Abstract: The high CO₂ content and various fluid components in oil and gas columns of reservoirs of Jupiter oilfield in the Santos Basin is the main reason why there has not been a consensus on the phase and forming mechanisms of the reservoirs yet.Based on a four-parameter-block diagram,a ternary diagram of fluid component,and a multiple parameter method with φ₁,Z₁ and Z₂ values,we tried to understand the phase of reservoirs in Jupiter Oilfield.The result shows that the reservoirs are condensate reservoirs with oil rings.The condensate was found to share similar molecular characteristics with that of the crude oil in the reservoirs:Pr/Ph value between 1.58 and 1.88,C₂₆/C₂₅ tricyclic terpane value ranging from 1.15 to 1.22,high abundance of C₂₇ sterane relative to C₂₉ sterane,hopane to sterane ratio value greater than 4 and TPP value greater than 0.5,and carbon isotope value of whole oil at -25‰.All these geochemical parameters indicate that both the condensate and the oil be originated from the Cretaceous Upper Barremian-Lower Aptian lacustrine source rocks.Both the condensate and the crude oil have similar maturity and correspond to peak oil generation stage.The maturity of alkane gas,sourced from the Upper Barremian-Lower Aptian lacustrine source rocks,corresponds to the peak oil generation-early condensate forming stage.The CO₂ is volcano-mantle type and its later influx transformed the reservoirs that formed at the early stage through evaporative fractionation,resulting in the present condensate reservoirs with oil rings.The evaporative fractionation is supported by geochemical parameters.The influx of inorganic CO₂ added some new content to the theory of the evaporative fractionation.

Key words: evaporative fractionation, condensate reservoir with oil ring, CO₂, hydrocarbon accumulation, Cretaceous, Jupiter oilfield, Santos Basin