IDENTIFICATION OF TWO TYPES OF EVAPORATIVE CONDENSATE IN EARLY PALEOZOIC CARBONATE RESERVOIRS IN THE TAZHONG REGION, TARIM BASIN, CHINA

Evaporative fractionation is a complex of phenomena which involves multiple processes including advection, phase separation and migration of vapor phase with enhanced burden condensable liquid, considered as a widespread evolutionary process in petroleum basins. Plenty of evaporative condensates in numerous basins were analyzed for the distribution of different components in vapor/liquid phases, EOS of gas condensate, and prediction of exploration potential of an area. The discovery of deep buried Cambrian-Ordovician non-thermal genetic gas condensates in Tazhong region of the Tarim basin provides a new opportunity for understanding the geochemical behavior of different components in multiple processes of evaporative fractionation. By the calculation of n-alkane mass depletion, analysis of slope factor for pseudo-components, quantification of biomarkers and diamondoids, we identified two types of evaporative condensate in Tazhong region. One type of the evaporative condensate is characterized by depletion of n-alkane lighter than tridecane, E7<1,E3<1,C₁/C₂ ranges from 30 to 160, P4/P8 correlated negatively with C₁/C₂ (E7,E3,C₁/C₂ and P4/P8 were introduced by Thompson(2010) to quantify the modification of gaseous and gas–liquid components in advection and evaporative fractionation), and MAI ranging 70-95, MDI ranging 40-60.The data indicates dry gas charged into oil reservoir and alternated to be high GOR condensate reservoir. Dry gas charging results in mass depletion of n-alkane and enrichment of low weight diamondoids in condensate. This type of condensates is mainly located in the east area of Tazhong region. The other type of the evaporative condensate is characterized by slightly n-alkane depletion, E7>1,E3>1,C₁/C₂ ranging 0-50, P4/P8 irrelevant with C₁/C₂, MAI ranging 70-82, MDI ranging 43-50. It indicates that wet gas carrying condensable fluid charged into reservoirs and caused less n-alkane mass depletion and no enrichment of diamondiods, and they mainly distributed in the west area of the Tazhong region. The study shows that dry gas advection leads to more n-alkane mass depletion and higher concentration of diamondoids in reservoir than wet gas charging, which means that geochemical characters of evaporative condensate may strongly influence by gas properties.