Study On Accumulation Process And Main Controlling Factors Of Undersalt Samandepe Condensate Gas Field In Amu Darya Basin,Central Asia

Condensate oil is a high-quality oil and gas resource,and the accumulation mechanism of condensate oil has always been a concern of oil and gas geologists.The Amu Darya Basin,located in Central Asia,is a world-renowned oil and gas basin and an important area for China’s oil and gas cooperation with foreign countries.The basin is famous for the development of large/giant subsalt condensate gas fields.However,there about the genesis of condensate oil,and the research on the controlling factors of condensate oil accumulation is relatively weak.Taking the Samandepe large-scale subsalt condensate gas field in the northeastern margin of the Amu Darya Basin as an example,this paper uses oil and gas geochemistry and basin simulation techniques to study the trap evolution history,hydrocarbon generation history and oil-source correlation of condensate oil and gas,analyzes the genesis and accumulation period of condensate oil,establishes the accumulation model of condensate oil and gas,and discusses the process and main controlling factors of oil and gas accumulation.Get the following understanding:The Samandepe condensate gas reservoir is an anticline structural gas reservoir with reef-bank reservoirs.It is a lithologic trap from the end of the Gimmerian to the Paleogene,and was transformed into a structural trap in the late Paleogene.The gas reservoir core sees ancient reservoir asphalt.The carbon number of the main peak of the n-alkanes of the condensate oil biomarker compound is mainly n C₁₅,C₂₇sterane is greater than C₂₉sterane,abnormally high pregnane,tricyclic terpane/pentacyclic terpane 1.79-3.49,and carbon isotope characteristics and oil source comparison show that the condensate oil is derived from the Upper Jurassic marine type II source rock,and the condensate oil maturity（Ro）value is1.03%-1.27%.The dryness coefficient of natural gas is 0.95-0.98,which is derived from the middle-lower Jurassic high-mature coal source rocks with different oil and gas sources,while the thermal evolution of the upper Jurassic type II source rocks is in the oil generation stage.Therefore,it is judged that the condensate oil belongs to the“gas invasion”secondary condensate oil genesis.The accumulation process of Samandepe condensate gas field is“early lithologic reservoir,late structural condensate gas reservoir”two stages of accumulation.At the end of Cretaceous,the oil generated by the Upper Jurassic type II source rock is filled with the reef lithology“large trap”,forming a lithologic ancient reservoir with low oil filling coefficient.At the end of the Paleogene,the excessive dry gas generated by the coal-bearing source rocks of the Middle-Lower Jurassic filled the anticline structural traps,“gas invaded”the ancient oil reservoirs,and the light hydrocarbon components in the oil were dissolved in the natural gas to form condensate gas reservoirs.The two sets of source rocks dominated by oil and gas generation under the salt are the main reasons for the formation of condensate gas.The superimposed area of inherited paleo-uplift and platform margin reef beach is the main controlling factor of“gas invasion”condensate oil accumulation.There are 32 figures,6 tables and 62 references in this paper.