Phase-controlled molecular fractionations in migrating petroleum charges

An experimental approach has been used to make a preliminary examination of the possible effects of partitioning processes distributing molecular markers between vapour and liquid petroleum phases during petroleum migration in the system gas-saturated oil/condensate-saturated gas. These effects are discussed with reference to several traditional petroleum geochemical parameters. Some source and maturity related geochemical parameters are dispersed from the inherited source values when a petroleum system undergoes a transition from a single phase gas-condensate to a two phase system and it is probable that partitioning is one of the many processes that contribute to the scattered data sets observed in natural systems particularly those associated with gas/condensates. In addition to affecting gross composition and gas and gasoline range composition, physical partitioning of petroleum components between vapour and liquid phases in deep reservoirs containing rich gas condensate vapour and a liquid petroleum phase in equilibrium can affect even high molecular weight components, selectively partitioning lower homologues into the vapour phase. This may disperse to various degrees any source inherited signature in source facies or maturity interpreted from parameters such as aromatic hydrocarbon distribution, gasoline range parameters, or sterane carbon number distribution. Migration history, or the frequency and extent of multiple equilibrations between vapour and liquid phases in deep high pressure reservoirs followed by selective release of one phase to a shallower position to produce a two phase petroleum system is proposed as an important variable in the geochemical interpretation of once equilibrated petroleum systems. It is concluded that phase related effects must be considered in addition to accepted source and maturity considerations as controlling the molecular composition of two-phase petroleums derived from single-phase precursors. This is particularly so for rich gas condensate systems.