Textural and Compositional Controls on Reservoir Quality of the Cretaceous Caballos Formation in the Putumayo Basin, Colombia

ABSTRACT The evaluation of geological attributes such as mineralogical composition, sedimentary texture, diagenetic processes, and petrophysical properties is crucial for understanding the storage, flow, and recovery potential in hydrocarbon reservoir rocks. This study focuses on the Cretaceous Caballos Formation in the Putumayo Basin, a foreland basin that borders the intramontane Upper Magdalena Valley basin to the west and the Llanos Orientales basin to the northeast. The main objective is to determine how compositional, textural, and diagenetic characteristics influence petrophysical properties, specifically porosity, permeability, and capillarity. Surface exposures of clastic rocks associated with this formation were analyzed at the asymmetric and faulted Santa Ana anticline, near the Costayaco field. Core samples were collected, and thin sections were prepared for detailed sedimentological analysis, focusing on mineral composition, grain size, sorting, roundness, and sphericity. Additional analyses investigated compaction processes, pseudomatrix formation, dissolution, and cementation by silica, carbonates, and iron oxides. Subsequently, petrophysical tests were performed on both the lower and upper segments of the formation. Variations in porosity and permeability were correlated with geological attributes through statistical analysis using the Pearson correlation coefficient, quantifying the relationships between textural characteristics (grain size, shape, and sorting) and petrophysical properties. The results indicated a significant positive correlation between porosity and permeability in both segments, at the individual sample level and within the regional context supported by well data. Additionally, a relationship between pore throat size and grain size significantly influencing permeability was observed. Diagenetic and compositional analysis revealed that the presence of clay matrix and pseudomatrix, resulting from sedimentary lithic deformation and glauconite alteration, negatively impacted pore space connectivity, thus reducing reservoir quality. Furthermore, a progressive quartz enrichment from the lower to upper segments was identified, suggesting greater compositional maturity attributed to tropical humid conditions during the Albian–Aptian. This enrichment facilitated the preservation of petrophysical properties in specific petrofacies, enhancing hydrocarbon storage and flow potential. Recognizing surface analogues, as addressed in the introduction, becomes essential due to the frequent lack of detailed subsurface information. Surface analog studies provide valuable insights into reservoir characteristics and diagenetic processes, thereby significantly improving the certainty and reliability of subsurface reservoir models used in hydrocarbon exploration and production. Ultimately, integrating surface analogues into geological modeling contributes substantially to reducing uncertainties and optimizing exploration and production strategies.