceous strata that migrate upward along reverse faults, such as the La Salina, Arrugas, Infantas, and Casabe faults. The Umir Formation, which is the regional seal between the Cretaceous and Cenozoic units, isolates the overpressured connate fluids while permitting limited vertical migration through reactivated faults (Rivera et al., 2025). Hydrocarbons and connate waters migrate together as multiphase fluids, but different physical processes govern their movement. Hydrocarbons, being less dense and more buoyant, tend to migrate preferentially along structural pathways, while hydraulic gradients and overpressure primarily drive the movement of brine. This process creates localized mixing zones that can be detected through isotopic anomalies, such as enriched ⁸⁷Sr/⁸⁶Sr ratios. This interplay between structural architecture, stratigraphic compartmentalization, and hydraulic gradients defines the complex dynamics of groundwater within the MMB.

Geochemical studies by Rangel et al., (1996, 2000), Ramón et al., (1997, 2001), Sarmiento (2011), and Thompson-Butler et al., (2019) confirm that the hydrocarbons found in the MMB originated from Cretaceous source rocks, mainly the La Luna Formation, as indicated by biomarker analyses of saturated and aromatic fractions. These findings support the hypothesis that connate waters migrate alongside hydrocarbons from the La Luna Formation into Cenozoic reservoirs.

Once the connate waters are emplaced, they mix with meteoric water, modifying the hydrogeochemical system. Whilst this mixing process is facilitated by fault-controlled connectivity, it occurs differently over time and through various mechanisms when compared to hydrocarbon migration (Bjørlykke, 1988; Mearns and McBride, 1999). In this case, water movement is less influenced by buoyancy and is primarily driven by pressure differentials and rock-fluid interactions.

Depending on the fault system, the polyphase history of some (e.g., La Salina) may restrict their actual hydraulic connectivity to the Mugrosa Formation reservoirs or deeper strata, despite evidence hinting at it as a pathway for fluid migration (e.g., Castelblanco et al., 2025). As previously mentioned, overpressures in deeper Cretaceous compartments restrict the vertical percolation of meteoric waters; however, seismic pumping may facilitate this process (Simpson, 2024). For example, pressure/permeability tests or other production data from adjacent fault systems could provide additional evidence to support or refute the model proposed in this study.

The hydrogeochemical results obtained here align with the hydrogeological model proposed by Cañas et al. (2019) and Piña et al. (2022), which also identified a primary infiltration area located in the eastern MMB, with groundwater flow directed westward. While Cañas et al. (2019) proposed this flow path for Neogene formations, our study extends this flow pattern to the Paleogene Mugrosa Formation (Fig. 9). The infiltration area for this formation is located in the eastern region at higher elevations, while the discharge area is situated near the Magdalena River. Piña et al. (2022) highlighted the lack of structural knowledge and groundwater monitoring in the MMB, and suggested that shallow aquifers associated with Neogene formations exhibited connections between surface and groundwater due to the hydraulic gradient and potential fractures. Our findings provide evidence that meteoric water infiltration occurs at significant depths (between 793 and 1951 m) in the MMB. Additionally, ionic, stable isotope, and radiogenic analyses confirm that groundwater chemistry evolves due to water-rock interactions as it moves through the hydrogeological system. These results also suggest possible connections between regional groundwater flows and deep connate waters from Cretaceous formations, migrating toward the surface through fault systems, a hypothesis that warrants further investigation.

While the regional groundwater flow pattern (east-to-west) dominates the Mugrosa Formation’s hydrodynamics, localized brine systems may arise from structural and stratigraphic heterogeneities. These compartmentalized subsystems, potentially driven by overpressure differentials across the Umir Formation seal, could explain geochemical anomalies (e.g., elevated ⁸⁷Sr/⁸⁶Sr, Cl^- spikes) that deviate from regional trends.

Ultimately, the low water cut levels in production wells throughout the study area indicate complex fluid dynamics, as seals facilitate the selective movement of hydrocarbons while limiting the migration of large volumes of water. Alternative mechanisms for water infiltration and migration from deeper Cretaceous formations into Paleogene reservoirs include the tortuosity of the succession and seal leaks of the regional Umir Formation. All these aspects should be investigated in future research.

6. Conclusions

A key emerging challenge in mature hydrocarbon-producing basins in Colombia, exemplified by the Middle Magdalena Valley Basin (MMB), is the practical application of analytical techniques capable of distinguishing between naturally infiltrated meteoric water, connate formation water, and fluids introduced through enhanced oil recovery (EOR) operations. As EOR processes become increasingly prevalent, they introduce additional geochemical and isotopic complexities to the signatures of groundwater and formation fluids, complicating the interpretation of basinal fluid origins. This problem requires the development of robust methodologies to characterize and trace fluid sources within the reservoir system accurately

This study therefore provides new insights into the hydrogeological and geochemical processes governing groundwater flow in the Mugrosa Formation of the MMB. Geochemical and isotopic data indicate a regional groundwater flow system with recharge occurring at the foothills of the Eastern Cordillera and discharge near the Magdalena River. Groundwater in the eastern MMB shows a stronger meteoric influence, as indicated by higher carbonate and bicarbonate concentrations and less evolved chemical signatures. In contrast, western areas exhibit more evolved waters with elevated chloride (Cl^-) concentrations, as well as enriched δ¹⁸O and ⁸⁷Sr/⁸⁶Sr isotopic ratios, suggesting longer residence times and greater rock-water interaction. Mixing between meteoric and connate waters occurs mainly along structural features such as thrust faults and folds. Connate waters, derived from deeper Cretaceous units, migrate upward along fault planes with hydrocarbons and interact with meteoric water in Paleogene reservoirs, altering their geochemical and isotopic profiles.

Acknowledgments
The authors acknowledge the Ministry of Science, Technology, and Innovation of Colombia (Minciencias) for its financial support through Grant No. 757 of the 2016 National Doctoral Program. Gratitude is also extended to the Petroleum Geochemistry Laboratory at the Industrial University of Santander and the Thermochronology Research Laboratory at EAFIT University for their valuable analytical support. Special thanks are due to Engineer Jorge Sáchica (ECOPETROL) for his collaborative contributions. The authors further express their sincere appreciation to the anonymous reviewers whose insightful comments and constructive suggestions greatly improved the clarity and overall quality of this paper.

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