A molecular thermodynamic model has been considered for the formation and growth of nonionic aggregates of surfactant molecules in a nonpolar solvent in the absence of water. The model implies fluctuating coexistence of micelles having different shapes without activation barriers between them. The work of micelle aggregation has been derived for a solution in heptane using molecular dynamics data. In the considered model, it is assumed that, for any aggregation numbers, the minimal aggregation work depends not only on the aggregation numbers and surfactant monomer concentration, but also on two independent shape parameters characterizing the deviation from the spherical shape of an aggregate. This approach provides a unified description for both disc-shaped and cylindrical micelles.