Contrasting Oil Properties and Formation-Water Chemistry in Lower Cretaceous and Jurassic Reservoirs of the Uytas Field, Precaspian Basin

Shallow supra-salt oils in the Precaspian Basin change with depth, but the controls are rarely tested. This study aims to quantify the vertical contrast in oil properties at Uytas on the south-eastern basin margin, separate its thermal and compositional parts, and identify what controls the formation waters, using ten oil and fifteen water analyses from twelve wells. The 24.0–28.0 °API oils are medium-gravity crudes, viscous in place because the reservoirs are cold. Estimated dead-oil viscosity at reservoir temperature is 234–764 mPa·s in the Lower Cretaceous and 83–112 mPa·s in the Jurassic, with no overlap in density, viscosity, saturates or aromatics (exact permutation p = 0.0048–0.0095). At a common 20 °C the 4.52-fold contrast falls to 2.40, so it is partly thermal. Only salinity separates the waters (7.4–31.4 against 46.1–94.9 g L−1), consistent with recharge in the Cretaceous and dissolved halite in the Jurassic. Regional evidence points to one oil family from sub-salt Palaeozoic source rocks; the remaining contrast is consistent with mild alteration of the shallow oils, which molecular and isotopic analyses can distinguish from multi-stage charge. Needing only routine appraisal data, the approach suits supra-salt fields across the Precaspian Basin and small fluid datasets in other salt basins.