Abstract: This paper presents the charts of compositional gradient distribution of different types of reservoir fluids based on the data from 21 groups of fluid compositions that change with depth reported in literatures at home and abroad. It is preliminarily identified that the compositional gradient of C₇⁺ in the volatile oil system varies in range of 0.01~0.05 mol%/m. Using the isothermal model of compositional gradient to predict the oil-gas contact, the light and heavy components and the physical parameters of crude oil in volatile oil reservoir, it is shown that the maximum rate of change occurs right near the oil-gas contact, displaying the distribution and variation of the fluid component and property with depth. Using the iso-compositional and isothermal gradient model to predict the PVT attribute, the relation chart of the crude oil volume coefficient and the solution gas-oil ratio with depth of reservoir is established for estimation of the range of geological reserves. For PB reservoir as a case, by using this simulation method, it shows that the re-estimated OOIP and OGIP are 5.38% and 3.9% higher than the original estimation values, respectively.

Key words: compositional gradient, volatile reservoir, geologic reserve, OOIP, OGIP, state equation