Abstract:Generally speaking,the description of a favo urable oil trap needscharacteristics(only four ones are taken in this paper),and each trap has n corresponding membership grades= ul,u2,u3,…,un.These characteristics play different roles in the trap respectively;therefore,they have different weights a,,a:,a3,…,an.Summing the product of membership grade and the corresponding weight,we have u.when a u% M, where M is a defined threshold value, we can consider trap favourable.Hydrocarbon traps can be classed and evaluated in mS of the threshold value M.The example of Dongpu depression is given to show how the the ternnethod is applied to the evaluation of hydrocarbon traps.