An extra-heavy crude oil downhole upgrading process involves the downhole addition of a hydrogen donor additive under steam injection conditions. Physical simulation experiments show an increase of approximately 3 degrees API in the treated crude oil gravity, a three-fold viscosity reduction, and approximately 8 wt% decrease in the asphaltene content. Compositional-thermal numerical simulations were performed and the results showed a good match between calculated and experimental API gravities of the upgraded crude oil for all conditions studied.