Abstract This paper presents the key of success for the development of a complex prospect, the Sarr dolomitic reservoir in the Kharir field, located onshore Yemen. The drilling of horizontal wells was considered to limit the early water encroachment observed on vertical producers and to improve both the oil production and the final recovery. A careful data acquisition program, a close cooperation between all the parties involved in the drilling process, with the implementation of common decision procedures, allowed geosteering in optimized reservoir targets, while preserving the safety of the horizontal drilling operation. The Kharir Sarr field is a multi-layered oil bearing dolomitic reservoir interbedded with tight limestones. Variable petrophysical properties characterize the dolomite in this field, following the geometry of elementary sedimentary facies, their stacking, their structural position, and finally their diagenetic history. On top of the structure, the reservoir is interpreted as dolomitized patchy biostromes, very difficult to target and to appraise with vertical wells. Monitoring the data acquired in real-time, referenced to pre-job-modeling, provided the key elements for cautious decision-making on trajectory definition. The expertise developed in geological steering has contributed to boost oil production and add significant field reserves.