Abstract: The overall horizontal well waterflooding development of carbonate reservoir is without precedent in Oman. This paper introduces a case of China’s first use of the such a technology to successfully develop the carbonate reservoir with highly depleted, thin?thickness,high pore and low permeability in Oman?D oilfield. The reservoir and fluid characteristic indicates that this oilfield is suitable for horizontalwell waterflooding development, and the overall strategy of long?horizontal section, small well spacing, toe?to?heel reversed deployment and top?production and bottom?injection was determined by reservoir numerical simulation. Pilot test was first implemented. After the successful practice, based on fine geological model and guaranteed by geosteering horizontal drilling technology, the overall horizontal well injection?production pattern was extended and perfected. In the process of horizontal well injection?production optimization, the water injectionperformance was monitored by abundant dynamic and static data, and the displacement of oil by water was tracked by streamline simulation, then the injection and production volumes were modified to reach the goal of evenly displacing oil by injected water. The case study indicates that effect of horizontal well waterflooding development is remarkable. Taking Block?B as an example, its incremental oil was 220×104 t as of end of 2011, and water cut increased stably. The predicted ultimate recovery would be 44%