As a unique and one of its kind, the overall objectives of this study are oriented towards investigating the evolution of the Lower Cretaceous strata at the Kharir Field in the Sayun - Masila Rift Basin, Yemen besides studying the reservoir quality and associated hydrocarbon resources. This study, in particular, deals with the subsurface facies sedimentology of the basin and mainly concerned with: characterize the sedimentary facies and the depositional paleo-environment of the basin besides establishing facies depositional model for the sedimentary strata in this basin, investigate the intensity of paleo-weathering, sediment transportation processes, depositional history and source rock origin and to infer the effect of the depositional regime and the diagenetic processes on reservoir rocks quality of the study basin. This research will spot the light on the characteristics of the reservoirs and should be especially useful for all Geologists, Sedimentologists, Petroleum and Reservoirs Engineers as well as all Oil and Gas expats.