Genesis and evaporative fractionation of subsalt condensate in the northeastern margin of the Amu Darya Basin

Abstract

This paper studied the origin and the maturity of the Jurassic subsalt condensate in the northeastern margin of the Amu Darya Basin and analyzed its formation condition and mechanism through conducting geochemical analysis of biomarkers, carbon isotopes and light hydrocarbons of condensate and source rock. Combining geology and geochemistry analysis, this paper compared the components of natural gas and associated condensate, and conducted oil-source correlation as well as analyzed condensate maturity in nine gas reservoirs. The genesis and geological conditions of condensate in the northeastern margin of the Amu Darya Basin were analyzed and a geological model of condensate accumulation was thereby established. The results show that natural gas in the northeastern margin of the Amu Darya Basin is dry gas with residual oil and oil intrusion. Parameters, e.g., saturated hydrocarbon distribution, sterane and terpane biomarkers and isotopes, suggest that the condensate was primarily derived from the Upper Jurassic marine source rocks. Mature condensate was identified using maturity parameters, e.g., heptane and isoheptane, light hydrocarbons 2,4-DMP/2,3-DMP(Dimethyl Pentane), C29sterane isomerization and aromatic hydrocarbon methylphenanthrene index. The relationship between light hydrocarbon paraffinicity and aromaticity indicates that the condensate was derived from evaporative fractionation. Comprehensive analysis draws the conclusion that condensate at subsalt reservoir in the northeastern margin of the Amu Darya Basin was contributed by evaporative fractionation. Oil generated by the Upper Jurassic marine source rocks at early stage was concentrated in the subsalt carbonate strata. Considerable dry gas was generated by Middle and Lower Jurassic coal-bearing source rocks in the late stage migrated to the Upper Jurassic reservoirs and displaced crude oil due to the absence of interlayers. And the light hydrocarbons in the oil were dissolved in natural gas and therefore formed condensate. The evaporative fractionation under subsalt was contributed by: (1) two sets of source rocks, e.g., one is gas-prone and the other is oil-prone, (2) no obvious barriers between these source rocks, (3) excessive natural gas generated.