Abstract: The crude oil mass fractrion is an important index to determine the oil sand effective thickness and judge if the oil sands possessing commercial recovery value. This paper analyses the mass fraction in oil?bearing core sampling and inversely calculates the limit of the fraction by means of the core sample occurrence, inverse and analogy, based on which the effective thickness of the oil sands drilled pay zone in Fengcheng area in Junggar basin is determined comprehensively. The estmated OOIP of the Cretaceous in Fengcheng oil sands is about 2 741.69×104t