The GOI (grains containing oil inclusions) index is used to distinguish oil zones, oil-water zones and water zones in sandstone oil reservoirs. However, this method cannot be directly applied to carbonate rocks that may not have clear granular textures. In this paper we propose the Effective Grid Containing Oil Inclusions (EGOI) method for carbonate reservoirs. A microscopic view under 10× ocular and 10× objective is divided into 10×10 grids, each with an area of 0.0625 mm×0.0625 mm. An effective grid is defined as one that is cut (touched) by a stylolite, a healed fracture, a vein, or a pore-filling material. EGOI is defined as the number of effective grids containing oil inclusions divided by the total number of effective grids multiplied by 100%. Based on data from the Tarim Basin, the EGOI values indicative of the paleo-oil zones, oil-water zones, and water zones are >5%, 1%–5%, and <1%, respectively. However, the oil zones in young reservoirs (charged in the Himalayan) generally have lower EGOI values, typically 3%–5%.