Assignment standard for hydrocarbon-bearing probability of traps is the basis of quantitative assessment hydrocarbon potential of traps and has played a very important role in exploration risk analyses.Four key reservoiring elements including trap

charging

reservoir and preservation were chosen to be the major factors to represent hydrocarbon-bearing probabilities of traps with the products of their individual probabilities.For traps composed vertically of several target re-servoirs

the hydrocarbon-bearing probability refers to the probability of founding oil or gas in at least one target reservoir of the trap.Each of the four factors was further divided into two subfactors-occurrence subfactor and effective subfactor-based on different reservoiring controlling mechanisms and risk analyses of various oil or gas reservoirs.By applying the Barrel Effect principle to our calculation

we proposed that if all the key parameters of the subfactors were independent from one another

then the hydrocarbon-bearing probability would be the product of probability of key parameters;on the other hand

if the subfactors were related to one another

then the probability would be the product of probability of key parameter with the maximum risk multiplying that of other subfactors.Based on the relevance

availability

controlling le-vel and quality of data

we established the principle for probability assignment and set up standards for assigning the probabilities to the factors and subfactors and a 5-level hydrocarbon-bearing probability system.According to the system

the bigger the probability of trap contains hydrocarbon

the lower the geological risks

and vice versa.Implementation of the method in Tarim basin has been proven positive.