Factors controlling the proportionality of vanadium to nickel in crude oils

The proportionality of V to Ni in crude oils is determined by the environmental conditions in which their source rocks were deposited. Thermal maturation, migration, and reservoir alterations may change the concentrations of these two metals by addition or subtraction of more labile portions of a crude oil, but their tenacious bonding with high-MW organics suggests that their proportionality to one another should remain unchanged. Eh-pH diagrams offer an explanation of the factors controlling their proportionality and its relationship with S contents. Three Eh-pH regimes are proposed for the natural system. Regime I represents conditions under which Ni⁺² is available for bonding and vanadium is unavailable because of its quinquivalent state. Crude oils expelled from source rocks deposited within this regime are expected to have $\text{V}\text{(Ni + V)}$ less than 0.10 and low S (<1 weight percent). Regime II represents conditions under which Ni⁺²-Nickelous cations and vanadyl cations are available with vanadyl cations being hindered in part by the formation of hydroxides and nickelous cations being hindered in part of metastable sulfide ions. Source rocks deposited within this regime expel crude oils with low S contents and V-Ni fractions that range from 0.10 to 0.90. Regime III represents conditions under which vanadyl and trivalent vanadium cations are available for bonding but Ni⁺² may be partially hindered by sulfide complexing. Source rocks deposited within this regime expel crude oils that have high S contents and V-Ni fractions greater than 0.50.