Abstract: During the oil and gas exploration, sampling operation with the wireline formation tester(WFT) is the most direct and effective method in identifying oil, gas and water layer correctly. This paper mainly discusses how to distinguish the data of WFT. Comparing the chloride concentration of mud filtrate samples by WFT from the invaded zone or transitional zone of pure oil reservoir with that of samples by conventional sampling operation at the same position, it shows that the water samples obtained by wireline sampling from the pure oil reservoir all are mud filtrate. It is concluded that it is necessary to compare the chloride concentration of water samples with that of drilling fluid used at the same depth in order to distinguish whether the formation water is contained in the samples or not. This conclusion further explains how to distinguish samples by wireline sampling, eliminates the doubts caused by filtrate sampled from the pure oil reservoir, and makes the explorationists' regain the confidence in wireline sampling operation.