A molecular model for simulating the aggregation of asphaltenes and resins in crude oils on a mesoscale is proposed. The asphaltene molecules are treated as discotic seven-center Lennard-Jonesium molecules, the resins are modeled as single spheres, and the surrounding crude oil is modeled as a continuum, characterized by a screening factor, and defined using a combination of its Hamaker and dielectric constants. The parameters for the model are obtained by coarse-graining the potential energy surface obtained from model atomistic simulations of pairs of asphaltenes and resins. Canonical Monte Carlo simulations are performed with this model, and effects of temperature, asphaltene, and resin concentration are studied parametrically. The results agree with experimentally observed tendencies. The asphaltene is seen not to conform to a linear aggregation model, but exhibits a more complex multimodal aggregation pattern. The screening constant of the crude oil, which ultimately controls the aggregation, can itself be related to other measurable quantities such as the refractive index.