An electron microscopy study of crude oils and maltenes

An electron microscopy study of four Venezuelan crude oils and their corresponding maltenes has been performed, using the combine freeze fracture-transmission electron microscopy (FFTEM) technique. All samples showed the presence of spherical primary colloidal particles, with average diameters in the range of 7-9 nm and an apparent Gaussian distribution. Large aggregates of primary particles, with the longest length being similar to200 nm or more were observed in some crude oils. These aggregates could be broken down to smaller aggregates or to primary particles by stirring and/or heating the crude oil. This indicates that the binding energy of these aggregates is weak, on the order of kT. Dilution with benzene up to 80 times does not dissolve the colloids completely but does reduce its diameter. On the other hand, the addition of heptane (up to 3 volumes) leads to an increase in diameter, which suggests the adsorption of resins upon dilution. In agreement with previous findings,3 these results were observed to be consistent with the model for the colloidal particle, where low-solubility fractions are mainly located at the core and soluble fractions prevail at the periphery.