Ordovician reservoir is the karstified fractured/vuggy carbonate rock in Block 12 in Tahe Oilfield, in which oil, gas and water are distributed complexly in regularity. According to the analysis of the physical property of oil, the characteristics of hydrocarbon distribution in plane and vertical, and the oil-water interfaces, it is recognized that the oil is commonly overweight crude oil with high viscosity and high content of wax and sulfur. The oil density exhibits a distribution trend, in planar, of low values in the eastern and western parts and high values in the central part of this area but the oil density is little relative with depth in vertical. The oil-water interface is inconsistent and decreases with decreasing the Middle Ordovician top from east to west. The areas with high oil yield focuses along the main fault belts. It is shown that the hydrocarbon distribution is without the control of nowadays structural positions, instead of those factors such as the regional geological background, the distribution of fault belts, the development extent of reservoirs, the process of the hydrocarbon accumulation and unconformity.