This paper describes a successful steam-injection vertical expansion recently implemented in California＇s Midway Sunset field with slim-hole injection wells. Slim-hole injectors were chosen over conventional injectors to reduce capital investment. The savings allowed drilling of additional injection wells, which were used to optimize steam placement on the basis of reservoir architecture. Two vertical expansion designs were compared at a fixed cost with either conventional or slim-hole steam injectors. The conventional well design had 10 injection wells and three temperature-observation wells. The steamdrive targets were two steeply dipping turbidite depositional lobes containing multiple, vertically segregated sand bodies. Selection of conventional well locations was based on the upper primary reservoir compromising injection location in the lower secondary reservoir. The slim-hole plan, consisting of 38 injectors and 10 temperature-observation wells, was selected because of its advantages. The capital cost for the slim-hole wells was about the same as the conventional program. Approximately three slim-hole injectors could be drilled for the cost of one conventional injector. With additional wells, injector locations were optimized for both the upper primary and lower secondary reservoirs. These additional injection wells provided improved profile control and steamflood management of the individual sand bodies within each reservoir. As a result of the vertical expansion, incremental production has increased steadily since project startup.