Saturation-Height Modeling from Drainage and Imbibition Capillary Pressure Data: Uncovering Saturation History and Structural Evolution

Fluid distribution in hydrocarbon reservoirs at static equilibrium is governed by the balance between capillary and gravitational forces. Saturation-height functions (SHFs), derived from and calibrated using laboratory capillary pressure (Pc) data, are commonly used to estimate initial hydrocarbon volumes and to initialize dynamic reservoir models. In deep-water turbidite sandstones with complex faulting, relying solely on drainage Pc curves may misrepresent saturation profiles. Observations such as paleo-residual oil below the oil-water contact (OWC) and sharp transition zones above it suggest a complex saturation history, including imbibition due to OWC rise or structural downshifting. This study demonstrates the importance of developing and utilizing both drainage and imbibition SHFs and validating against alternative methods such as core-and Archie-based water saturation (Sw_core and Sw_Archie, respectively). This workflow enhances static reservoir characterization, links saturation history to structural evolution, and reduces volumetric uncertainty. Drainage experiments simulate initial water expulsion during hydrocarbon charge, while imbibition reflects water re-entry from later geological or production events. Mercury injection (drainage) and extrusion (imbibition) data from representative core plugs were used to construct rock-quality-dependent SHFs. The workflow includes selecting representative Pc curves across target stratigraphy and rock quality, applying clay-bound water corrections, normalizing to J-function, and converting to height above free water level at reservoir conditions. Each curve was fitted using Leverett-J, Brooks-Corey, and Thomeer models via SciPy least squares and ranked by average correlation error. Fitting parameters were correlated with porosity (φ), permeability (k), and √(φ/k) to derive continuous SHFs above free water level (FWL) in the reservoir. A separate correlation was developed for the paleo-residual interval below FWL using lab core water-flood end-point saturations. Saturation trends in various zones and wells were integrated with structural and fault displacement maps to link variations to specific faulting events. Among the three SHF models tested, Leverett-J yielded the lowest average fitting error. Drainage and imbibition Leverett-J SHFs were applied across all wells to calculate corresponding saturations (SwH_Dr and SwH_Imb, respectively) and validated against core and Archie Sw. Archie Sw generally agrees with core Sw in thick sandstones and is used to validate SHFs in non-cored wells. SwH_Imb aligned well with core Sw across the entire cored interval, while SwH_Dr predicted higher Sw, especially within 150 ft above the FWL. This pattern was observed in four wells located within two downthrown fault blocks, whereas the drainage SHF model provided a better match in wells situated in the structurally higher block. These results suggest that structural movement and faulting can explain the observed imbibition saturation profiles without requiring a significant upward shift in the FWL. Leverett-J saturation-height functions (SHFs), derived from both drainage and imbibition capillary pressure data, effectively capture saturation hysteresis and support the interpretation of structural overprint in deep-water reservoirs. Imbibition SHF best matches core and Archie Sw in downthrown fault blocks, while drainage SHF gave better alignment in wells in structurally stable blocks. These patterns support a scenario in which imbibition is mainly caused by downward structural movement into a stable FWL, rather than by a considerably deeper paleo-FWL. This scenario also provides evidence that faulting occurred after or during hydrocarbon accumulation which is consistent with the current petroleum system maturity models. The results highlight the value of applying both drainage and imbibition SHFs to reconstruct saturation history and understand structural evolution and obtain more accurate estimates of fluid saturations in complex faulted settings.